Mine had this issue after my first time playing with it. Luckily a friend of mine hade a 3.5 and let me swap parts until I found the problem.
- in my case, after swapping the solenoid manifolds out I was able to stop the leak. I got on the phone with the shop I bought the gun from and they sent me a new manifold. Ever since then it's held air just fine.
Try swapping parts with your buddies 3.5 until you find the part that's causing a leak.