Most modern ATVs have hydraulic brakes that rarely need servicing, but once in a while I do have to work on them. I have found that they can be stubborn to refill with fluid and bleed if the system has been opened up and all fluid drained out. These photos show the brakes on a Yamaha Grizzly, but most machines work the same way.
After reassembling the system I connect a hand operated vacuum pump to the brake bleeder using a piece of clear tubing.
Fill up the reservoir with fluid, put a little vacuum on the bleeder, and slowly open the bleeder.
The vacuum will pull the brake fluid and any trapped air through the system. The clear tubing will show the air bubbles escaping. When the bubbles stop and there is only clean fluid coming out the brakes are properly bleed.
It is possible to bleed the brakes without the vacuum pump, using only the master cylinder to push the fluid through, but I have found that it can take a very long time. The hand vacuum pump is an inexpensive tool that speeds up the process.