pump problem on 1377

[chucky]

Hi All
Hoping for some help and advice.
When i pump my 1377 i get no build up of resistance no matter how many pumps i do, there is air going into to valve but i only get 2ft/lb. if i partially pump the piston and release the handle the piston pushes the cocking lever back towards the muzzle so i assume it is holding air. i then placed the tube complete with valve and piston in place into a bath of water, i pumped the piston and there appeared to be no air bubbles and after a few pumps in the water i could not push the piston and further, really strong resistance.
any ideas what is going on as i cannot get more than 2ft/lb or pump resistance under normal circumstances.
this is a real weird one.
thank in advance.

[ped]

check valve isn't sealing in the valve~strip and clean valve