Taipan Taipan Veteran help

A few years ago i set the Vet to shoot the NSA 29gr @ 936fps, very accurate and never had any problems at that speed, last time I went hunting I noticed the speed increased about 10+ fps I decided not to adjusted down, after taking several shots I heard a big pufff no more shooting for me that afternoon.
I know some guys shoot their Vets pass 950fps, mine doesn’t have a power plenum neither the power spring so I don’t know what could cause the valve stem to break 🤔
Few years back bought a .25 Long, grouping for JSB 33.95s wasn't great, tightened up groups with one turn in on HS. Shocked at "jolt" gun now had on firing, backed off HS 2X till "jolt-free," good groups. Seeing your story can't help but think what if I'd kept shooting with HS at one full turn in. WM
 
A few years ago i set the Vet to shoot the NSA 29gr @ 936fps, very accurate and never had any problems at that speed, last time I went hunting I noticed the speed increased about 10+ fps I decided not to adjusted down, after taking several shots I heard a big pufff no more shooting for me that afternoon.
I know some guys shoot their Vets pass 950fps, mine doesn’t have a power plenum neither the power spring so I don’t know what could cause the valve stem to break 🤔
Question answered. Thank you.
 
Yes, that ball bearing sits on top of the imbedded spring at the rear of the housing. It evidently helps with the cocking. If I have time later today, I will disassemble mine and post a pic.
In case you need a visual on the little ball bearing
IMG_9718.jpeg