Help! Low brake pedal

[donwhale]

Have a 91 vette with low brake pedal. I have replaced master cyclinder,booster,pads, hoses, front calipers, and bled the brakes and brakes are still low. Any help will be greatly appreciated!

 

[Mac]

It sounds like you are running out of things to replace. That's gotta be frustrating... :ugh

Mac

 

[KOPBET]

Assuming you have done everything else correctly (did you bench bleed the master cyl?), you might try activating the ABS several times and re-bleed. You can usually activate the ABS with hard braking, especially if the pavement is damp or wet. I like to find a large empty parking lot, but a secluded less traveled road would work. Just be safe doing it. Hopefully there is not too much air in the system for you to activate the ABS. Otherwise, I'd start again from the basics, E.G.-bench bleed the master again, and power bleed the remainder of the system. Make sure the bleeder screws are not allowing the system to suck air through the threads, get some sealer if necessary.

 

[Hot Rod Roy]

Make sure the bleeder screws are not allowing the system to suck air through the threads, get some sealer if necessary.

KOPBET, there's no way air can be sucked through the bleeder screw threads if you're following the correct bleeding procedure! The seal for the bleeder screws is the tapered seat, not the threads. The only time the bleeder screws should be loosened is when there is pressure in the hydraulic system, so air is prevented from entering the system.

Since the m/c was replaced, there's a chance the push rod in the booster is too short. It's adjustable as necessary, to match the m/c. Before taking anything apart, check for correct operation by removing the m/c cap, then pressing down slowly on the brake pedal (engine on or off- it doesn't matter). You should see a disturbance in the fluid just as the pedal begins to move. This disturbance in the fluid is caused by a jet of fluid from a small orfice in the m/c bore. If the pedal has to move very far, the push rod is too short. (By the way, if you don't see the disturbance, the push rod is too long, which will cause the brakes to drag as the fluid warms up.) This is a pressure relief port to allow the hydraulic pressure to go to zero when you take your foot off the brake pedal.

I'm presuming the calipers are installed with the bleeder screws at the top of the calipers.

 

[Kevo]

Does parking brake work? Rear caliper piston is actuated by cable to apply. Piston is supposed to self adjust hydralically as brakes are applied. If it doesn't piston will be too far away normally to clamp pads to rotor and pedal can very low. Other posibility is brake pedal is not fully returning...something (added electrical stuff many times) blocking it under dash. You do have the correct front calipers? Base and J55 calipers are not interchangable.