jockellis

Hello. My power brake booster is supplying no stopping force and I hear a hissing noise when I apply the brakes. It goes away the instant the engine is turned off so I figure it is from the vacuum side. The check valve? But with the hood up, standing outside the car with my foot on the brake I hear nothing. That might be, though, because of engine noise or I'm not able to push hard enough on the brake.

ezone

If you HEAR hiss inside the car, it's probably a bad booster.

Check 3 steps of basic troubleshooting in this link
power-brake-booster-test.asp

Seamus O

Do you have a vacuum gauge?
Do you have access to service info? Repair manual?
I would recommend vacuum testing all your lines, starting by testing your brake booster.
Place a vacuum gauge hook up between the check valve and booster, run the engine to normal vacuum 18-20Hg. Shut the engine off and see if the check valve vacuum goes to atmospheric pressure its a bad valve.

A vacuum leak anywhere in the system can cause power brake issues. Do you have any other issues?

jockellis

Thanks. That worked. Changing out the power brake booster was a tough job. Luckily, my drivers side floor needed cleaning anyway so taking out the seat served two purposes. Now it is on to fixing more problems.

Wankenstein

Thanks for the follow-up.