Leyland daf 45/150 handbrake not releasing after driving?

Any ideas please? After driving along the motorway for an hour I came off and had to stop at traffic lights, put handbrake on and when I released it to drive off the brakes were stuck on. Air tanks were reading 9 on both as they usually are. This has happened a couple of times on and off and the only pay I can release them is to pump the foot brake until air tanks are empty then rev engine to build them back up. It's then ok. Passes mot fine.