I bought a used 2000 Trailboss. I changed the oil for the first time. I used a Fram PH6607 filter and two quarts of Polaris Premium 4, 0W/40 Motor Oil. I drove the quad a block to let it warm up. When I turned off the engine and checked the oil at the dipstick there was a significant air pressure release from the dipstick hole, like you get when you open your automobile gas cap on a hot day. I though maybe this was the air from the oil filter that was displaced. I drove it around a little more and the same thing happend. Is this normal?

 

Sounds like the oil tank vent hose is plugged. On my 500, the hose is on the top pointing towards the engine. There is no way pressure could build in the tank because that hose is always open, right to the air box. I don't think the TB is any different but someone with that machine might be able to tell you.

 

Mystery solved. "Kinked" crankcase breather hose at air box.