Hello, I hope someone can help me out here. I have a 91 Firebird convertible with a lo3 and a little over 100k on the clock that we have owned for over 10 years. Well about five years ago the car went into storage and over the last year has been brought out of hibernation and has recently been made ready for the road again, or so I though.

The brakes worked well but have developed a problem. Here's the deal, the brakes work great...at speed. At any speed above coasting it will stop on a dime. Or if you are giving the car the least bit of gas the brakes work flawlessly. But if you are just idling, like say you drop it in gear to back out of a parking space the pedal goes soft and to the floor, brake light on the dash comes on and brakes are almost non-existent. Same if you are coming to a stop by coasting and ease into the brakes. Soft, to the floor, and non-existent, but you give it the least bit of throttle and they firm up and work like a champ. Also while the car is cold they work great at coasting or less speeds but the moment it warms up they go to crap unless you are giving it a bit of throttle.

Stats on the car is that it has iron caliper rear discs that it has had from before we even got the car. Has new calipers on the front, new rubber lines, new master cylinder (bench bled), new fluid through out. The brakes have been bled and bled and bled some more. I have put over 64oz of fresh fluid through the system and I am confident that there is no air in the system. The booster checks out fine, it pumps up with the motor off and holds pressure, the check valve only works one way as it should.

I am pulling my hair out trying to solve this problem. This car is my wife's summer cruiser and I want to get it straightened out and made safe for her but I can not figure out this problem. Any insight would be greatly appreciated.

 

Only part of the braking system that can be effected by engine RPM would be the booster. Check your vacuum supply or just switch it out

 

I would have said the booster vacuum as well. But a wild guess, maybe the diferential valve is at fault?

 

Just following up, wondering how you made out? In reading my first response I meant to say distrbution valve could be an issue. Sorry about that!

Would like to hear how you made out on this? hopefully all is fixed!

 

Sadly it is not fixed yet. I took the master cylinder off and re-bench bled it and they whole system again and it is still the same.
Checked the vacuum yesterday and off the motor we are pulling almost 20cfm at idle so more than enough. Need to make a tee and see what it is pulling when the brakes act up. I have not thought much about it being the booster because I have always heard when they go bad the pedal gets real hard to push not really easy to.
But a friend pointed out a possibility last night. Maybe it is not the diaphragm that went bad in the booster, maybe something else has failed, broke, or bent like a pushrod or such and when vacuum comes up it puts enough pressure on it to make it all line up enough to work and when the vacuum drops off it comes out of enough alignment and no longer works. So going to order up a new booster and see what happens I guess. Wouldn't hurt to replace it since the booster on the car is 23 years old. Going to cross my fingers and hope for the best.

 

Yes replace it none the less. I had to replace mine, was original and was very hard. I had a similar issue with my JEEP wrangler and EVERYTHING was new, booster, all lines, master, wheel cylinder in the rear drum brakes, as well as the brake and etc. BLED it a bunch of times, could not get it working way to soft.

Turned out to be a leak in one of hte lines I replace. No indication of fluid, just air getting in. Mechanic found it when he power bled the system. Something you may want to do if all else fails.

 

Welp this has been solved. Turned out the new master cylinder was bad. Got a replacement from Napa and all seems to be good now. *crosses fingers*

 

Awesome!! Great to hear that. Good luck moving forward dude!