a5 leaking after polish job

Follow this video.
http://www.youtube.com/watch?v=-5e2PnY8dC4

You may still need to take the power tube out and remove the valve out and dissassemble the valve body, and check and see if the rubber on the inner valve is not scratched.

Taking apart a valve is not something I would necessarily recommend for a new player. It is rare that the o-rings or seals inside the valve go bad. I have had several A-5's that  were 5+ years old, and none of them leaked. More than likely leaks are attributable to bad tombstone o-rings, bolt resting on the valve pin (you should cock the marker prior to airing up), dirt or other debri in the marker, lack of oil, or low air causing the bolt not to reset and again pushing against the valve pin. There is no design flaw in the A5 that causes leaks.

Disassembling the marker and learning the parts is always a good idea and I recommend it to every marker owner. You can't fix it, if you don't know how it works. The exception to this is taking apart the valve. Unless it's leaking or causing problems, there is really no need to ever take it apart. The seals rarely go bad in my experience and there is more chance of mucking it up than doing any good. When it comes to the valve, if it isn't broke, don't fix it. As for polishing the internals, other than making it look pretty, I have seen no real evidence that this really improves performance either. Again, just another opportunity to screw something up if you don't know what you're doing.