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mac350 09-06-2010 06:00 PM

brake problem
the problem i am having is that the brakes wont hold. i press the brake as hard as i can and try to bring up the rpms. at like 1500 rpms or so the pedal starts to drop and the brakes let loose. the car just wont stop under high rpm. anybody have any ideas.

LATECH 09-06-2010 07:25 PM

If the pedal is dropping then the master cylinder is worn and bypassing pressure. however that would be most noticeable while waiting at a light for instance while just holding the brake with light pressure. If they are power brakes it is possible the booster is dropping vacuum from an internal leak and at 1500 rpm there is not enough vacuum produced to keep it assisting the drivers foot pressure. In other words the booster assist is going away and the required pressure from the driver is greater making it tough to hold the brakes. Maybe the check valve is no good or not holding. I could be in left field with all of this.It is just what came to mind.If the pedal is actually dropping it would be more likely to drop with light or medium pressure than with maximum pressure. Just my thoughts

Chevolla 09-06-2010 07:53 PM

Is this a problem that just started?Have you made any mods before the problem started?Is this a new car you just puhased?

mac350 09-06-2010 08:42 PM

OK this is an 87 formula. it has 400+hp. it has an lt4 hot cam and i do have a vac can on it. i did have a problem were the brakes weren't holding so i put a vac can on it and it solved the problem. the car sat to a couple of years. i was starting it every month but didn't drive that i am getting it ready for the track, this is on of the problems i have come across. i was thinking booster but i figured i would ask for advice first.

LATECH 09-06-2010 08:44 PM

I would say check the check valve then test the booster

mac350 09-06-2010 11:28 PM

this might be a dumb question but how do i test the booster

Chevolla 09-07-2010 03:12 PM

There are no dumb questions.

h With the engine off,press brake pedal several times to deplete all
vacume.Then hold light foot pressure on pedal,start the car,if power brake is working pedal will fall away under foot pressure.If no action is felt when engine is started,power brake is not working.

To check hydralics, deplete vacume again.Hold pedal.If the pedal gradually falls away under foot pressure,hydraulic system is leaking.

Start car rev to about 2500 rpm turn engine off.This builds up vacuume.Wait about 90 sec then try brake. you should have two or more vacuume assisted pedal applications.If not check valve is crap or you have a leak in the vacuume system.

mac350 09-07-2010 03:19 PM

thanks for the info. one of the things i did change today was the check valve and it is fine. i will do those other tests that you said to see what i come up with. thanks again.

mac350 09-22-2010 10:40 PM

OK i changed the Master and the booster. i bled the **** out of them and no luck i am back to square one. what next

that are the symptoms of a bad proportioning valve

mac350 09-23-2010 08:22 AM

could it me my brake hoses

i wouldnt think so because i have Earls stainless hoses

what do you guys think.

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