Cocker Pump Leaking Down the Barrel after Shot

[shooter311]

I have a cocker pump with a Platinum Slice Kit, and AutocockerParts.com full lower tube kit. It was shooting fine, but then I tried using it with 12 gram co2, and I couldn't get it above 230s fps. So I just put HPA on it and for some weird reason I still couldn't get it above 230s (I messed with the reg too much?). I swapped out the hammer spring with the next stiffer spring (Inceptions Designs Spring Kit). I then take it out and it is shooting 320s. Something seemed odd right off. Anyway I adjust the reg so it is shooting 270s everything seemed perfectly fine. Then When I was done using the handheld chrono I go to shoot the rest of my paint in the hopper. I notice if I hold the pump forward after the shot air leaks down the barrel, if I shoot without holding the pump forward it is fine.

I swapped the springs back and I am shooting 250s but with the same issue. I am assuming that the valve is not sealing up, but I don't understand why it would make such a difference holding the pump forward. What am I missing?

Thanks!

[punkncat]

Check your cup seal face as well as your valve rings to be sure they are holding. Not an unusual problem as I have had it happen with LP sprung semi rigs.
You may need to respring for a harder valve spring and rethink the hammer spring tension.

[maggot]

Your leak is happening all the time, but you'll only notice when the pump handle is forward because that's the position where the bolt allows air directly down the barrel.

Check your cupseal and o-ring on the valve for damage, then sweet-spot your setup. Should resolve it. If not, you need to re-think your springs.

[iamthelazerviking]

Cup seal most likely. Maybe jam nut too. Also may need to sweet spot reg again.

[Falcon16]

Check your jam nut, make sure it's tightened down to the point you'd need to use something to get leverage on the tool to back it out, if that's tight I'd be taking your valve out and changing your cup seal. The other thing it sounds like is that your valve spring may be too soft or may have come off the nose of the cupseal