You stated that you tested the newly rebuilt pump before you installed it. I don't know how much truth there is to it but I was taught to never hand operate the lever on the pump to "test" it. The theory is that the lever only moves a small amount on the cam and the diaphragm can be damaged by moving it too far.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine