Brakes Locking up Question

2015 Mack pinnacle cxu612 single axle. I would start the truck build air pressure push in the brake valve and the brakes release. As soon as I put in gear they lock down. I have to pump the brake pedal to get to hem to release again.

 

It’s an automatic transmission. Brakes release before I put it in gear. But when I put it in gear I hear a 1 second air release from the brake valve then they are locked down. I put the back window an see the slack adjusters in the lock position. And the knob does not pop out. Brakes are adjusted I’ve have a Michanic to set them but he couldn’t figure out the problem. Just trying to do all I can do before sending it to the shop. When you say cam roll over you referring to the S- cam right. If so I should remove the wheels an check them while someone is applying the he brakes. Thanks for the info.

 

Sounds good thanks for the info