Help me diagnose a brake issue. (1 Viewer)

[Azca]

Replaced the master cylinder today. While adjusting the push-rod for depth on the brake booster, air was noisily expelled when applying pressure to the rod to adjust.

Used a pressure bleeder to bleed the brakes. There is no LSPV as it has been removed. All hoses were replaced within the last year and rebuilt calipers at the same time.

Problem. Brakes are firm but fade the more you use them and all but dissapear under a very heavy stop. Takes a few moments to get them back afterwards. I do the foot on the brake and turn the engine off test for the brakes, takes about 30 seconds (the first time I tried it) to start to rebound. The second time, after the hard stops, took about 10 seconds. Foot on the brake and starting, it immediately softens and reacts normally.

I am about 100% sure it is the brake booster but would like the mud brain trusts opinion before dealing with it.

 

[baldilocks]

Did you bleed the MC itself before bleeding the rest of the system?

 

[Azca]

Did you bleed the MC itself before bleeding the rest of the system?

Yes. Took it out after the initial run and bleed the whole system again. Same result.

 

[Rifleman]

Yes. Took it out after the initial run and bleed the whole system again. Same result.

I think what Boldilocks was asking you above, did you bench bleed your new master cylinder to remove all the air trapped in it, BEFORE you installed it on your truck.

 

[Azca]

Yes, bench bleed and then bled the entire system twice. Almost a gallon of fluid.

 

[landtank]

not sure why you had to adjust the push rod. But brake fade is usually caused by the pads dragging on the rotors causing excessive heat.

I'd double check the push rod and make sure you are adjusting it properly.

 

[Azca]

Adjusted because I am an idiot. The last time I put it together, I could not find my micrometer. Guessed and WAY off. Put up with it till it appeared that the MC was going. That issue was long brake travel and the pedal would sink to some degree just holding normal pressure on it. Booster passed all the normal tests prior to me replacing the MC. New issue only occurred after I replaced the MC.

Just tried the booster test again. Started to rebound less than 10 seconds after startup. I am sure that is the problem now.

I used this tool to adjust the BB push rod as I am tired of dinking with the micrometer.