It’s possible your end cap vent is plugged. All kinds of strange things can happen when the vent is plugged. Take the cap off and fill the cap with water. If it doesn’t run out the hole, it’s plugged. Get a toothpick and stick it in there and pop it open. Then rinse it out with lots of warm water. Then make one of these: https://venusformen.com/forums/topic/easy-fix-for-sticky-end-cap-valves/
Mart may be right about the air control box. To test if it’s working properly, take the box off the hose and blow air into it with your mouth. No air should pass through. Press the green button and air will go through. Then try to suck air out. No air should come out until you press the red button. If it works any way other than that, you’ll need to replace it. If it’s working properly, check both hoses for leaks and make sure they’re tight at both ends like Mart said.
If you know it’s not the box, and you’re still having problems, and what Mart said doesn’t work, you might have to get inside the unit and change the position of the push rod on the drive disk. You’ll notice the disk has 5 threaded holes on it. Move it one hole closer to the shaft of the motor. That will decrease the amount of air being moved in and out per stroke and should make the stroke shorter so it doesn’t go up as high or down as far. If it’s still too much, move down another one.
If you bought that second hand and ordered a new receiver to fit you, that position on the disk might not match the size of receiver you have. The match has to be correct to work properly.
The disk looks like this:
Disc
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