Air Venturi G4 won't pump

[.20cal fan]

I have a G4 pump that won't pump air into my Hammerli's cylinder.  I rebuilt it with a kit and still no go.  Air gets pushed out through the probe which i can hear and feel but no where else, which is a good thing.  When I attach it to the cylinder and pump a couple of times it is very easy going then bam, it won't even go to 100bar and when I let the pump handle go, it will rise about 1/2 way up.  When I try to lift the handle all the way up to get a full stroke, it won't allow me to and will go back down to the 1/2 points...about.  The previous owner used it twice, which I have no reason not to believe.  Why twice?  His tank came in.  I used it about 10 times.  Slow strokes, didn't over heat it (to the best of my knowledge), upon the rebuild, I did not notice/see any blown, damaged, worn and/or burnt/melted orings.  I have no other means to charge my Pneuma and I hope someone can help me.