Hello folks, someone may have an idea. The brakes on my trusty old forklift locked on recently so I removed & cleaned the drums & swapped some springs. Reassembled to find the same result, one pump of the pedal and the brakes locked on. Looked at the booster, it has a little diagonal diaphragm valve on the side of the hydraulic stage with a button in the centre of the diaphragm end cover. I pulled that and released the brakes with a hiss of air. It's not practical to keep doing that! I wonder if I'm up for a booster assembly replacement or if it's a sign of another problem. Booster hoses etc look OK and the master cylinder rod has some slack. Thanks very much!