Soft brake pedal then hard on second press (1 Viewer)

[0tterPop]

Long time lurker that needs help! I have a 2003 LX with 120k miles. All in all the rig is in amazing shape lexus dealer maintained until 95k all services performed. Recently I noticed that my brake pedal will feel soft on the first push then it will stay stiff like it should. I will drive for awhile and it will happen again, sometimes a few times a day sometimes only once. I bled all the brakes and checked the fluid and all seems well. I have tried to find similar issues on here with no luck so far. I'm thinking something with the master cylinder perhaps the check valve any ideas?? I'm already about to throw a new front diff in it after an unfortunate trail riding session so it would break my heart if it was something serious.

I truly appreciate any help I can get I absolutely love Thomas my LX

 

[f5driver]

I had similar symtoms. The lock washer sheared on my right front hub which allowed the nut to loosen.

 

[Skidoo]

A number of folks have fixed 2nd pump issue with a MC rebuild, as mentioned here.
Master Cylinder Rebuild - DIY

 

[Cargo1234]

Did the rebuild and sorted out the same brake issue for me

 

[0tterPop]

Appreciate the help all

 

[CLEcruiser]

Some also speculate that the pedal adjustment after the rebuild could be the actual solution, so maybe try the adjustment first.

 

[2001LC]

Was this happening before bleeding?

After bleeding did you test brake booster motor/pump run time?

Brake booster motor/pump run time test (pressurizing accumulator) procedure:
IG key off.
Pump brake pedal 40 times. (Same as topping procedure, which instructions are embossed on reservoir)
Check that fluid level is now at max line (no higher or lower)
IG key to on. Start timer.
Time how long brake booster motor runs. Spec is 30 to 40 seconds.

If over 40 seconds, you likely have air in line or a leak. Leak or weeping at "a" bleeder is very common after bleeding. This especial true if bad or no rubber bleeder caps, or bleeders were every over torqued.

BTW: What makes you think a new front diff is needed? I ask, because so often, issue with wheel hub flange or it's snap ring makes one think they have a broken front diff.

 

[TEXAS 57]

I have the same issue with my 99 LX. I have ordered part # 04493-60330 and plan on doing the rebuild.
Should be the same part # for all 100 series.

Here is a link to a rebuild thread: Master Cylinder Rebuild - DIY

 

[0tterPop]

@2001LC

Thanks so much for this procedure! I will try it this weekend. The pedal started feeling like this before bleeding, I noticed it about a month or so ago. I live down a very rough dirt road I would leave my house (brakes felt fine on start up) then drive about a mile to the main road. By the time I come to the stop sign the brake pedal travels much farther, if I pump it once its firm again. I can drive on regular roads for some time before noticing the pedal going somewhat soft again, even with further travel the brakes always perform as expected.

When I bled the brakes this past weekend I didnt really notice any air but I did notice 2 of the 4 valves had no covers on them (Is this a concern?)

As far as the Diff I got turned around and ended up on a pretty advanced trail at Rush Springs Ranch in MO. While trying to make it through (Couldnt go back because I had gone down a series of drops where I drug my rear bumper pretty well and there was no way I could clear the same drop with my front bumper.) So I was trying to crawl on some loose rock and was giving it some gas trying to be as cautious as I could (admittedly I became frustrated) The car then started making this loud grinding noise while giving it throttle and the wheels turned. It sounded like serious metal on metal not like a bad synchro but a pretty loud terrible sound. At that point I winched it to a flat spot on the trail removed the driveshaft and hubflanges and towed it home just in case after limping it down.

If you start the video at 6:10 you can hear it all go south.



For anyone who sees the video be nice I was an idiot that day. Would love to hear that its something besides the front diff but Im guessing thats it. (this whole video was just winching Tomas's fat but out of a gully.

 

[0tterPop]

@2001LC

Thanks so much for this procedure! I will try it this weekend. The pedal started feeling like this before bleeding, I noticed it about a month or so ago. I live down a very rough dirt road I would leave my house (brakes felt fine on start up) then drive about a mile to the main road. By the time I come to the stop sign the brake pedal travels much farther, if I pump it once its firm again. I can drive on regular roads for some time before noticing the pedal going somewhat soft again, even with further travel the brakes always perform as expected.

When I bled the brakes this past weekend I didnt really notice any air but I did notice 2 of the 4 valves had no covers on them (Is this a concern?)

As far as the Diff I got turned around and ended up on a pretty advanced trail at Rush Springs Ranch in MO. While trying to make it through (Couldnt go back because I had gone down a series of drops where I drug my rear bumper pretty well and there was no way I could clear the same drop with my front bumper.) So I was trying to crawl on some loose rock and was giving it some gas trying to be as cautious as I could (admittedly I became frustrated) The car then started making this loud grinding noise while giving it throttle and the wheels turned. It sounded like serious metal on metal not like a bad synchro but a pretty loud terrible sound. At that point I winched it to a flat spot on the trail removed the driveshaft and hubflanges and towed it home just in case after limping it down.

If you start the video at 6:10 you can hear it all go south.



For anyone who sees the video be nice I was an idiot that day. Would love to hear that its something besides the front diff but Im guessing thats it. (this whole video was just winching Tomas's fat but out of a gully.

 

[2001LC]

@2001LC

Thanks so much for this procedure! I will try it this weekend. The pedal started feeling like this before bleeding, I noticed it about a month or so ago. I live down a very rough dirt road I would leave my house (brakes felt fine on start up) then drive about a mile to the main road. By the time I come to the stop sign the brake pedal travels much farther, if I pump it once its firm again. I can drive on regular roads for some time before noticing the pedal going somewhat soft again, even with further travel the brakes always perform as expected.

When I bled the brakes this past weekend I didnt really notice any air but I did notice 2 of the 4 valves had no covers on them (Is this a concern?)

As far as the Diff I got turned around and ended up on a pretty advanced trail at Rush Springs Ranch in MO. While trying to make it through (Couldnt go back because I had gone down a series of drops where I drug my rear bumper pretty well and there was no way I could clear the same drop with my front bumper.) So I was trying to crawl on some loose rock and was giving it some gas trying to be as cautious as I could (admittedly I became frustrated) The car then started making this loud grinding noise while giving it throttle and the wheels turned. It sounded like serious metal on metal not like a bad synchro but a pretty loud terrible sound. At that point I winched it to a flat spot on the trail removed the driveshaft and hubflanges and towed it home just in case after limping it down.

If you start the video at 6:10 you can hear it all go south.



For anyone who sees the video be nice I was an idiot that day. Would love to hear that its something besides the front diff but Im guessing thats it. (this whole video was just winching Tomas's fat but out of a gully.

The timing test, gives indication of air bubbles or leaks. If it passes you can assume no leaks.

Missing or bad brake bleeder caps, allow moisture in bleeder along with its seat area in caliper. I find the rear bleeder get "the worst of it". They sometimes are so corroded/rusted that orifice is plugged. The bleeder becomes pitting and must often be replaced. In really bad cases the caliper needs replacing, as seat is damaged.

When I'm bleeding, I remove any bleeders that I suspect are corrode and recondition them. I torque bleeder to factory spec of 8ft-lbf. If they weep I'l increase torque ~1ft-lbf at a time up to ~12ft-lbf in attempt to stop weeping. If by about 12ft-lbf they don't stop weeping, I'll replace them. If a new OEM bleeder also fails/weeps, I'll replace caliper. So a bad or missing $2 bleeder cap can take out a caliper.

Also keep in mind if reservoir not holding fluid level (read topping instructions embossed on reservoir), this indicates a leak. Frequent booster motor/pump running, without depressing brake peddle, may indicate a leak.

If timing test to spec and no leaks are found in/at bleeders, brake lines, calipers and/or master (these last two, can be very hard to see). Then, its likely master cylinder leaking internally. Rebuilding (seal kit) or replacing master takes care of this.

A few other possibilities that come to mind are accumulator or pressure switch failing.

Front diff.
If you inspected both axle, hub flanges, snap rings and CV's of front driveshafts, and all good. Then likely you did take out front diff. In which case draining diff gear lube through a screen, you'll likely find many large pieces of metal (a few is normal).

 

[0tterPop]

Well just took off the right front and it looks like I have a bad piston in the caliper that is causing the caliper to drag on the rotor... glad I havent let this go on very long. Checked the pressure test pump went on in 37 seconds so within spec. I'm hoping if I have a sticky piston thats causing my pedal issues

 

[TEXAS 57]

Would be a nice cheap fix...

 

[Topoguide]

@Otterpop did repairing the caliper(s) fix your problem? I am experiencing a similar issue that I am trying to narrow down. My issue is that after the brakes are applied, and being used to keep it stopped at idle, that the pedal will sometimes go soft. A quick pump and it's fine again.