galletti

Hello,
I've searched and searched but I can't find a corect answer for this and before I start swapping parts, I'd like some help please. I have an 88 IROC convertible automatic. I have LS1 brakes on all 4 corners which were working fine for a while. All of a sudden one day, my brake pedal got really hard and braking power went down. Answers I've found for this are bad proportioning valve (will change anyway), bad master cylinder, bad booster or need for bleeding.
The other part of the problem (and the reason I posted on this forum) is that after sitting at traffic lights with the brake pedal on for a while, the car will stall after releasing the brake pedal. You can imagine how embarassing this is since it stall exactly when the light turns green!
The engine is pulling good vacuum at the manifold (21 inches). All in all, it sounds like the booster is the culprit but I don't know how to verify this. Thanks for any advice.

Lo-tec

Don't know the exact way to diagnose a booster, but I would think you could put a vacuum on it with a mitivac and see if it leaked down. When my booster crapped out, pedal got real hard and wouldn't stop well at all. With your stalling problem I think it points to the booster, though.

galletti

Thanks for the quick reply. So I should disconnect the vacuum hose from the manifold and put a vacuum on the booster? How much vacuum should I try? Will too much damage it if not already? How do I know it's the booster and not the little valve to which the hose attaches?

Lo-tec

The little valve is a check valve. It keeps the vacuum in the booster. You can check that by blowing in each end. You should only be able to blow in one direction. As far as vacuum, I would put 15-20" on the hose that runs to the booster and see if it holds or leaks down.

galletti

Thanks for all the help. I checked the valve on the booster and it only blows one way. The manifold is pulling about 21 inches of vacuum steadily. I put 20 inches of vacuum on the hose coming out of the booster and it held for over 5 minutes. That, I believe tells me the booster and valve are good and that the brake problem may be a clogged master cylinder or proportioning valve. I'm already changing the prop valve since I went with LS1 brakes all around. However, is there a way to clean out the master cylinder or check if it is clogged? Is there a way to repair it? I'm sorry for all these questions but my shop manual is in storage while my house get remodelled?