New Zealand Datsun Club

One rear brake has been jamming on my 1600.(std rear brakes apart from finned alloy drums)
This is happening on the right rear from time to time after applying the brake pedal.
I have had the drums off a few days, nothing obvious I could see.
Shoes still have 4-5mm of lining on them.
Shoes and wheel cylinder seem to move freely.Drums are within tolerance.
Hand brake levers and cables are free and working.
Reversing while the brake is locked does not release the shoes.
Has any one had this issue before,or know what may be causing the problem?

Are you sure the shoes are in the right way, there is a lead shoe and a follower and if this is wrong ive found the jam on in my past experience. Do the pistons in the slave cylinders move back ok ?

I'd check hoses.
I've recently seen a collapsed hose preventing fluid return