Have had pop-off issues since the beginning. Saw a discussion here Umbrella Valve mentioning an “Umbrella Valve” would be an option but I never found an examples of them being tried out and working. I eventually ordered a pack, intended for Harleys of all things! Valve
Unfortunately the vent hole on my existing endcap was too large to secure the umbrella valve. I ordered a new endcap, then realized there’s a built in reinforcement on the endcap which prevents a seal with the umbrella valve. Put it all away and forgot about it.
Last week I’m working in the shop and realize I left the endcap sitting on the table saw. Usually very good about securing adult entertainment products. Go to put it away and find ANOTHER endcap. Turns out the one on the saw was the cap for a tube full of plastic clamps I was using for glue jobs in the shop. Out of curiosity I checked and it was the exact size as the Venus supplied 2 1/2″ endcap, without the reinforcement! They’re called “Pro-Series” 22PC Spring Clamps. Clamps
There’s a small “nipple” in the center of the endcap from the molding process. Carefully cut it flush with an X-acto knife. Drilled a small hole in the center and worked the size up slowly until the valve would be captured. Drilled a single, small hole halfway between the edge of the center hole and the perimeter of the valve flapper. Tried it out and still popping off. Drilled a second and finally a third hole and it finally worked! Adjusted the air and it worked exactly as it was supposed to!
This endcap won’t work on every receiver obviously, but I would imagine all the endcaps are an off the shelf product and that there’s a substitute out there without the reinforcement. Once you have the right cap, play with the number of vent holes, maybe the size of hole, until it works right. Take your time as you can’t go back and undo anything!
I’ve made a similar kind of valve before for Venus using a patch of round rubber and a bolt. But the best valves I have found are duck bill valves which you can buy on Ebay: https://www.ebay.co.uk/itm/302841651412
Slightly harder to fit to an endcap, but if you have a 3d printer you can make these: