M-923 Brakes locking up

Would appreciate if anyone can help with troubleshooting the brakes on an M-923 (1984).

The truck builds up excellent air pressure to 130psi. The brakes release and it will drive. However, after applying the brakes about 5 or 6 times, they lock up and will not release... even when caged.

If you drain the air from all four tank drain spigots, and wait about 10 minutes, you can start over.

I've removed the brake pedal, opened it up, and cleaned and lubricated the plunger... in case it might be sticking. Didn't help.

Feels like it's an air issue getting past a diaphragm or an o-ring somewhere.

Any thoughts?

Otherwise... my trips around town to pick up 5 tons of groceries are limited to just 6 stops

Thanks!!

Make sure the vent on the Passenger side front Gladhand is open and not plugged up (and if has a shutoff valve installed make sure it's open)...

Go read this thread http://www.steelsoldiers.com/showthread.php?t=124778
And next time search a little bit