Strange brake behavior, cannot get good performance

[Fughuert]

New master plugged port bled, tried two with same results. Less than 1/8th” of plunger depression achieved.

New calipers all the way around bleeders right way up, bled a bunch of times, not getting any air bubbles anywhere. No ABS on my Jeep.

Front calipers/pads/rotors are from black magic. Rear are all new kit but not from BMB. All new soft brake lines whole way round.

The issue I’m having is the pedal just goes to the floor and I have undriveable braking performance. If I pump the brakes they do not ever firm up, unless I keep a foot on the gas pedal and then pump the brakes, then they will firm up and the brakes will start working near the top of the pedal stroke.

Doing the latter I was able to lock up both front tires(33s) for a few feet from about 35mph, if I release the gas the brake pedal travels to the floor and I lose braking strength, literally unlocking up the fronts. I repeated this three times.

Is this a bad booster? I always figured that would only cause a hard to press brake pedal.

 

[Chris]

@mrblaine

 

[macleanflood]

Might want to compare the original booster and the new one...the rod that actuates the booster...the engagement point might be different?

-Mac

 

[mrblaine]

New master plugged port bled, tried two with same results. Less than 1/8th” of plunger depression achieved.

How did you adjust the booster pushrod inside the booster or check it?

New calipers all the way around bleeders right way up, bled a bunch of times, not getting any air bubbles anywhere. No ABS on my Jeep.

How are you doing the bleed, step by step?

Front calipers/pads/rotors are from black magic. Rear are all new kit but not from BMB. All new soft brake lines whole way round.

The issue I’m having is the pedal just goes to the floor and I have undriveable braking performance. If I pump the brakes they do not ever firm up, unless I keep a foot on the gas pedal and then pump the brakes, then they will firm up and the brakes will start working near the top of the pedal stroke.

Doing the latter I was able to lock up both front tires(33s) for a few feet from about 35mph, if I release the gas the brake pedal travels to the floor and I lose braking strength, literally unlocking up the fronts. I repeated this three times.

Is this a bad booster? I always figured that would only cause a hard to press brake pedal.

No idea what that is. Most vacuum is at idle, revving it up should drop the vacuum and do the opposite. Why did you change the master?

 

[Fughuert]

How did you adjust the booster pushrod inside the booster or check it?

How are you doing the bleed, step by step?

No idea what that is. Most vacuum is at idle, revving it up should drop the vacuum and do the opposite. Why did you change the master?

Didn’t know adjusting the pushrod was a thing and I haven’t touched it.

RR RL FR FL, hose over bleeder, crack bleeder partner depresses pedal, close bleeder, release pedal.

I also tried a motive power bleeder to the same results. I am anal about keeping the master topped after trying so many times, I know it isn’t running dry.

I get air bubbles eventually then do a few bleeds witnessing good fluid. I have the key on and make sure the brake light doesn’t trip, it has on occasion and I reset the shuttle.

I replaced the master because after parking it for a few weeks the brake pedal went soft and I started having longer stopping distances before I parked it, I tested the old master and it wouldn’t bench bleed.

 

[mrblaine]

Didn’t know adjusting the pushrod was a thing and I haven’t touched it.

RR RL FR FL, hose over bleeder, crack bleeder partner depresses pedal, close bleeder, release pedal.

I also tried a motive power bleeder to the same results. I am anal about keeping the master topped after trying so many times, I know it isn’t running dry.

I get air bubbles eventually then do a few bleeds witnessing good fluid. I have the key on and make sure the brake light doesn’t trip, it has on occasion and I reset the shuttle.

I replaced the master because after parking it for a few weeks the brake pedal went soft and I started having longer stopping distances before I parked it, I tested the old master and it wouldn’t bench bleed.

Your throttle test is sitting still, raise the engine RPMs, and then you can get a brake pedal by pumping it up? Then if you let it drop to idle, the pedal goes to the floor?

I know you said, but check the bleeders again for being on top. I've seen several folks say they were, say they checked them, and when I had them send over pictures, they weren't.

 

[Fughuert]

Your throttle test is sitting still, raise the engine RPMs, and then you can get a brake pedal by pumping it up? Then if you let it drop to idle, the pedal goes to the floor?

I know you said, but check the bleeders again for being on top. I've seen several folks say they were, say they checked them, and when I had them send over pictures, they weren't.

Ok I went out and retested. Vehicle not moving throttle input or not, pumping does nothing, brake pedal does not firm up. I believe the throttle input is not the variable.

I think I thought it was a factor because on a long straight I could keep on the gas and left foot pump the brake and after a few pumps the brakes actuate near the top of pedal travel and deliver decent stopping power. If I was not on the gas the vehicle would stop before the pedal could firm up, doing little pumps doesn't do anything, I need to do a hard jab a few times. Everything points to air in the system but I don't understand how it won't bleed out.

I was able to lock up the front tires again for a solid 15 feet after doing the pumping. I’ve included photos of my bleeders please tell me I’ve not been wasting my time.

 

[mrblaine]

Ok I went out and retested. Vehicle not moving throttle input or not, pumping does nothing, brake pedal does not firm up. I believe the throttle input is not the variable.

I think I thought it was a factor because on a long straight I could keep on the gas and left foot pump the brake and after a few pumps the brakes actuate near the top of pedal travel and deliver decent stopping power. If I was not on the gas the vehicle would stop before the pedal could firm up, doing little pumps doesn't do anything, I need to do a hard jab a few times. Everything points to air in the system but I don't understand how it won't bleed out.

I was able to lock up the front tires again for a solid 15 feet after doing the pumping. I’ve included photos of my bleeders please tell me I’ve not been wasting my time.

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Bleeders are correct, so that points to the booster push rod. If the push rod is too long, it won't let the pistons come back far enough to take in fluid to push out to the brake cylinders.

 

[Fughuert]

Ok I adjusted the booster push rod in about half the distance it was previously at to achieve .030 from the master face with the o-ring removed to the booster face based on an old post of yours

It is better, and the brakes will now chirp the tires as it decels from about 45 without needing pumping up, but the brake pedal goes all the way to the firewall to achieve and does not do much between top of stroke and bottom 1/4 of stroke.

I will try to turn the pushrod in and shoot for .020 gap then bleed again and see if it improves or gets worse.

 

[mrblaine]

Ok I adjusted the booster push rod in about half the distance it was previously at to achieve .030 from the master face with the o-ring removed to the booster face based on an old post of yours

It is better, and the brakes will now chirp the tires as it decels from about 45 without needing pumping up, but the brake pedal goes all the way to the firewall to achieve and does not do much between top of stroke and bottom 1/4 of stroke.

I will try to turn the pushrod in and shoot for .020 gap then bleed again and see if it improves or gets worse.

Probably be a good idea while you have the master off to go ahead and bench bleed it again.

 

[Fughuert]

Alright I got the flu and had some other projects to attend to but I just messed with it again, bottomed out the booster pushrod, made it way worse. Set it back to .020 gap and got back to the previous results.

Rebench bled the master, no issues getting a stiff plunger. Bled the brakes again, cycled about 5 qts to be sure and it's about the same but better.

I get ok braking at around 3/4 pedal travel now, I can still pump it to improve pedal feel, and stabbing them will go to the firewall. It's not at the point I would let someone else drive it but is ok around town. Not really sure what else to try at this point.

 

[Fughuert]

well I don’t really know what happened if it was placebo, me being used to my cars brakes and acclimating to the jeep or what, but driving it around a few hundred miles felt like brakes got better did a gravity bleed and got a few bubbles.

I can consistently lock the front tires from 50mph with ease. Was able to have them on the verge of locking up 5 times in a row with minimal fade on 33” tires.

Issue was definitely the booster plunger being too long.