Brakes won't release just like you're pressing on the brake. .
Not familiar with a JCB backhoe but you could start at the master cylinder if nothing else obvious is jammed up.
Depending on year this may have power brakes which mean you have hyd pressure at a lower pressure operating a charge valve via the pedal or you have conventional master cylinder try to slacken off bleed nipples on axles if the master cylinder wont allow the oil to return it will be pressurised and then should free if no pressure and still locked up try jacking up suspect axle and see if you can spin wheels but you will need to release the hand brake .
I can send you the factory manual if you like.
Interested in hearing more about this. And getting that repair manual (and operator's manuel if anyone has it), just bought a 1994 210SL a couple weeks ago. Great condition, love it so far but I got a hydraulic oil leak coming from under the cab, i'Ll have to look where it comes from. Also, under a little load, the bucket or fork drops down quite fast... shouldn't it stay right up? At least for a minute before starting to slowing drop...
I wonder if the original poster got his brake problems fixed. lockeed, ,the bucket shouldn't drop like you said, the hydraulic leak may have something to do with that. Let us know if you found the leak, also, you should start a new thread with your problem's, highjacking a thread of someone else gets confusing.