I have just disassembled my Avenge-X 25 cal to the point of almost every part removed and separated except the trigger assembly - it wasn't my idea of fun but rather necessity - it was leaking from several locations and I couldn't charge it.
The firing valve is a bit of a task to remove and replace.
Degass the gun entirely if its a tube type (or remove the bottle) and dry fire the gun to make sure there is no pressure left.
Remove the stock and either the tube or the bottle - you may want to remove the barrel too - its held in by two screws in the picatinny rail and slides out the front (making the rest of the process easier) and in the process, I found that one of the two screws was loose (solved the shifty aiming problem in the process)
The valve is going to come out the front of the gun -
Remove the Connecting Seat by loosening the set screw on the side that holds it in and pull it forward.
Behind the connecting seat is the air reservoir plug #2 and the valve return spring - this unscrews from the front but I had to modify a screwdriver to fit perfectly in the oval slot in the plug and unscrewing it took a lot of torque. I used a t-handled driver. It's a large brass plug and the slot is in the bottom of the front recess. An undersized driver could chew up the slot and make it even harder to remove (or impossible) and they won't sell you the main block - so don't mess up the plug trying to unscrew it. You would have to make a tool that fits the recess and used a pin in the hole in the flange if the slot doesn't work.
After removing the plug and spring, you will see the turquoise colored valve at the bottom. but it has to be pushed out from the back
The valve spring bracket, spring and hammer (tubular piece that impacts the valve) all have to come out the back of the gun. You do not have to remove the adjuster from the bracket.
I removed the trigger assembly first - two screws on opposite corners.
The bracket is held in by two screws on either side of the spring adjuster and then the adjuster will come out, the spring and hammer will also fall out.
This exposes the back of the valve and the valve stem so you can push it out the front.
There is a set screw on the side of the gun that holds the valve in place - it needs to be loosened but you may as well remove it as you will have to look in that hole to be sure the valve is in the correct position before locking it in place.
A very firm push with a dowel rod will push the valve assembly out the front of the gun - including three o-rings, the valve head and stem and the valve seat.
The replacement valve has to go in at the correct angle - the port has to be facing up and equally important, the notch that the screw sets into to hold the valve must be facing exactly sideways for the screw to hold it correctly. I blackened the bottom of the grovoe with a pencil to make it easier to see.
The assembly pushes back into the gun from the front and after pressing it in, you need to look through the screw hole to make sure you see the bottom of the notch and that its square to the screw - it may take a few tries to get it correct but its important the port be straight up - which it will be if the screw is centered on the bottom of the notch.
The rest of the parts go back in reverse order - its not hard but its not a fast project or one I would want to do in the field.
Its probably worth it to replace the o-rings you encounter as you go but several are very hard to get back in place - I found dried out and deformed o-rings and several that were brittle and broke into pieces while trying to remove them (and were usually the ones leaking as well).
I did have to re-tune my settings after replacing the valve -
Hope that helps - my valve seat did have damage and was one of my leaks - with o-ings replaced and the valve repaired, the gun holds 4300 PSI for days
