Worm,
The problem is probably the poppet inside the valve body. They generally do not wear, sometimes a little fly spec of dirt will get in there, and hold it open.
Another thing to check, that a lot of people overlook.... if you were adjusting hammer stroke in, and you increased the hammer spring tension too much, it could hold the valve open at rest. An easy way to tell, is this:
Charge the gun to 3000 PSI. As you said, you can hear hissing. Cock the bolt completely back. If it still hisses, it's a valve problem. If it stops, you have too much preload on the hammer spring.
On my Field Target Tech Channel, I disassemble a Marauder, and another episode I put it back together. It's a .177, but the mechanics are all the same.
Hope this helps,
Tom Holland