The 140, 1400, early 760 & the 130 pistol all work the same way. When the gun is pumped, it sets up the sear to hold the cap on the valve. When fired, the cap pops rearward releasing the air. A spring then pushes the assembly forward closing the valve, ready for the next pump cycle. Taking any of these guns apart without an idea how it works will stump Joe average man, it is a very different style valve than the hammer fired now used.
I have had both good and bad results resealing these guns. Attention should be payed to the pump head seal and the cap quad seal. Mac-1 may soon have a replacement piston head seal that is better than what is currently available.
A healthy 1400 should do 635fps at ten pumps. Some better some worse.
Overall a decent pumper when it has a good barrel. Dave