No Brakes When Applied Hard

[IkeWhitt]

I have a 95 Lumina 3.1 and I discovered to my surprise the other day while driving through the Wal Mart parking lot that I have no brakes when I apply the pedal quick and hard. The car keeps rolling and I hear this noise under the dash like air coming from a hose and the pedal doesn't move down, like it's stuck at the top. Almost like there is a hugh rock under the pedal. When I let off the pedal and apply them normally they work fine. I replaced the pads and bled the system and they do OK except when I try to stop very quickly. I don't think I have the ABS system. Anyone ever experienced this before?

[Forscythe87]

Check your brake booster hose.

[jeffcoslacker]

With the motor off, pump the brake several times to deplete the vacuum reserve. Then start the engine while holding the pedal firmly. You should feel the pedal drop immediately as the engine begins to run. Try to notice if you hear the booster hissing or anything as it builds vacuum again.

Check the hose to the booster, as suggested.

If you don't have any reserve that depletes as you pump with the engine off, or is slow to build boost, it may have a faulty check valve. This is designed to hold some vacuum in the booster in case the engine dies, enough for several application, and to maintain supply during low vacuum conditions of engine operation.

If it fails, you are only getting boost as supplied by the motor at that moment, there is no vacuum being built and stored...so if you make a sudden huge demand of the booster, or say hit the brakes suddenly coming directly off a hard throttle application, you'd have little or no boost available. In this case, the sound you heard could be the expended booster trying to rebuild vacuum.

At any rate, I wouldn't play around with it..get it taken care of. You CAN stop without boost, but you'll have to just about stand up out of the seat with both feet on the pedal...

[Bearwulf]

Do you have an abs light that comes on when first turning on the key? I have had this happen with my '93 Lumina w/abs, turned out to be a broken connector @ the R. front sensor. With that one it would happen with a soft or hard pedal application, but happened only intermittently.
However, I do have to go along Jeffcoslacker about the check valve.

[IkeWhitt]

Thanks for the info. Is that booster hose under the hood or dash?

[jeffcoslacker]

Originally Posted by IkeWhitt

Thanks for the info. Is that booster hose under the hood or dash?

http://auto.howstuffworks.com/auto-p...wer-brake1.htm

Under the hood, master cylinder is bolted to it, and it is in turn bolted to the firewall.

Pretty good basic info click the link....

[IkeWhitt]

I installed a new check valve and everything seems to be working fine now. The only one they had at Auto Zone had a smaller diameter than the booster hose so I had to put a clamp on it. Thanks for all the help!!