First, I would call Brown & see what they say.
My gun had a similar problem; it would just hiss air, but would work more often in a horizontal position than vertically.
Holding the trigger down, fill the thing thru the air fitting with air tool oil, or better yet, Marvel Mystery Oil; some even use a solvent like regular paint thinner. Let it sit for a couple minutes. Crank up the pressure to about 90 PSI, put it against a wooden block and hammer away. If it is dirty inside, you will get some black goop. You may want to do this a few times before going on to the next step.
If that doesn't work, you can disassemble the barrel assembly which is not technically hard to do. You have to expand the ring at the back of the barrel to take it off--it's the piece with the slot in it. Then, being careful about how things go together, it is easy to disassemble the barrel assembly. Clean everything. I soaked parts in paint thinner for a day or two, but that may not be necessary. Putting it back together is easy except for the snap ring.