Hi all,

I have been working on the brakes on my 88 firebird (front and rear disc) and I have had this problem for a while:

When the engine is off the brakes feel great however once the engine is on the pedal loses most pressure and become soft. When driving, the car only slows down when the pedal is pressed all the way to the floor, and even then it is not a strong stop.

• I have replaced basically all of the brake components (brake booster, proportioning valve, brake lines, brake hoses, calipers, brake pads, rotors)
• I've bled the master cylinder after my last modification (brake booster) as well as bled the brake lines from the calipers.

Does anybody have any insights? Do I need to replace the master? Caliper alignment or any sort of calibration?

If any clarifications or images are needed just ask. Thanks in advance!

 

And this was done with the MC off the car, in a vise, held level, right???

If not, that's what it needs. We call it "bench bleeding".

 

No. I did it still in the car using plastic fittings which screw into the master cylinder on one side and on the other have a rubber tube that I put back into the reservoir. Kind of like this:


Is there a difference between this and a regular bench bleed? If so, what is the procedure for a bench bleed? At that point, does it make sense to just replace the MC given that it is near 40 years old?

Thanks a lot.

 

On the bench you can deep stroke her. On the car you might not be able to go deep. If you are short stroking, then you won't get all the air out.

 

The master cylinder is tilted nose up in the car. I think the reason for bench bleed is because an air pocket is captured in the front of the master cylinder if done in car. Leveling the master cylinder (off the car) allows that air pocket to escape. Procedure is same as shown in your graphic.

You want to avoid over stroking the cylinder as it has a chance of damaging the o-ring if bore is not uniform.

Fill master cylinder with fluid after re-installing and then open the brake bleeders. Gravity alone will do much of the work for you. Not everybody has to do this next step, but I also purge air from high in the system by cracking the fittings at master cylinder while someone is placing constant pressure on brake peddle. Seems I just can't push that air out any other way (but I don't have stock lines either).

 

No one will ever do this but I'll repeat it just for laughs. Buy 2 plugs for the master cylinder and install them while it's on the car. Step on the pedal, if it's as hard as a rock MC is good, if not take it off and bench bleed it.

 

I dealt with this recently. I bought this kit from Harbor Freight and pulled a couple big bubbles out of my MC with it still in the car. Highly recommended.