Not familiar with the big truck brakes but some things to check if they apply. If the parking brake is part of the rear brakes make sure the cable is retracted all the way, that can sometimes spread things out and make it difficult to get apart. If the shoe retainers have pins that go through the backing plate sometimes those need to cut off to get things apart.


38 Chevy 1/2 ton being resto-moded
65 AC Cobra replica
USAF vet