Does anyone have issues with the parking brake not holding the truck on F250's with disk brakes in the rear?

 

Have you checked the linings ? ( thickness/adjustment )

 

If the brake cable is rusted, likely the cable is rusted inside, therefore sticking.
Exactly my problem right now with the parkiong brake.

 

They could either be out of adjustment, in which case you can crawl underneath and remove the rubber plug and adjust them up, or you may have rear hub seals leaking on them, making them slip. This is a common problem. If you see any oil on your inside rear tire or smell rear end oil around there, that's prolly the problem.