Apologies for resurrecting such an old thread. It's just that this is the thread I discovered when googling leaking Belmont valves and I thought I would add my tuppenceworth at the end for anyone in future who has had the same problems as me. Belmont valves are well worth repairing as the seals can be repaired/replaced without draining the system down. It is an easy 5-10 minute job.
The o-rings are notoriously expensive - ~£1.50 each +P&P. I think they used to be made from butyl, but a modern upgrade would be red silicone. They are Imperial British standard size BS012 (internal diameter = 9.25mm, cross section = 1.78mm). These can be bought from RS for ~£6 for 50 +~£5 P&P, but if you just want a few then e-mail me and I can send you some (with a bit of silicone grease) for 50p each plus 50p P&P. Alternatively you can just fix a leaking valve with a bit of PTFE tape.
This is how I serviced my valves:
1. Before starting, loosen the valve by winding the radiator off then on and off again, then leave half on. Wrap an old towel round the valve as it will leak slightly
2. Next, you undo the screw on the wheelhead, and lift off the wheelhead.
3. Then lift off the thin plate that strengthens the wheelhead connection to the spindle.
4. The top lock nut needs taking off – unscrew this in the anticlockwise direction. Clean all the parts as you are taking them off as they will go back on easier.
5. Unscrew the spindle in the clockwise direction – you can use the wheelhead to do this. As it comes out it will leak slightly, but should stop (or at least slow down to a tiny dribble) as the spindle lifts out. If it continues to gush, put the spindle back in, screw it up and down a few times and try again.
6. Carefully remove the old red o-rings from the spindle using a small screwdriver.
7. Clean everything and either put new o-rings on with a bit of silicone/valve grease, or wind 3 turns of PTFE tape round the o-ring grooves and hold in place with the old o-rings.
8. Replace the spindle and locknut and wheelhead.
Hope this helps.