If you are talking about your 2001 XC on Freightliner chassis, it should be equipped on the rear with spring brakes that are released by air.
Do you have drum brakes or disc brakes on that? I would think for that year it would be drum brakes.
A few things that might have happened.
1: You did not allow the air pressure to build up over 70#s before trying to move the unit. A brake "maxi" pot should apply at approx. 60lbs and fully release about 70lbs.
2: As these spring units wear, the pushrod may have enough mis-alignment to hang up on the housing, or you may have a broken spring in the pot.
3: You could have an air leak or punctured diaphragm at the brake pot primary. Are you loosing air?
4: Your unit should be equipped with automatic slacks that may be contacting something that is hanging in the area? Air hoses, wiring?
5: Has the unit been greased lately? The S cam shaft will have two grease fittings. One at the hub and one at the slack adjuster pivot. Do not overgrease.
The slack adjuster may also have a grease fitting and will use a normal chassis grease.
Do not use moly-disulfide loadedgrease fittings.
grease or oil because it may shorten
automatic slack adjuster service
life. Do not use pressure-release
6: There will be an air relay valve that supplies the spring brake with air when you push the park button on the dash. This valve could be at end of life.
If you have limited knowledge of 'AIR BRAKES' Pete Snidal has detailed a good, easy read, review here for Flxible owners and I would say it is a must read for anyone new to Air Brakes.
Article: The Brake System – An Airbrake Primer – Flxible Owners International