Brakes ...... Overdue write-up. (3 Viewers)

[MissouriLC]

Hard to see, but look a little wet inside motor. If so, that also reduces life. There's a breather hole to motor and drain for pump. They must not be clogged. Or fluid is sucked into motor.

Hey Paul (and any other brake gurus): TL;DR - I got the brand new MC assembly in in Saturday, and now I am getting ABS system DTC codes C1223 "ABS Control System Malfunction" and C1246 "Master Cylinder Pressure Sensor".

So, I received the brand new, complete MC assembly on Friday and got it installed Friday night. It only took me about two hours total, including setting the pedal height per the FSM and getting the interior put back together. On Saturday, I filled and bled the system with Toyota DOT 3 brake fluid (took exactly 6 12-oz bottles). Much easier to bleed than last time, perhaps due to the new MC being much more pressure tight. When I finished manual bleeding, I did the air bleeding procedure via Techstream--twice on each front line and once on each rear line. Checked the motor run-time after 40+ pumps of the brake pedal, and it's only 26 seconds! I struggled to get it to 40 seconds on the old MC.

I didn't have a chance to drive the car on Saturday, but I Sunday I went to church and afterwards to Harbor Freight to get some stuff to service my A/C. I was intermittently getting the ABS, VSC, and TRAC lights illuminating. They would come on, go off after a few minutes, very random. When I got home, I hooked up Techstream to check the codes and found the C1223 and C1246 codes mentioned above. No alarms, brakes feel great. When I had Techstream hooked up, the warning lights were NOT on. I shut off the car, unplugged and re-seated the pressure sensor connector, got back into Techstream with the car started again, and looked at the live data. Again, warning lights were NOT on at this point. Live data showed the pressure sensor responding with the pedal depressed. While sitting there, I couldn't get the lights to come on, so I cleared out the DTCs.

This morning, as I was waiting in line to drop the kids off at school, the lights came on again. I was sitting stopped, in Drive, with the brakes depressed for approximately 40 seconds when the lights popped on. Left school, lights went off, and stayed off until I was stopped at a long stoplight near work. Again, after about 40 seconds of the brake pedal depressed, the lights came on. Got to work, they had gone off again. Hooked up Techstream, and saw the same two codes.

Now, I haven't been able to trigger the lights when the car is in park, and every time I have been in Techstream so far, the lights have been off, so I don't know what the live data will show. I'll have to try and replicate that condition and report back.

Have you experienced anything like this on a new MC install? I would be disappointed if the brand new MC came with a bad pressure sensor. Any thoughts? Thank you in advance!

Best,
Tim

 

[MissouriLC]

Okay, so I looked up the Diagnostic Procedure for that DTC in the FSM. It states:

"1 Check output value of the master cylinder pressure sensor.
PREPARATION:
(a) Connect the hand-held tester to the DLC3.
(b) Turn the ignition switch ON and push the hand-held tester main switch ON.
(c) Select the DATA LIST mode on the hand-held tester.
CHECK:
Check that the brake fluid pressure value of the master cylinder pressure sensor displayed on the hand-held
tester is changing when depressing the brake pedal.
OK:
Brake fluid pressure value must be changing.
OK Go to step 4."

The sensor passes this test. So...

"4 Check whether or not the ECU terminal STP input voltage is changed when the
stop light switch is turned on and off."

What is this test? I am assuming this is the Skid Control ECU, under the steering column? I'll see if I can find the pin outs to see to what terminals they are referring. Any experience with this test? Thanks!

 

[MissouriLC]

Okay, so I looked up the Diagnostic Procedure for that DTC in the FSM. It states:

"1 Check output value of the master cylinder pressure sensor.
PREPARATION:
(a) Connect the hand-held tester to the DLC3.
(b) Turn the ignition switch ON and push the hand-held tester main switch ON.
(c) Select the DATA LIST mode on the hand-held tester.
CHECK:
Check that the brake fluid pressure value of the master cylinder pressure sensor displayed on the hand-held
tester is changing when depressing the brake pedal.
OK:
Brake fluid pressure value must be changing.
OK Go to step 4."

The sensor passes this test. So...

"4 Check whether or not the ECU terminal STP input voltage is changed when the
stop light switch is turned on and off."

What is this test? I am assuming this is the Skid Control ECU, under the steering column? I'll see if I can find the pin outs to see to what terminals they are referring. Any experience with this test? Thanks!

Update... with additional diagnostics.

On my lunch hour, I ran some additional diagnostics. My thoughts were going to something with the brake light switch, since the FSM diagnostics have the brake light switch and circuit as part of this. Regarding my question above, I also found that this is part of the diagnostic for C1249 "Stop Light Switch Circuit". So here's what I've done.

1. I confirmed the adjustment of the brake pedal and stop light switch; switch operation, brake pedal height from asphalt sheet on floor pan, and brake pedal free play. All adjustments are in factory spec as outlined in the FSM. I did not attempt to adjust the stop light switch, since it appears you have to remove the steering column to do so.
2. In order to complete the above, I had to remove the Skid Control ECU. After confirming the brake pedal and stop light switch was in spec, I reconnected the ECU, started the car, and "4. Check whether or not the ECU terminal STP input voltage is changed when the stop light switch is turned on and off." The STP pin-out is the green-white wire on the ECU connector right next to the big ECU connector. When installed, this is the connector that's second from the bottom. I checked voltage between the female pin on the plugged-in connector and body ground with the engine running. It showed 13.35 or 13.36 V with the pedal not depressed, and 13.31 V with the pedal depressed. Both figures in within the in-spec range of 10 to 14 V.

So, for the C1246 error code diagnosis:

1. Step 1: "Check output value of the master cylinder pressure sensor."
   "OK: Brake fluid pressure must be changing."
   PASSED
   "OK: Go to Step 4"
2. Step 4: "
3. Check whether or not the ECU terminal STP input voltage is changed when the stop light switch is turned on and off."
   "YES: Check and replace skid control ECU."

So, my skid control ECU is bad? Or intermittently so? If so, that is incredibly frustrating! It was working perfectly fine prior to replacement of the master cylinder. I will probably start a new thread, but any advice is, as always, appreciated. Thanks!

 

[ClassyJalopy]

Update... with additional diagnostics.

On my lunch hour, I ran some additional diagnostics. My thoughts were going to something with the brake light switch, since the FSM diagnostics have the brake light switch and circuit as part of this. Regarding my question above, I also found that this is part of the diagnostic for C1249 "Stop Light Switch Circuit". So here's what I've done.

1. I confirmed the adjustment of the brake pedal and stop light switch; switch operation, brake pedal height from asphalt sheet on floor pan, and brake pedal free play. All adjustments are in factory spec as outlined in the FSM. I did not attempt to adjust the stop light switch, since it appears you have to remove the steering column to do so.
2. In order to complete the above, I had to remove the Skid Control ECU. After confirming the brake pedal and stop light switch was in spec, I reconnected the ECU, started the car, and "4. Check whether or not the ECU terminal STP input voltage is changed when the stop light switch is turned on and off." The STP pin-out is the green-white wire on the ECU connector right next to the big ECU connector. When installed, this is the connector that's second from the bottom. I checked voltage between the female pin on the plugged-in connector and body ground with the engine running. It showed 13.35 or 13.36 V with the pedal not depressed, and 13.31 V with the pedal depressed. Both figures in within the in-spec range of 10 to 14 V.

So, for the C1246 error code diagnosis:

1. Step 1: "Check output value of the master cylinder pressure sensor."
   "OK: Brake fluid pressure must be changing."
   PASSED
   "OK: Go to Step 4"
2. Step 4: "
3. Check whether or not the ECU terminal STP input voltage is changed when the stop light switch is turned on and off."
   "YES: Check and replace skid control ECU."

So, my skid control ECU is bad? Or intermittently so? If so, that is incredibly frustrating! It was working perfectly fine prior to replacement of the master cylinder. I will probably start a new thread, but any advice is, as always, appreciated. Thanks!

That is not fun. Let me know the part number on the skid control ecu, I might have spare one I could just send you

 

[2001LC]

Update... with additional diagnostics.

On my lunch hour, I ran some additional diagnostics. My thoughts were going to something with the brake light switch, since the FSM diagnostics have the brake light switch and circuit as part of this. Regarding my question above, I also found that this is part of the diagnostic for C1249 "Stop Light Switch Circuit". So here's what I've done.

1. I confirmed the adjustment of the brake pedal and stop light switch; switch operation, brake pedal height from asphalt sheet on floor pan, and brake pedal free play. All adjustments are in factory spec as outlined in the FSM. I did not attempt to adjust the stop light switch, since it appears you have to remove the steering column to do so.
2. In order to complete the above, I had to remove the Skid Control ECU. After confirming the brake pedal and stop light switch was in spec, I reconnected the ECU, started the car, and "4. Check whether or not the ECU terminal STP input voltage is changed when the stop light switch is turned on and off." The STP pin-out is the green-white wire on the ECU connector right next to the big ECU connector. When installed, this is the connector that's second from the bottom. I checked voltage between the female pin on the plugged-in connector and body ground with the engine running. It showed 13.35 or 13.36 V with the pedal not depressed, and 13.31 V with the pedal depressed. Both figures in within the in-spec range of 10 to 14 V.

So, for the C1246 error code diagnosis:

1. Step 1: "Check output value of the master cylinder pressure sensor."
   "OK: Brake fluid pressure must be changing."
   PASSED
   "OK: Go to Step 4"
2. Step 4: "
3. Check whether or not the ECU terminal STP input voltage is changed when the stop light switch is turned on and off."
   "YES: Check and replace skid control ECU."

So, my skid control ECU is bad? Or intermittently so? If so, that is incredibly frustrating! It was working perfectly fine prior to replacement of the master cylinder. I will probably start a new thread, but any advice is, as always, appreciated. Thanks!

Did you disconnect battery, while working on master, ECU or for that matter any electrical plug!
Not doing so, there's a risk of shorting an EUC, blowing a fuse or setting off an air bag.

I'd also look very close at wires and pins. Make sure non damaged or pins bent.

 

[MissouriLC]

That is not fun. Let me know the part number on the skid control ecu, I might have spare one I could just send you

Hi, Classy: Yes, it's frustrating... mainly because of the very random nature of it! I appreciate that generous offer! Part number is 89540-60160 (Computer Assy, Skid Control W(VSC)).

Please see my update in response to Paul's post below. Thanks!

 

[MissouriLC]

Did you disconnect battery, while working on master, ECU or for that matter any electrical plug!
Not doing so, there's a risk of shorting an EUC, blowing a fuse or setting off an air bag.

I'd also look very close at wires and pins. Make sure non damaged or pins bent.

I did have the battery disconnected, so hopefully no chance of shorted-out components or blown fuses. However, I did not inspect the fuses yet. I will do so.

Here's an update:

• After my lunchtime diagnostics, during which I had to unplug the skid control ECU to check the specs on the brake pedal adjustment, I had to drive the car about 30 miles to pick up the kids from softball practice and return home. No lights.
• This morning, I drove another 30 miles round trip to take the kids to school and then go to work. The lights came on only once, when I was stopped at a stop light for about 3 minutes. They went off almost immediately after the car was moving again.
• Compare this to yesterday, when I was getting the lights going on and off more than a dozen times, all while stopped (in drive with the brakes applied) for more than 30 seconds or so.

So I am thinking maybe some dirt or contamination in the ECU plugs or MC plugs? The pins on the ECU all looked good, but I will clean the pins and connectors with electrical contact cleaner. I am puzzled by the fact that unplugging and re-plugging the ECU seemed to improve the problem somewhat (although maybe that is just wishful thinking!)

Another random thought: the diagnostics clearly demonstrate that there is some inter-relation between this and the brake light/stop light switch circuit. While doing some google research, I came across a thread on the 4Runner forum about a similar issue with a 4th gen 4Runner (so, contemporary to the 100-series). This thread stated that 4Runners have some circuit for an optional trailer brake controller that is activated by a signal from the stop light switch, through the ABS computer. Basically, the ABS computer gets a signal that the stop light switch is closed, which then signals the optional trailer brake controller to apply the trailer brakes. I am not sure if the 100-series has something similar. So, could the brake pedal height adjustment have something to do with this? Here is why I ask.

When I removed the old MC, I observed that there were approximately 4 threads of the plunger push rod exposed on the inside of the clevis. The new MC was adjusted out of the box with about 5 threads of the push rod exposed. When I installed in and measured the pedal height from the asphalt sheet per the FSM, the pedal height measured about 7.0625" (7-1/16) from the asphalt sheet (spec range is 7.232 to 7.626"). I adjusted the pedal to a height of 7.5". I am not sure how the shift interlock is activated, but I notice that before, I only had to slightly push the pedal to get the shifter to unlock, whereas now I need to push the pedal farther and harder. Could the brake pedal adjustment be too high, thus affecting the operation of the stop light switch in some way (brake lights still com on, but it takes more pedal movement to turn them on). Again, the FSM diagnostics indicate that these two circuits (MC pressure sensor and stop light switch) are somehow inter-related in the skid control ECU. I'm just not sure how.

 

[ClassyJalopy]

I did have the battery disconnected, so hopefully no chance of shorted-out components or blown fuses. However, I did not inspect the fuses yet. I will do so.

Here's an update:

• After my lunchtime diagnostics, during which I had to unplug the skid control ECU to check the specs on the brake pedal adjustment, I had to drive the car about 30 miles to pick up the kids from softball practice and return home. No lights.
• This morning, I drove another 30 miles round trip to take the kids to school and then go to work. The lights came on only once, when I was stopped at a stop light for about 3 minutes. They went off almost immediately after the car was moving again.
• Compare this to yesterday, when I was getting the lights going on and off more than a dozen times, all while stopped (in drive with the brakes applied) for more than 30 seconds or so.

So I am thinking maybe some dirt or contamination in the ECU plugs or MC plugs? The pins on the ECU all looked good, but I will clean the pins and connectors with electrical contact cleaner. I am puzzled by the fact that unplugging and re-plugging the ECU seemed to improve the problem somewhat (although maybe that is just wishful thinking!)

Another random thought: the diagnostics clearly demonstrate that there is some inter-relation between this and the brake light/stop light switch circuit. While doing some google research, I came across a thread on the 4Runner forum about a similar issue with a 4th gen 4Runner (so, contemporary to the 100-series). This thread stated that 4Runners have some circuit for an optional trailer brake controller that is activated by a signal from the stop light switch, through the ABS computer. Basically, the ABS computer gets a signal that the stop light switch is closed, which then signals the optional trailer brake controller to apply the trailer brakes. I am not sure if the 100-series has something similar. So, could the brake pedal height adjustment have something to do with this? Here is why I ask.

When I removed the old MC, I observed that there were approximately 4 threads of the plunger push rod exposed on the inside of the clevis. The new MC was adjusted out of the box with about 5 threads of the push rod exposed. When I installed in and measured the pedal height from the asphalt sheet per the FSM, the pedal height measured about 7.0625" (7-1/16) from the asphalt sheet (spec range is 7.232 to 7.626"). I adjusted the pedal to a height of 7.5". I am not sure how the shift interlock is activated, but I notice that before, I only had to slightly push the pedal to get the shifter to unlock, whereas now I need to push the pedal farther and harder. Could the brake pedal adjustment be too high, thus affecting the operation of the stop light switch in some way (brake lights still com on, but it takes more pedal movement to turn them on). Again, the FSM diagnostics indicate that these two circuits (MC pressure sensor and stop light switch) are somehow inter-related in the skid control ECU. I'm just not sure how.

Adjusting the brake pedal height, while finicky, isn't too much work. I'd say try adjusting the pedal a little lower to see if this would solve your issue.

 

[2001LC]

To be clear: This was a new booster assy w/master (the whole brake master assy) from Toyota? P/N matched to your VIN #.

Great you disconnect battery before R&R. Than there should not be any change/damage to circuitry, relays, ECU or fuses.

Make sure all wire housing block connected securely (clipped in). Be good idea to check all pins in wire housing also to see if dirty, bent or broken.
Make sure master is seated tight against fire wall with it's gasket in place and it's 4 securing nuts torque to 11ft-lbf

Brake Master comes with plunger clevis/pedal pre adjusted. I've yet to see one that needed adjusting when installing. @ClassyJalopy suggestion is a good one. Set back to where it was. What we're really concerned with is pedal free play. Which is just 1mm to 6mm. Again I never see this out of adjust when installing a new master. Make sure to check, brake tail lights off, then on as just as pedal depressed, taking up free play.

Also make sure fluid level, is correctly set to max mark.

 

[MissouriLC]

To be clear: This was a new booster assy w/master (the whole brake master assy) from Toyota? P/N matched to your VIN #.

Great you disconnect battery before R&R. Than there should not be any change/damage to circuitry, relays, ECU or fuses.

Make sure all wire housing block connected securely (clipped in). Be good idea to check all pins in wire housing also to see if dirty, bent or broken.
Make sure master is seated tight against fire wall with it's gasket in place and it's 4 securing nuts torque to 11ft-lbf

Brake Master comes with plunger clevis/pedal pre adjusted. I've yet to see one that needed adjusting when installing. @ClassyJalopy suggestion is a good one. Set back to where it was. What we're really concerned with is pedal free play. Which is just 1mm to 6mm. Again I never see this out of adjust when installing a new master. Make sure to check, brake tail lights off, then on as just as pedal depressed, taking up free play.

Also make sure fluid level, is correctly set to max mark.

Yes, brand new whole master cylinder assembly, P/N:
47050-60041
Brake Booster Assy, W/Master Cylinder
W(VSC),PNC 89637D=TYPE A

(Note, I didn't order it from ToyotaPartsDeal.com, I ordered it from my local dealer for about $100 more plus tax, free overnight delivery... it helps to be close to the parts center in KC!)

Last night, I disconnected the skid control ECU (battery disconnected!) and all the connectors on the MC assembly. I cleaned all of the connectors with electrical contact cleaner. Then, I readjusted the brake pedal so that there's now about 3-4 mm of free play before the brake lights come on. This puts it at just about 7 inches above the floor, which is lower than the FSM states it should be adjusted to. I'm pretty sure the clevis is at approximately the same position as when I took the MC out of the box.

Hooked everything bag up, started the car, reset idle, and connected Techstream. I ran active test on the ABS/VSC module (which cleared the codes) and then, for s***s and grins, reset the zero point calibration. Taking the kids to school this morning, I threw no codes (yet...) So, I am hopeful that the issue is resolved. I won't be able to say for sure for at least a few days, given how intermittent it is, but knock on wood it is resolved. I'll hook up Techstream later today to see if any ABS/VSC codes are pending.

So, if it is indeed fixed, it seems like there is (was?) a conflict between what the MC pressure sensor was reading (i.e. some pressure) and the fact that the brake lights weren't necessarily on because the pedal was adjusted too high, and it wasn't depressed far enough to turn on the lights. The skid control computer seems to resolve these two signal against one another, and if they're not in sync, it's not happy. I'm assuming the stop light switch/circuit also controls the shifter interlock, because that feels normal again. I'll keep an eye on it and report back.

 

[2001LC]

200LC made between 07.1999-08.2000, which is only years with type A wire housing connector. So must it be the correct master.

 

[MissouriLC]

200LC made between 07.1999-08.2000, which is only years with type A wire housing connector. So must it be the correct master.

Yep, mine's a 10/1999 production date. All the wiring connectors are identical to the unit that it replaced.

Knock on wood, but I drove another 16 miles of stop and go, and no lights have returned.

 

[MissouriLC]

200LC made between 07.1999-08.2000, which is only years with type A wire housing connector. So must it be the correct master.

Thinking about this come more... obviously the skid control ECU collects a lot of data to manage the ABS, VSC, and TRAC functions (vehicle speed, engine speed, throttle position, brake pressure, deceleration, yaw, steering wheel position, etc.) Paul, do you know if the stop light switch actually measures brake pedal stroke--as in, how far the brake pedal is depressed? When I was measuring the voltage of the STP pin out, it was just light off voltage and light on voltage--two figures. But, is there another pin out that measures voltage based on pedal position--i.e. like a potentiometer?

That switch seems like it has more going on than a simple on/off switch for the brake lights...

 

[ClassyJalopy]

Thinking about this come more... obviously the skid control ECU collects a lot of data to manage the ABS, VSC, and TRAC functions (vehicle speed, engine speed, throttle position, brake pressure, deceleration, yaw, steering wheel position, etc.) Paul, do you know if the stop light switch actually measures brake pedal stroke--as in, how far the brake pedal is depressed? When I was measuring the voltage of the STP pin out, it was just light off voltage and light on voltage--two figures. But, is there another pin out that measures voltage based on pedal position--i.e. like a potentiometer?

That switch seems like it has more going on than a simple on/off switch for the brake lights...

I will be very surprised if the brake light switch also measures the pedal stroke. The ABS, VSC and A TRAC are all based on wheel speed sensor input and are usually not concerned at all with how much pedal is depressed!

 

[DanInDenver]

Brake Master Cylinder Sub-Assembly with Solenoid
#4702560042
Brake Master Cylinder Assembly With Fluid
#4705060041
https://autoparts.toyotaofdecatur.com//products/part-number.html/4705060041
What's the difference between these two? With fluid is slightly more expensive. Truck is a 2000 LX470

 

[MissouriLC]

Brake Master Cylinder Sub-Assembly with Solenoid
#4702560042
Brake Master Cylinder Assembly With Fluid
#4705060041
Brake Master Cylinder Assembly with Fluid #47050-60041 | Autoparts.toyota.com - https://autoparts.toyotaofdecatur.com//products/part-number.html/4705060041
What's the difference between these two? With fluid is slightly more expensive. Truck is a 2000 LX470

Year of manufacture for your truck. 60041 is correct for 07-1999 through 08-2000 production dates (which would typically be a M/Y 2000). 60042 looks to be for M/Y 2001 - M/Y 2002 (not sure of the exact production dates). The ABS wiring harness connector on the master cylinder changed between the two... the first is rectangular/oval, which then changed to (I think) a triangular connector. Whichever one you have, the other one won't physically connect.

Paul (@2001LC) has handled more of these over different model years and knows more about this than I do, but you need to check the part against your VIN/date of manufacture before ordering.

 

[elemenohpee]

I just replaced the whole master cylinder unit with a brand new OEM one. The motor on mine was going out and the wires were pretty heavily corroded.

The new one is making a "pssshhh" noise whenever I depress the brake pedal. I don't remember my old one making this noise at all and I haven't seen many people mentioning this here. Anyone else have this sound? Could there still be air in system causing this or maybe something wrong with my new assembly?

Here's a video I found that sounds almost exactly like mine – maybe just slightly louder but it's hard to tell from the video. I can only hear it in mine with the windows up driving on smooth roads.

 

[2001LC]

Thinking about this come more... obviously the skid control ECU collects a lot of data to manage the ABS, VSC, and TRAC functions (vehicle speed, engine speed, throttle position, brake pressure, deceleration, yaw, steering wheel position, etc.) Paul, do you know if the stop light switch actually measures brake pedal stroke--as in, how far the brake pedal is depressed? When I was measuring the voltage of the STP pin out, it was just light off voltage and light on voltage--two figures. But, is there another pin out that measures voltage based on pedal position--i.e. like a potentiometer?

That switch seems like it has more going on than a simple on/off switch for the brake lights...

VVVVVV

I will be very surprised if the brake light switch also measures the pedal stroke. The ABS, VSC and A TRAC are all based on wheel speed sensor input and are usually not concerned at all with how much pedal is depressed!

I agree. I've never seen anything to indicate pedal stock is measured.

Brake Master Cylinder Sub-Assembly with Solenoid
#4702560042
Brake Master Cylinder Assembly With Fluid
#4705060041
Brake Master Cylinder Assembly with Fluid #47050-60041 | Autoparts.toyota.com - https://autoparts.toyotaofdecatur.com//products/part-number.html/4705060041
What's the difference between these two? With fluid is slightly more expensive. Truck is a 2000 LX470

Likely "fluid" a Toyota web page error. Toyota web page info and P/N, well. Suck! But by using VIN #, you're assured of correct part.

I like www.partsouq.com for accuracy. It about as good as it gets, other than getting from Toyota parts Department at Dealership.

 

[MissouriLC]

I just replaced the whole master cylinder unit with a brand new OEM one. The motor on mine was going out and the wires were pretty heavily corroded.

The new one is making a "pssshhh" noise whenever I depress the brake pedal. I don't remember my old one making this noise at all and I haven't seen many people mentioning this here. Anyone else have this sound? Could there still be air in system causing this or maybe something wrong with my new assembly?

Here's a video I found that sounds almost exactly like mine – maybe just slightly louder but it's hard to tell from the video. I can only hear it in mine with the windows up driving on smooth roads.

Mine makes--and did with the old MC as well--a bit of a whoosh or "psshhh" sound as well--although not as loud or a "squirty" sounding as the noise in the video. I've always chalked it up as a bit of air escaping the rubber boot on the pushrod when the pedal is depressed. I suppose it could also be noise from the pedal return spring, which has a piece of foam stuffed inside the coil to reduce noise. The FSM offers the "hint" of applying a little grease where the spring hooks in to both the pedal and the bracket under the steering column.

If there was fluid leaking out of the back of the MC, it seems you would have other issues, such as low pressure, long run times, and you'd notice fluid being lost from the reservoir and likely on the carpet as well. Air in the system would not make this noise, as it would be getting compressed with the brake fluid. This sounds like air leaking which, if coming from inside the MC, would mean fluid leaking as well.

The questions are, does your pedal feels consistent?; you're not losing brake fluid (reservoir level staying consistent)?; and are your pump run times are in spec (27 to 30 sec on a brand new, properly-bled MC assembly after the 40 pedal strokes with the IG off)?