Silca Impero Leak?
 

I tested this new pump today, and when I did, the seal formed, but the handle was shot backward to its full extension by the air pressure, and I could not pump. . What is going on, here?
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https://cimg0.ibsrv.net/gimg/bikefor...b649e98641.jpg
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[MENTION=160128]1989Pre[/MENTION] - The tire valve did not close. I have tried to pump a tire up by letting the weight of the wheel, when loose, rest on the pump by the valve stem while pumping. This can happen in this situation.

That worked fine. Thanks.

The mechanics are that the end of the Presta valve is inserted too far onto the valve. There is no check valve in the pump, so any force pushed back on the handle is from the tire pressure.

Campag metal pump heads generally do not do this- the cavity is deep- if problem was with one then the rubber gasket is worn.

Silca ends were shallower and mor of a problem.

repechage is correct. I find that the standard Silca head is a bit tricky to get just right and have changed out my pump heads to the Campagnolo head with the little legs. No fooling around , just pump up the tire . I HAVE had to massage the leather piston to get good pressure though. These pumps are for roadside use when you are in a bind, not for every day use. I actually like the Zefal a bit better but hey, the classic look of the Silca especially when matched to the frame color , it just don't get any better'n dat!