Brake pedal goes to the floor when truck is not moving - 97 Tahoe 4x4

[jahworks123]

Hello there. I have a 97 Tahoe that I've trying to diagnose a brake problem on. So far, front brakes have been replaced, twice, rear brakes, once, system bled twice.

The problem I'm having is that the pedal goes to the floor when sitting at a stand still. It makes a hissing sound (like air is being let out of a tire) and just goes to the floor. The brakes work fine, and it has decent pedal under normal driving conditions. I imagine that in an emergengy situation where the pedal would have to be pressed hard for a stop, the pedal would most likely do the same as at rest.

I know that a bad brake booster would make the pedal hard, but would it also make it soft? Also, I think there's some kind of valve connected to the booster, what is it called? Is that the Brake Check valve? Maybe I should start by replcing that first??

[jahworks123]

After replacing the booster and MC, I found a leak in one of the front calipers, and replaced both of them. No more hissing, but pedal still went almost to the floor before engagement, even after multiple, full system bleeds and bench bleeding the MC. Turns out that the push rod that came with the after market brake booster was about 1/8" too short, and non adjustable. I fabricate my own rod using a 3/8" thick bolt. I basically cut the head off the bolt and made it slightly longer than the aftermarket push rod. Pedal engagement is now at about 1/2" of travel. I'd imagine that a brake booster from the dealer would have adjustable push rods, but I have not verified that myself. Maybe at some point I will try the dealership to see if they sell the booster push-rods separately.

[Blue Bowtie]

Thanks for the update. I recall having read that before, possibly in a TSB. It seems that some MC push rods are not quite optimal, but I can't recall what vehicles are involved. I know some of the wrong wheel drive sedans were having the opposite problem. The brake pedal would climb after use due at a push rod that was too long.