The brass cap should be screwed on as tight as possible by hand.
If it's too difficult to push onto the valve, a little lube (saliva works fine) might help. For the first two or three times you use it it will seem too tight.
On the other hand, if this is a well used pump, you might need a new head gasket.
That is, the pump head might need a new gasket.
They cost a couple bucks and last about a year or two.
If you use smooth stemmed tubes ike the Michelin Airstop you might get three or four years use.
When they wear out, they leak and blow off under pressure no matter how hard you tighten the brass head.
And as long as you're working on the pump, you might want to spend another three bucks for the leather pump washer.