I think you will find that modding the valve to "improve" the flow does not pay benefits in .177 cal, in fact it may very well hurt the performance.... In .22 cal it's a totally different matter, you need the flow.... In .177, however, the theoretical benefit to improving the flow is outweighed (in my experience) by the fact that you are increasing the volume between the valve seat and the pellet.... That reduces the pressure of the initial pulse which in the smaller caliber is extremely important.... When I am working on .177 pumpers (or CO2), I do NOT hog out the valve exhaust port, the transfer port, or even the barrel port unless I'm building a gun capable of in excess of 800 fps.... Every time I've tried, I've lost performance, particularly in guns under 700 fps....I would seriously doubt you were NOT dumping all the air at just 10 pumps.... Most 1377s can be pumped ~14 times with an F-T-P without retaining air.... and with the stock piston it's almost impossible to develop enough pressure to retain air unless you have put in a stronger valve spring and/or a lighter hammer spring (ie the opposite of what most people do).... If you are down on velocity compared with before, my guess is your mods to the valve are the problem.... Bob