I have a rebuilt M936 that has been sitting for a few years, ran great before. I had caged the rear brakes to move it around the yard with my loader and a chain. Got her up and running the other day and went to drive her out of the shop with the brakes still caged and they wouldn't budge. Uncaged them and still the same, i had the cab lift bolts up so i figure that was causing an issue, nope. The only thing i know for sure is that when primary air builds up they are off but as secondary builds they start to lock up. Any ideas? Thanks