I think you're looking at that funky behavior that makes people say "my classic valve has RT".
I've always associated it with a sticky on/off, but like ta2maki said, the sear could be involved as well. Basically, you're getting gas into the dump chamber after you let off the trigger, but before the sear can latch up completely.
Since it's more like a performance issue, watch the videos heinous pointed out and look at a few things on your marker in particular;
1) bolt spring length - should be about 1/8" or more over the tip (a little longer is better for this problem).
2) sear tip is not rounded/worn
3) rail bushing in place (always)
4) rear frame screw is snug when assembled
5) on/off pin is .750 (make sure no one has ground it down) and it is free to move.
6) sear pin/axle fits properly (sometimes you find a "substitute")
7) on/off top o-rings in good shape
It's not that you expect to have trouble will all that stuff, it just all needs to fit together and old mags can have a lot of "history". HPA runs at a higher dump chamber pressure than CO2 in the same mag, and it's sneaking by the on/off where the CO2 doesn't.
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some things are stickier than others