1987 s4 tii. Brakes work well, but I notice a hissing when I turn off the vehicle. It is between the booster and master cylinder, where there shouldn’t be pressure. Where is a good place to get a replacement, or is an upgrade needed?

 

I don't think many places carry replacments. RockAuto has a service where they will rebuild your core, but you have to send it to them and wait for it to be rebuilt.

 

It sounds like the diaphragm is not holding vacuum. After the hissing stops how does the brake pedal feel? Is it instantly rick hard? If you depress the brake pedal and start the engine, dies the brake pedal drop slightly?

 

Just make sure the check valve in the hose supplying the brake booster is installed and functioning correctly.

The OEM hose has the check valve built in and is not visible.

You might know this already but just putting it out there. A failed check valve will let air rush by it.

 

sorry to reply late, that’s interesting, my check valve is clearly visible… maybe I need to check that.

 

I will check this. I know that if I spam the pedal, the rpm rises. I’ll get back to you on this

 

both of these are yes.

 

It does sound like the booster is holding vacuum at least. If you spray some soapy water on the booster and on the area in between the brake master cylinder and the booster, do you notice bubbles anywhere indicating a leak?

 

i haven’t done that, but I’ve hooked up an evap tester to the booster, and smoke came through. I also held my finger down between the two, there was a noticeable audio change in the hissing, meaning my finger was on the leak.

 

brake booster in under vacuum. That test would not produce bubbles.