A buddy called today with no brakes on his son's 110 4 wheeler. Stopped by to take a look. It has a single disk, dual piston disc activated by the brake handle on the bars. There is no resistance when pulling ther brake lever. Disconncted the brake line from the caliper and pumped the handle. Just spurts of very little brake fluid and air. Stuck my finger on the end of the brake line and no pressure. Could the piston on the master cyclinder be bad?

the seal around the piston, bad bore, what did the fluid look like I have seen were guys have gotten a pedal just by changing the fluid, I don't have that kinda of luck....it would be a bad bore......
may just have some dirty diaper stuck in the seal of the piston....try cleaning it real good.....

If you cannot build up pressure in the line and you have sufficient fluid, the master cylinder is at fault.

thats what it sounds like to me also.