Look, here is the deal, RAPTORAZ and RaptorOwner are right on the money. It has nothing to do with brakes or anything else (it's a solid axle, if one side turns the other side turns the same RPM.) If your axle is cocked in the frame it will drive the quad in one direction. (drive steer) Like someone said, it goes without saying, you have to be on flat ground, same traction and the shock setting should be the same.
If you have more roll out on one tire it will cause one tire to roll more than the other and push your quad in the other direction. This can't be checked by air pressure. It has to be checked with a stagger gauge or a tape measure. 1/2" can make a big difference in roll out. Just jack the quad up and wrap a cloth tape around the tire.
Since this all started when you changed axles, I would give that a good look-over and see if it is cocked in there.
If all else fails, keep putting air in the right rear. I guarantee it will straighten out with more tire on that side.

 

it depends where your doing these wheelies too. you know some paved roads arent flat. like youll be on your side of the road and it will pull to the right so youll go to the other side of the road and it will still pull to the right now try going down the opposite side same direction. your quad is problably fine.

 

when i used to have ablaster, and i got new tires one time, the ppl put them on and i went for a ride, and it pulled real hard right. i let out all the pressure in theleft tire and it still drifted strong right. i switched tires(right to left, left for right) and then it pulled hard left. so i guessed they bent my rim some how