Crosman 13xx/2289 Forearm Pops Open

[Moke]

My Crosman 2289 Backpacker has developed an ism where the forearm pops open under pressure. I found this out when I laid the rifle across my knees, and suddenly got nicked in the junk by its forearm popping open. I now have a velcro band loop that I slide up the forearm after pumping to keep it in check.

Causes?
Fixes?

Don't open the image if a dead rodent offends you. please and thanks.

 

[EquivalentDelta]

The pump is held closed by the face of the piston meeting the valve in the pump tube.

If your o-rings or piston head are worn, then you’ll see this issue.

 

[Simplemann]

Sounds like it might be a good time to replace the piston with an upgraded adjustable one??