Kenworth T300 hard brake pedal

Got a 2001 Kenworth T300. Driver tells me that the pedal randomly gets hard, he has to pump it and then it catches and then the brakes lock up on the passenger side rear. I drained the water out and there wasn't much. I didn't see anything obvious. I installed a new treadle valve and it was fine for a day. Now it's happening again. Anything I can test or something i'm overlooking?

Kenworth T300 hard brake pedal