Quote Originally Posted by Shed tuner View Post

Mr V, just another thought - have you tried sliding the piston down the tube without the seal fitted, to make sure it's not a tight front bearing, or the piston itself, that is the problem ?

HTH - JB
Yes, I did try that, just to check that the new bearings were not causing a problem.
My main reason for thinking that there is no problem with leaks elsewhere is that the gun was doing 11.3 before I changed the piston assembly. When I strip it to fit the used seal I will check all the other possible problems suggested by you and other contributers to this thread.