I have a leak on my c h system , it's a pressure type valve near the cylinder after the automatic valve (with the little lever) the leaky valve has a small red cap on it . Its just before the water pump.
Why would it suddenly start to leak (allbeit slightly )
To change it how much do I have to drain down
Thanks

Is this a hot water system or a furnace system?

If it is a boiler make sure you check the operating pressure and if to hign it can cause the valve to weep.