That sounds like an old rifle...leather seal on piston. They were not powerhouses to start with so it may just need to have the seal oiled...a mix of 50/50 neats foot oil and silicone oil seems to be whats used now..put 5 to 10 drops of a good leather oil in the port with the rifle cocked, then release the spring without firing the rifle and stand it upright over night, see if it gets any faster, first few will either be really low or about the same fire it about 10 times then let it sit over night again...the oil/grease behind the seal may also be dry so a relube won't hurt, don't use a moly type grease bad for leather. If you do that and roughen the outside of the seal slightly it will work better. From your sign on name you might be from the UK....if not I can rework the rifle for shipping costs or if you are in the UK shipping will be more than the rifle is worth unless you have a collector you know....
