Check valve pin mishap.

[Mike71]

Did the pin mod to some crosman check valves.
Factory cup valve works great.
Brass Flat Top valve works great.
I did it for one of the aluminum flat top valves and it worked for one firing, then springy feel to the pump.
Take it apart and the check valve pushed far enough in the pin came out of the hole and got stuck inside, couldn't find the hole to get back out.
Done it 3 more times.
What would cause it to push that far in ? Pin is under sized for the hole so it moves very easy and allows air to pass freely, I didn't have alot of lube that may have done it. I put in a stiffer spring and it did it the 4th time so I took it out and all is good.
But man it was building pressure and shot hard when it did.
Anyone have this problem  ?
First pic is the setup. Second is after I took it out and didn't see the pin. Opps.

[Mike71]

This is my factory cup style and brass flat top, both pins are a tighter fit but move freely.

[Mzq284]

That's odd, Mike. Maybe the pin is too tight of a fit and the air pushed it in? How close is the fit? My 1322 is .003 difference, it hasn't given trouble.

Got the package, I'll work on it Friday and get it back in the mail

[Mike71]

My brass flat top has a tighter fit than the one giving me trouble. So odd as it has to push the pin completely out of the hole and off to the side a bit, then it's stuck open, I'll try and turn the pin down a smidgen more and see what happens.

[Mike71]

Think I've fixed it, made a new 760 check valve with pin and had to shorten the spring a bit, had to polish the inside seat surfaces, seemed to work great till I blew out an o-ring on the piston, put a new one in, lubed it up and wow.. at 11 pumps it retained some air, dumps it all at 10.. lots of pressure, had to shim the hammer spring to get that. Perfect, I wanted as much pressure as I could get and dump at 10 but no more, wish I had a chronograph to see what it's shooting at, but it's alot more than factory I know that.
Happy with it now..

[JPSAXNC]

When you put a pin the check valve hole, you reduce the size of the air path, This causes the air temperature to rise high enough to cause the pin to expand and seize in the hole. I use to make flat top valves with a .017 hole, the incoming air was so hot it would burn a hole in the check valve.

[Mike71]

I'm thinking of getting some Delrin rod and making one with that.
But mine seems to be working now..