I'm at my absolute wits end- Fading Brake pedal despite best efforts (1 Viewer)

[IanB]

I got my new MC today and installed it, tried to bench bleed but the kit I bought only had one of the fittings I needed and there's 2 ports, so I just pushed the plunger about a dozen times till fluid squirted out the fittings, and put it on the truck. I tried using my new Motive Power Bleeder but after 20 minutes of messing around there was no fluid coming out the bleeders when I cracked it, 15psi in the system. So I pulled that all apart and had a friend pump the brakes for me. I didn't check in time and actually sucked the reservoir dry on my 4th bleeding point, so stupid. Started all over again and bled them all again. Took it for a drive and it's perfect.

The reason I mention all this, my symptoms matched yours perfectly, I F'd up enough that if anyone should be having problems it should be me, and a new MC is what fixed my issues completely. I really think that's what everything is pointing to. Atomicshawn's point about dry bleeding the MC possibly damaging it is valid IMO. Maybe see if you can get your hands on a used one to try?

 

[Chester]

Parked the truck with good brakes, got in and the pedal went to the floor. No leaks, bled the system and ordered a master cylinder.


We're picking up some plugs and caps in the morning. Intention is to isolate in a stepwise fashion. We're also prepared to bug out the ABS modulator. We're on the second replacement master cylinder, but a plunger kit is on it's way.

 

[Howard705]

^ Good next step. Not accepted but can use old crimped off brake lines to cap too-starting at m/c to isolate axle-abs will add major complication. Can also vise grip hoses at wheel/axle if somewhat gentle. Have used this method in past on other vehicles too. Any chance of borrowing parts off another 80 to swap/test-sounds like you have enough brake fluid to bleed another afterward.

 

[R0b0con]

Does anyone have a flow diagram of the entire brake system for the 80?

Found it, i think. Does this look right? Should be he bottom one

better late than never? if you still want the diagram, but this may not have ABS in it.

 

[musthave]

better late than never? if you still want the diagram, but this may not have ABS in it.

Thank you for sharing! Yes very helpful.

 

[musthave]

We started the day by configuring a makeshift SST for bleeding the ABS unit. There is not much chance that there is air still in the ABS unit. We bled the ABS unit with the SST attached and bleeding breaks at the same time.

Checked the piston rod depth into the master cylinder and it is perfect.

Moved the LSPV to the lowest possible point.

Bled in accordance with the FSM. Have the same soft spongy fading pedal.

Absolutely no air bubbles while bleeding from any point.

Absolutely no leaking anywhere along brake lines.

Tomorrow morning we replace all brake lines and installed new LSPV.

Difficult to believe that a rebuild master cylinder was bad. And that a brand-new OEM master cylinder could be bad.

So also ordered a master cylinder rebuild kit to rebuild the one from Napa.

 

[blkprj80]

Got any details on the homemade SST?

 

[musthave]

Followed what pinhead said. Simple.

 

[Dave 2000]

^ Good next step. Not accepted but can use old crimped off brake lines to cap too-starting at m/c to isolate axle-abs will add major complication. Can also vise grip hoses at wheel/axle if somewhat gentle. Have used this method in past on other vehicles too. Any chance of borrowing parts off another 80 to swap/test-sounds like you have enough brake fluid to bleed another afterward.

Suggested this I think it was 47 (or was it 67) posts back. This is should be easy to work out but the OP is running around in circles and no longer following a procedure, ultimately he will disappear up his own arse.

When you press the brake pedal there are many things going on, pedal to pushrod, pushrod to master, air in master, fluid passing seals in master, hoses expanding (seen it a dozen times), air being compressed, OP points out dirty coloured fluid, could well be condensation has screwed the ABS unit, condensation is not compressible. I have not heard anything about the wheel bearing check I have to assume all that is ok. I did advise clamping off the rear circuit and trying that, I am not suggesting driving down the motorway with 50% of the brakes cut off but, you can duplicate the problem in the drive.

When you get a problem like this you have to go at it in a logical manner, the rod between servo and master for example, 1/6th of an inch too long and you can find as the car warms up the brakes have applied by themselves.

Because this is driving me around the bend and god knows what it is doing to the OP shall we start over?

I think it is a two port master cylinder right? Block the front port, and bleed the system and see if you can get a decent pedal, yes or no? If yes then we know the fault is at the front. If no then clamp of the rear flex to the axle better pedal yes or no, yes then bulging pipe or air. If no then look at the LSPV even temporarily bypassing it and so forth. Logical or ad infinitum!


regards

Dave

 

[Chester]

Suggested this I think it was 47 (or was it 67) posts back. This is should be easy to work out but the OP is running around in circles and no longer following a procedure, ultimately he will disappear up his own arse.

When you press the brake pedal there are many things going on, pedal to pushrod, pushrod to master, air in master, fluid passing seals in master, hoses expanding (seen it a dozen times), air being compressed, OP points out dirty coloured fluid, could well be condensation has screwed the ABS unit, condensation is not compressible. I have not heard anything about the wheel bearing check I have to assume all that is ok. I did advise clamping off the rear circuit and trying that, I am not suggesting driving down the motorway with 50% of the brakes cut off but, you can duplicate the problem in the drive.

When you get a problem like this you have to go at it in a logical manner, the rod between servo and master for example, 1/6th of an inch too long and you can find as the car warms up the brakes have applied by themselves.look at the LSPV even temporarily bypassing it and so forth. Logical or ad infinitum!


regards

Dave

All input is appreciated.

"Spongy brakes feel like you have a sponge on the pedal". Pin_Head

 

[Chester]

Got any details on the homemade SST?

Followed what Pin_Head offered.

I just took a look at the FSM page BR-58. You can activate the 3 solenoid valves and run the pump motor on the ABS actuator by disconnecting the two plugs at the actuator. Then you connect pins 1, 3 and 4 on the 6 position plug (wire colors light blue, light blue with green stripe, light blue with red stripe) of the actuator side to ground. Connect pin 1 of the 4 position plug (white with black stripe) to ground. Then connect pins 3 and 4 of the 4 position plug (black, red stripe, black, blue stripe) to battery voltage. The FSM implies that you don't want to run it this way for more than 15 sec.

Based on what I've been able to find on ABS from other vehicles, this "jig" doesn't put the 3 valve through all it's paces, but it suffices to allow cycling, which allows fresh fluid in, while expelling the fluid and or air trapped in the ABS reservoir, valves and piping, which is all we needed to do. Or not, because with the exception of the first time the ABS was caused to cycle several days ago, only clear fluid and no air was expelled from the ABS, or anywhere else for that matter. It did allow us to eliminate the ABS as a possible source of all this frustration, which is at least something.

 

[Pin_Head]

Have the same soft spongy fading pedal.

That really sucks.

 

[95ToyCruiser]

Hate to say it, but I'd delete the LSPV And add a proportioning valve in line (there would be a hell of a lot less line under the truck and make things simpler)

If that doesn't work, delete the ABS...there's really no point in even having it if your brakes suck so bad that you can't even lock them up...

I'd rather have good brakes and no ABS than ABS and s***ty brakes...

 

[musthave]

Hate to say it, but I'd delete the LSPV And add a proportioning valve in line (there would be a hell of a lot less line under the truck and make things simpler)

If that doesn't work, delete the ABS...there's really no point in even having it if your brakes suck so bad that you can't even lock them up...

I'd rather have good brakes and no ABS than ABS and ****ty brakes...

New LSPV will be here tomorrow, so we will try that first along with new lines. If it's a problem, delete may come sooner than later.

We have 10mm caps and plugs to isolate if needed.

The new parts don't bother me, I'll be good for another 15 years.

Thanks for suggestions

 

[DARKNESS]

Is it possible you installed the calipers upside down so that the bleeder is on the bottom allowing air to be trapped at the top of the caliper? Not sure if this possible or not.

 

[musthave]

I don't think it's possible to install upside down.

 

[Chester]

Is it possible you installed the calipers upside down so that the bleeder is on the bottom allowing air to be trapped at the top of the caliper? Not sure if this possible or not.

It may be possible, but they're installed properly.
Same with the Accelerator cable stay/ bracket on the MC studs, it might be possible to assemble upside down or in the wrong sequence, but it's not. Also it's possible to shred the plunger sweepers by dry pumping, we didn't. It's also possible to not completely bench bleed the MC, that didn't happen. May be possible to push the plunger too far or too hard, nope. We even put a nominal 10" Hg vacuum on any bleeder valve before we open it, and on the last partial stroke before we lock them tight. We exercise good shop practice, no oil or grease on parts, tools rags or hands. All fasteners get cleaned and threads get chased before re-assembly. It's just not solved yet, it's been rational step-wise to this point, generally, and when it's solved, there's a couple 6 packs of blonde belgians chilled just past frosty, waiting.

 

[DSRTRDR]

if you put new brake pads in, don't forget the "bedding in" procedure

 

[atomicshawn]

...I think it is a two port master cylinder right? Block the front port, and bleed the system and see if you can get a decent pedal, yes or no? If yes then we know the fault is at the front. If no then clamp of the rear flex to the axle better pedal yes or no, yes then bulging pipe or air. If no then look at the LSPV even temporarily bypassing it and so forth. Logical or ad infinitum!


regards

Dave

Sentiments proposed a bit abrasively perhaps, but makes sense to a lot of us it would seem. This procedure just seems so much more rational than simply throwing more parts at it. I haven't looked under my hood to see how difficult it would be. Is there some insurmountable obstacle keeping you from doing this? Or some other logic we're missing?

Edit: If I could add a step to Dave's procedure ^ it would be at some point to also bleed/block both ports at the same time just so you know the MC for sure is not the problem. It's probably fine, but "knowledge is that with which you cannot possibly be mistaken". I would hate for you to chase everything down the one side of the system just to find out later the MC was the problem all along.

 

[Dave 2000]

Sentiments proposed a bit abrasively perhaps,

Having spent many many years under a car and in the 'early' days when there was no such thing as 'forum support' (yes I am that old) and mechanics learnt very early to be as logical as possible when diagnosing any problem, you do get a little frustrated.

I hear you though, long day and was feeling a little 'fractious', had a good nights sleep and this morning, openly concede it was a little heavy handed ....'Dave offers apologies'....

Good idea about blocking all of the MC, the other thing I wanted to point out is, just how many flexible pipes are there on the 80? If they ALL have a little 'give' then this might be an issue on an ageing vehicle, I really am grasping at straws here, the OP will have a new 80 by the time he has finished.

regards

Dave