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.
Manual transmission? Explain lock down. Does your dash knob pop out?
Are you sure the brakes are adjusted and releasing. Sounds like cam roll over. IF the cams are rolling over you might end up having them lock up and not release.
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.