Brake Pedal Gradually Goes to the Floor

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
So an update from over the weekend. I had a long talk on Friday afternoon with the lead mechanic at Atlantic British. His assessment was as follows.

1) ABS air - Bleeding the ABS wouldn't be the issue. Any air introduced into the module through the standard bleeding of the bad master cylinder could also be removed with standard bleeding, which I've done in spades. If there happened to be air in the system that could only be removed by activating the system, then it would only present when the system activated, not consistently as is my issue. Additionally, air in the system in general would not present with a sinking pedal, but one that requires pumping to firm up (we noted this earlier in the thread). If the ABS is the issue, it would be an internal hydraulic leak between chambers or one presenting where I'm not seeing it. In either case, the unit would need to be replaced, as it's non serviceable.

2) Hydraulic - This is where he felt the root cause would be. Without visible leaks, he believed there were 4 possible explanations. 1) Hydraulic expansion or minimal leak (not yet visible) in the rubber system parts: caliper piston seals or rubber brake lines. 2) Pass through issue of the internal seals in the master cylinder 3) Seal leak internal to the brake servo 4) Internal leak between chambers of the ABS module.

So, here is the action plan and the results to this point.
1) Isolate the MC and servo from the rest of the brake system and test. I pulled the two brake lines and plugged with two bolts and o-rings to allow them to seat. With this set-up the pedal was completely firm and did not descend. So, the MC and servo should be officially ruled out and the issue further down stream.
2) I've ordered new seals for the remaining two calipers which I hadn't repaired yet, along with stainless steel brake lines. If figure those would be the logical next step. Simply because even if they don't solve the issue, it's still a good idea and not wasted money.
3) If the problem persists following that, it would seem I'm looking at the ABS.

Thanks so much for all the continued responses. They're really serving to keep my though process focused and moving in the right direction. I'll keep everyone updated.
 

glester

Well-known member
Jul 11, 2017
96
3
Seattle
So an update from over the weekend. I had a long talk on Friday afternoon with the lead mechanic at Atlantic British. His assessment was as follows.

1) ABS air - Bleeding the ABS wouldn't be the issue. Any air introduced into the module through the standard bleeding of the bad master cylinder could also be removed with standard bleeding, which I've done in spades. If there happened to be air in the system that could only be removed by activating the system, then it would only present when the system activated, not consistently as is my issue. Additionally, air in the system in general would not present with a sinking pedal, but one that requires pumping to firm up (we noted this earlier in the thread). If the ABS is the issue, it would be an internal hydraulic leak between chambers or one presenting where I'm not seeing it. In either case, the unit would need to be replaced, as it's non serviceable.

2) Hydraulic - This is where he felt the root cause would be. Without visible leaks, he believed there were 4 possible explanations. 1) Hydraulic expansion or minimal leak (not yet visible) in the rubber system parts: caliper piston seals or rubber brake lines. 2) Pass through issue of the internal seals in the master cylinder 3) Seal leak internal to the brake servo 4) Internal leak between chambers of the ABS module.

So, here is the action plan and the results to this point.
1) Isolate the MC and servo from the rest of the brake system and test. I pulled the two brake lines and plugged with two bolts and o-rings to allow them to seat. With this set-up the pedal was completely firm and did not descend. So, the MC and servo should be officially ruled out and the issue further down stream.
2) I've ordered new seals for the remaining two calipers which I hadn't repaired yet, along with stainless steel brake lines. If figure those would be the logical next step. Simply because even if they don't solve the issue, it's still a good idea and not wasted money.
3) If the problem persists following that, it would seem I'm looking at the ABS.

Thanks so much for all the continued responses. They're really serving to keep my though process focused and moving in the right direction. I'll keep everyone updated.

Thanks for the update. It's interesting reading through the details of this issue. Hopefully you get the the root of it in the steps AB provided. Good luck!
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Just an update. I received all the parts finally yesterday. The brake hoses were the last to arrive, shipping from NY. So, the work will begin this weekend:

Replace rear caliper piston seals
Replace front caliper piston seals
Replace rear rubber brake lines
Replace front rubber brake lines

While I'd love to bleed the system after each individual repair and then test to give a definitive answer on which, if any of those items solve the issue, I'm too lazy for that. So, I'll report back once it's all done.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
I finally got the chance this week to do the planned work. I replaced the pistons in seals in the remaining two calipers and installed stainless lines front and rear. Then I pressure bled all four calipers twice, going through roughly 1.25 liters of brake fluid in total. The rubber lines I removed all seemed fine, with no cracks or leaks. I did find a bit of jellied brake fluid between the wiper seal and fluid seal on one of the pistons in the rear caliper, indicating a minor leak.

The results of all the repairs was an increase in actually braking power, but no change in the performance of the pedal. I can still drive it to the floor under constant pressure and stopping power takes much longer pedal travel than it should. Pumping up the pedal does nothing to increase the pedal firmness, indicating that the system is clear of air.

I did a short road test where I took the temperature off all four rotors prior to driving and then again afterwards, with frequent braking. Prior to the drive, all rotors read at 67 degrees. After, both fronts were around 200 degrees. The right rear was 120 and the left rear 90. I'll retest tomorrow to see if I get the same discrepancy in the rear. If so, I guess it could mean an ABS module failure that's relieving pressure from that caliper? Although I don't feel the ABS engaging.

Bottom line, is I'm still at a loss and chasing the issue.
 

AbnMike

Well-known member
Apr 6, 2016
1,218
117
Western Slope, CO
Agree with German.

If you have even slightly worn bearings you get wheel wobble. Wheel wobble pushes the caliper pistons out.


To test it start the truck and press the pedal until it’s firm consistently.

Now drive. If your pedal then goes to the floor or longer travel one of your wheel bearings is probably loose.

If your pedal will never firm up, even when sitting still and pumping it (pump it a ton of times to get the pistons extended in case the above is occurring), then I'd look to the power booster thing.

I chased all kinds of problems with no leaking fluid and finally bought a new MC and Servo. Replacing both was a cake walk.
 
Last edited:

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
What about bearings? If rear bearings are worn, you can't feel it depressing wheel to feel it because of the shafts. But this small amount of movement necessary to accomodate disc can take a lot of pedal distance (and brake pads from a rear wheel wear unequal).

Regards

I thought of that. Here my reasons for dismissing it, at least for the moment. Let me know if my reasoning is flawed and I've missed something.

1) I don't have zero brakes for a portion of the pedal travel and then achieve normal brakes later in the travel. In other words, it's not the "OH crap, no brakes!" that you feel with pad knock back and then you find them after pumping the pedal. Light pressure on the pedal yields stopping power and there is stopping power spread over the entire range of the pedal movement. That movement is just elongated from normal and with reduced overall power for hard stopping. Granted, I would still get this with one or even two bearings causing those pads only to be pushed back.
2) However, I would expect a brake pull in one direction or another with this cause, as a uniform failure of the bearings seems unlikely. However, I don't have that.
3) I don't get the pedal back by pumping it, which would in theory move the pads back up against the rotors after being pushed back by bearing wobble. I'm able to move the pedal to the floor at a standstill repeatedly after pumping it up in between.
4) I just replaced the stub axle seal on both rear wheels and checked the bearings while I had those hubs apart, all felt good.
5) I'll recheck each wheel for play today, as I did that way back at the start of this process, but I don't recall anything out of the ordinary.

Thanks very much for the suggestion and let me know if you think I should rethink it.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Agree with German.

If you have even slightly worn bearings you get wheel wobble. Wheel wobble pushes the caliper pistons out.


To test it start the truck and press the pedal until it?s firm consistently.

Now drive. If your pedal then goes to the floor or longer travel one of your wheel bearings is probably loose.

If your pedal will never firm up, even when sitting still and pumping it (pump it a ton of times to get the pistons extended in case the above is occurring), then I'd look to the power booster thing.

I chased all kinds of problems with no leaking fluid and finally bought a new MC and Servo. Replacing both was a cake walk.

Thanks for the suggestions. You are correct, I never get a firm pedal at standstill. I can pump it up to feeling initially firm after 4 to 5 long, slow pumps, but then it will just descend under constant pressure. I agree that this would normally leave the MC or the booster. However, I ruled those out earlier by blocking off the master cylinder at the outlets and then testing the pedal with the car both on and off. In that case I had a firm pedal. I'm also clearly getting brake boost and the booster is holding vacuum when tested at the end of the tube running to the engine. So, it would appear to be an issue further down the line from the MC/Booster. Even though RAVE lists the MC and Booster as the only two possible causes of a descending pedal, I don't see that those are my issues, based on the symptoms and tests I've run.

If I've missed something in my thinking, please let me know. The more suggestions I get the more it helps me. Thanks everyone.
 

AbnMike

Well-known member
Apr 6, 2016
1,218
117
Western Slope, CO
However, I ruled those out earlier by blocking off the master cylinder at the outlets and then testing the pedal with the car both on and off. In that case I had a firm pedal.

I'm also clearly getting brake boost and the booster is holding vacuum when tested at the end of the tube running to the engine. So, it would appear to be an issue further down the line from the MC/Booster. Even though RAVE lists the MC and Booster as the only two possible causes of a descending pedal, I don't see that those are my issues, based on the symptoms and tests I've run.

Maybe they work well enough to hold when you have the MC blocked off but not well enough when you have the entire system in play? I have no idea if that even makes sense in physics world, but kind of does in my brain.

Sometimes I get to a point where throwing parts at a problem starts to make sense, especially if the parts I'm replacing are already 20 years old....
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Maybe they work well enough to hold when you have the MC blocked off but not well enough when you have the entire system in play? I have no idea if that even makes sense in physics world, but kind of does in my brain.

Sometimes I get to a point where throwing parts at a problem starts to make sense, especially if the parts I'm replacing are already 20 years old....

Point taken and I agree with you on the fact that at some point, you just have to eliminate as best as possible and try parts. I'm just trying to pick my financial battles wisely on this car. Believe me, I'd love to just start overhauling with the idea that if it doesn't fully solve the issue, at least it's a new part. I didn't expect the stainless lines to solve anything, but I new those would not only replace a part that was recommended to be replaced many miles ago, but would also be an improvement over the original.

My frustration and hesitation to replace the booster is that it passes every test, without any indication of failure to any degree:

~With the vacuum line disconnected, air doesn't come out when pressing on the pedal. Indicating the check valve is operating correctly.
~Holding the brake pedal with the car running, then shutting the car off and slowly pressing the pedal three times results in a gradually higher and higher pedal. Also, running the engine and pressing and holding the brake pedal, then shutting of the engine while maintaining pressure on the pedal doesn't result in a drop of the pedal once the engine is off. Both indicate the internal seals are airtight.
~Lastly, maintaining pressure on the brake pedal with the car off and then starting the engine results in a slight drop in the pedal, indicating the booster is providing proper assist. The only issue is that in my case the pedal continues to descend under constant pressure.

I've read that a bad ABS modulator can mimic the symptoms of a bad master cylinder. Essentially the valves which normally would release pressure from a caliper who's wheel is skidding are not fully closed under normal driving, creating the same pass through experienced with a bad MC. Anyone ever had this? Any way to test for it?
 

discostew

Well-known member
Sep 14, 2010
7,706
1,015
Northern Illinois
Air can get trapped in strange places. Have someone gently put pressure on the pedal and crack lines loose at the master cylinder. See if you get any air up there. Gently is the key word there. You could spray brake fluid to a painted surface.
 

discostew

Well-known member
Sep 14, 2010
7,706
1,015
Northern Illinois
Point taken and I agree with you on the fact that at some point, you just have to eliminate as best as possible and try parts. I'm just trying to pick my financial battles wisely on this car. Believe me, I'd love to just start overhauling with the idea that if it doesn't fully solve the issue, at least it's a new part. I didn't expect the stainless lines to solve anything, but I new those would not only replace a part that was recommended to be replaced many miles ago, but would also be an improvement over the original.

My frustration and hesitation to replace the booster is that it passes every test, without any indication of failure to any degree:

~With the vacuum line disconnected, air doesn't come out when pressing on the pedal. Indicating the check valve is operating correctly.
~Holding the brake pedal with the car running, then shutting the car off and slowly pressing the pedal three times results in a gradually higher and higher pedal. Also, running the engine and pressing and holding the brake pedal, then shutting of the engine while maintaining pressure on the pedal doesn't result in a drop of the pedal once the engine is off. Both indicate the internal seals are airtight.
~Lastly, maintaining pressure on the brake pedal with the car off and then starting the engine results in a slight drop in the pedal, indicating the booster is providing proper assist. The only issue is that in my case the pedal continues to descend under constant pressure.

I've read that a bad ABS modulator can mimic the symptoms of a bad master cylinder. Essentially the valves which normally would release pressure from a caliper who's wheel is skidding are not fully closed under normal driving, creating the same pass through experienced with a bad MC. Anyone ever had this? Any way to test for it?

I think what you might be talking about is an unwanted abs activation. That's usually caused by a loose wheel bearing but could be a wheel speed sensor too. It makes you feel like your going to glide right into something at slow speeds. But when you start to freak out and press the pedal harder the abs works and can grind it to a stop. I've heard crazy talk of taking out the whole abs system for that. Tightening up the wheel bearing and pushing the sensor back in is probably easier.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
I've been meaning to circle back to this and provide an update for a while. After suffering with this for awhile, but being able to drive the car due to the overall increase that in braking performance that the individual repairs provided, I noticed that I never seemed to be able to activate the ABS. That's not unusual in LA, where rain and slippery conditions are few and far between combined with the pedal issue causing a more elongated application of stopping power. However, it got me to start trying to get it to activate. Finally, we had some rain this past fall where I could legitimately make the attempts at reasonable speeds in safe areas. It still proved difficult and finally took absolutely stomping the pedal from 40mph. However, once that attempt was successful, the ABS engaged more readily and now I would get a firmness change in the pedal upon pumping it up.

So, I went back to and bled the system again and problem mostly solved. I still have a slight descent to the pedal when stopped on a steep downward incline, but without the vehicle beginning to creep forward.

All in all, I would say the initial cause of the issues was the leaking piston seals in the caliper. However, during the process of replacing those and bleeding the system, something got hung up in the ABS module that rarely activates due to the driving conditions I typically encounter. I really don't have a technical explanation for it, but there it is.

Hope that's helpful and thank everyone for the input.