brake noise

[saltydog]

I was having some brake problems with my 1987 GN a few months ago. Whenever you pressed on the brake pedal the brake light would come on for a second. The brake pedal then got very firm and the car was very difficult to stop. I swapped out the accumulator bowl and all of these problems stopped. However, now I can hear a constant hissing noise coming from the brake fluid reservoir and the accumulator bowl. The noise lessens when you apply the brake pedal. Is this the power master unit I'm hearing and how can I get the noise to go away?

 

[gofstbuick]

It's probably the PM motor humming. Key on engine off, the PM motor will pump up. Let it sit for a while and listen if the motor begins to hum. After 20 seconds it will be hot to the touch. If that is the case, the check valve is probably defective. This requires that the master cylinder get rebuilt. You can buy a master cylinder rebuild kit from GM and do it yourself. The rebuild kit has a new check valve in it. What color is the brake fluid? really black? If so, that stuff has coated the master cylinder pistons and every thing. The master cylinder bores are anodized, so it cannot be honed. If there are pit in the bore, put it back together with the old parts and go get a autozone PM unit. I highly doubt the bores will be pitted.