HW75 rebuild problem

I recently re-sealed an HW75. The re-seal was successful except when cocking it, the pumping resistance occurs on the opening stroke and it closes very easily. When it's cocked and fired, you get a little "pop" but no real force. This can be corrected by cocking it and then pressing the firing pin to release the small amount of air. Then, when you re-cock it, it functions correctly - for that shot only. To fire it again, you cock it, depress the firing pin to release the small amount of air, then you can cock and fire again. I've had it apart and reassembled it several times with the same result. Any suggestions?

Thanks, Jon