3 months ago, I bought a secondhand Gunpower pump.
I use it daily with no problems.
But over the last 4 days I have had increasing problem.
On the upstroke, about a third of the time, no air goes into the cylinder, resulting in the handle going down again under vacuum, with no resistance.
On the downstroke, a third of the time, there seems to be double resistance and the pressure goes up by about 4 bar on a single stroke.

This destroys my rhythmn and makes it hard work.

Any suggestions? Apart from get a dive bottle.