Does anyone else have hard brakes at low speeds (under 15mph). My brakes dont seem to be normal until I have higher RPM's getting the vacuum up there. The brake booster was replaced a year ago and I had the brakes bled and refilled with new fluid. It really sucks because honestly its not safe.

 

Start the truck, goose the gas a bit, don't touch the brakes. Turn the truck off, wait one minute then pull the vacuum check valve from the brake booster. You should hear air sucking in - if not then that check valve is bad or your booster went bad again (it's possible). If that checks out good then.... You notice any other oddities? Heater controls don't work unless you give it gas? Front axle lazy to lock into 4WD? Might be you have a smaller bad vacuum line or a line off. Check the smaller lines above & next to that booster.

 

Thanks for the input, I didnt think about the smaller lines and checking out all those controls that you said. I'll do that when its not raining out, hopefully in the next two days.

 

As far as vacuum from the manifold and RPM's go, this sure sounds bass ackwards to me. At idle, you pull the most vacuum. As the RPM's increase, vacuum decreases. As vacuum decreases, things that rely on vacuum (brakes in this instance) would tend to work not as well. This is why there is a reservior for the vacuum.

I'm discombobulated over this one.

 

I'm having the same problem on my 93 Jimmy. Had to do a panic stop to avoid squashing a prarie dog. (don't laugh, we love all animals) It took way too much effort to bring the truck to a full stop. Good check valve. No obvious vacuum leaks. I'm puzzled too.

 

Im still puzzled with this. I have been spending time during the week under, in and around my truck. Trying to follow the vacuum lines and I also can not see anything wrong with any of them.

I looked at the check valve on the vacuum booster and that seems to be functioning properly. Im about to just give up but I really dont want to have brakes I can not trust, in a emergency.

 

The theory (for me) between vacuum and RPMS is when the engine is turning higher RPMs and you let the throttle plate slam shut *that's* the highest vacuum you'll see. Also the faster the engine is running the more vacuum pulses per second you have to overcome a leak. Two addition tests I can suggest then: Disconnect the vacuum source for the small lines from the intake manifold and plug that off (the intake) with a short piece of hose and a screw. Take the truck for a test drive and see if the low speed braking improves. If it does you have a leak somewhere. If not, with the engine running, pull the check valve out of the brake booster and see if you have good vacuum (put your finger over it and see if it sucks). Might be the check valve is acting like a restriction.