The Crosman 1377 & 1322 had better triggers & used the knock open valve, the earlier 1300 had a blow off valve & the trigger pull got heavier the more you pumped it. They also had an annoying habit until they were run in of the quad ring valve not seating properly. This could lead to you firing & upon pumping up again the gun could occasionally discharge prematurely.
I have seen this happen on two 1300's of similar vintage. It is very sensible never to load a pellet & close the breech until after you have pumped it up.
While using mine about 45 years ago I had put one in the breech & was pumping it up. The swing of the handle was such that the side of your hand can go across the muzzle when pumping. At approx 10 pumps it went off & plugged me in the palm at the base of my forefinger. Didnt half sting i can tell you!
I had to pull the .22 pellet out with tweezers.
I never pumped a 1300 with the breech closed & one up the spout ever again!