P17 repair question?? Intermittantly wont hold air?

I have a P17 that has some issues. It will not hold air after it has sat for a day. If I put my finger over the valve hole that abuts the barrel and pump it several times, something will reseat? and them I can feel it holding air. I can then shoot for a session. I believe it is a part in the valve that stays open or reseats. I have taken it apart partially ie removed the cylinder lubed the ring but the issue is in the valve I believe. I have no problem trying to fix it (not an expensive gun) but it is an odd defect and would like to know what to look for. thoughts? thank you.

PS I have 2 p17s bought for about $30 each. I can hit a tuna can (4in target?) at 35 yards off a rest about 2-3 out of 5 tries when I get sighted in. Great fun.
 
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