Oldsmobile/Buick Repair: 1989 Olds 88 power brake system., vacuum lines, foot brake

Question
The brakes on our Olds seem to working OK when the engine is running, however, as soon as the engine is turned off, the power assist function is gone almost immediately. This was first noticed on our driveway, which is inclined, upon arriving home one evening and shutting OFF the engine, and then deciding I was to close to the garage door and needed to roll back slightly. I set the transmission selector to N, released the EMER brake, maintaining pressure on the foot brake.  Then lightened up pressure on the foot brake to allow gravity to roll the vehicle back. When I was in the desired location, and applied
down force to stop, there was no power assist available.  It was all brute force at this point.
When I did get the movement stopped, I was farther down the driveway than I wanted to be.  Had to restart the engine, the power assist function returned, and I was able to place the car where wanted. This condition is repeating. My first thought is that the vacuum resevoir check valve is not working. I did check to see if
any vacuum lines sevicing the resevoir were disconnected or leaking. They checked out okay.
Need assistance/help in isolating this problem. Thanks.


Answer
Hi,Joe

I think your diag of the brake booster check valve is very good,i have seen those fail many times and have the same symptoms as your haveing,another possibility is the booster itself has an internal leak,though rare ive replaced a few not only for your type of complaint but a hard brake pedal with engine running,those GM boosters are very strong units and fail very rarely,i would lean towards replacing the booster check valve,you should be able to have brake assist 2-3 pumps after the engine is off and no vacuum.if you still need help feel free to contact me,Thanks

ED