loss of brakes.

[buffblazer]

86 k5.

did a axle swap to 60 front 60 rear, drums in back, all new pads, lines and cylinders. no leaks in the system that i can see.

when driving, every once in awhile i will hit the brake and the pedal goes all the way to the floor. then i pump it and it comes back after a bit.

figured it was a vac issue so i replaced the brake booster. still have the same issue.

thoughts?

 

[4X4HIGH]

Bad master cylinder bleeding back through itself.

 

[buffblazer]

Bad master cylinder bleeding back through itself.

Will that have any internal leak between the master cylinder and the brake booster cause I didn't see anything leaking when I removed it to do the booster.

 

[4X4HIGH]

Will that have any internal leak between the master cylinder and the brake booster cause I didn't see anything leaking when I removed it to do the booster.

Not if it's only leaking within itself. Basically there is a piston with a couple seals on it and fluid can pass one seal and allow it to not work properly but the other seal keeps it from leaking externally. Sometimes they do leak at the rear seal and into the booster but not always.

 

[1BEATK5]

loose front wheel bearings will cause that also. when you turn, the loose wheel allows the brake rotor to push the caliper piston in, and then the brake pedal will go to the floor until you get the caliper piston pushed back out.

also give this a try.
while stopped, engine running, lightly push on the brake pedal and hold it there. push on it just enough to engage the brakes and wait, if after awhile you get a sinking feeling under youre foot, it prob has a bad master. sometimes they "seal" better when they are under alot of pressure, and only will loose pressure under light pedal application. i have had this same complaint from a few customers over the years, and sometimes could not duplicate it when just driving it regular.
this does not work well with hydroboost, or some newer gm masters, as they will fail the test and might not be bad

 

[Fordum]

Wheel bearings will do it like 1BEATK5 said, and that's an easy check.
But it is almost certainly a bad master cylinder like 4X4HIGH said.
Two reasons.

One, that is a classic symptom.

Two, it was probably caused by your replacing the axles.

You drove that truck no telling how many miles. All that time, the master cylinder piston was pretty much traveling in the same section of the bore.
Meanwhile, your brake fluid had collected some moisture, and the far end of the bore had slightly corroded and generally gotten rough.

Then, when you made your swap, you bled the brakes.

Pump up the pedal good and hard, open the bleeder and let the pedal go all the way to the floor.
Which pushed the piston into the rough part, damaging the seals.

The older brakes were not as prone to this because as they wore, the pedal got lower until the shoes were replaced or adjusted. So all the bore got used.

Its so common nowadays, if I have to replace a caliper or do anything that causes me to bleed the brakes, I will buy a new master cylinder and change it at the same time if its more than a year or two old.

Sure I am certain I have replaced a good cylinder or two, but its a little quirk of mine.

My vehicles steer and stop.
Running is optional.

Driving my old CJ with the single brake system through bad stuff, I got really really good at dealing with sudden brake loss.

Its a skill I do not desire to maintain........