Well here is a picture of the spring measurements. I would look into maybe it being a problem like what James_Blond suggested
"your may need to screw your main pump rod(the big one, not the one going to the bolt) in a little bit further into the pump handle. Just make sure you know where it is before you move it in case you need to put it back where it was. If it is entering the gun to far, you might be cause the valve to open slightly when pumping fast.
Started with a Splatmaster, moved onto a Nelspot 007, then came the Autocockers, then a STERLING Pump, next was a Bob Long defiant then a PM7. As of now I have 6 Sterling Pumps, 1 Phantom for SC, a CCM SS-25 and a Sovereign 3.