brake pedal hard after sitting awhile

Post by **JimBee** » 14 Jul 2013, 17:17

The brake pedal in my '93 850 feels "stuck" at the top after engine has been off for awhile. Sometimes after just an hour or two if put my foot on the brake it is like stepping on a solid block and the brakes don't work. As soon as the engine is started, with normal pressure to activate the brakes, the pedal descends to its normal position and the brakes work. What might cause this?

Post by **jimmy57** » 14 Jul 2013, 17:55

Check valve on booster or faulty booster. If it is the check valve then using a pair of needle nose Vise-Grips to clamp the vac hose to booster while engine is running. Shut off engine and wait 10 or minutes and apply brakes. If pedal is hard then it is booster. If pedal still feels normal then it is check valve.
The booster shouldn't leak vacuum with pedal released and when engine is off the check valve should hold vacuum in booster.

Post by **JimBee** » 14 Jul 2013, 21:03

Okay, so when I clamp the vacuum hose I'm emulating the check valve? Thanks, I'll give it a try.

Post by **jimmy57** » 14 Jul 2013, 21:18

Yes, You are assuring the vaccum holds in booster if it has a bad check valve. You have to be sure the clamping is effective.

Post by **FireFox31** » 14 Aug 2022, 19:21

I am having this problem and a related symptom on my 2000 V70 NA FWD. For over a year, when I shut off the car, I would sometimes hear a slow groaning sound coming from behind the steering wheel (the location of the brake booster). The sound would stop when I pressed the brake pedal and resume when I released it. After 30 seconds, it would stop. I could not reproduce this on demand and it happened infrequently.

A few days ago when I went to start the car, I stepped on the brake pedal and found it firm and not possible to depress. That lead me to this and other threads suggesting the vacuum line check valve is bad.

As recommended here, I clamped the line and the pedal quickly became firm. So this is a failed brake booster, not a failed check valve? The brakes work fine when driving. What about the booster has failed?

Or should I just replace the check valve and see what happens? I'm trying to properly diagnose this before throwing the $50 part at it. Thanks.

Post by **scot850** » 14 Aug 2022, 19:38

It is most likely the check valve. That bit of plastic does fail with age, just like most of my joints.........!

A junkyard part is fine. They don't fail all that often.

Neil.