Crosman made the flat-top valve before they switched over to the cone-shaped valve. The flat-top is undeniably more efficient than the cone-shaped. Does anyone know the reason for switching over to the cone-shaped? I would think Crosman did it for a reason. My curiosity is bugging for an answer.
On my Crosman 1377 phaze 2 the box says "easier" pumping action. When you pump a flat top piston, it is work. The cone shaped valve is much easier to pump. IMO. That is what I think about that. Buy a flat top piston and valve and put it in your 13xx gun and you will see. I have 2 1300 pistols and they have the flat top piston and the flat top valve. More effort to pump than my 1322. :) :)