Getting worse as time passes. Going to have to fix. Sometimes you just barely touch pedal and it will really grab, sometimes barking the back tires. I'm thinking I need a new foot valve. When it does that I can hear a shoosh of air like you just stomped the pedal. Then 10 seconds later you try it again and works smooth as glass. Seems like itis worse if you have not used the brakes for a time, like driven a few miles without braking. Replaced the rear relay couple years back so it is newer. Anyone have experience with this.
More than likely its the E-7 foot valve, once in a while the R-12 relay valve located in the middle of the rear axles can do that also. But normally the R-12 just leaks out the exhaust port. Cold weather can bring that on-did you use any air system antifreeze/alcohol in it? One thing I would check though is the inside of the air tanks are ok-not rusty. Rust deposits in air valve will cause an on going issue. The normal cracking psi used on relay valves like the R-12 is 4 to 5 psi. Truck Shop
I would think that if you can hear a shoosh of air that it is in the treadle valve and not in the relay valve. If it is causing the problem, it is too far away to hear anything. Unless the sound is coming from the air tanks under the step or something but I would think in the winter with the windows up you would still not hear it.
Had the same thing happen on a mid 80's KW. Would put you through the windshield if you stepped on the brake. Had to ever so lightly touch the pedal to not make it happen. Wouldn't happen every time also. Changed out the trundle valve and never had it happen again. Wait till you wedge yourself in there changing it, the words you will create.
Well now that might depend on how old one is, how long of wheel base, weather it has a bed on it and so on. I normally can hear a air can leaking on application. One thing I do anymore for customer trucks that have a foot valve below the floor boards, is ask the owner if he would like the foot valve changed while installing a new clutch. If it hasn't been done recently.
OK, that settles it. Order a new foot valve. I can see it will be fun to get it in there but will be worth it. Thanks to all . LB