My old (1980s with rear cocking handle and slide cover breech) 1377 wasn't pressurizing when pumping so I replaced the piston cup and valve with new ones from Crosman. I believe that I have the valve positioned correctly; threaded hole of the valve lined up with the smaller hole of the tube. Once reassembled, it is impossible to pump. There is so much resistance, I can only get the pumping handle down about 2/3 of the way and I fear I will break something if I try to force it down. What could possibly be wrong. Thanks in advance