Got a Big Bear 400 2WD in the other day, front brakes locked up. Bought and installed the following:
Replaced all brake lines from master cylinder to calipers
Rebuilt the master cylinder
Replaced caliper O rings
I can ride the 4 wheeler down the road, barely hitting front brakes. Brake pressure builds up, locking front brakes. Jack up the front end and cannot turn front tires. Remove tires and cannot push back calipers. Break the top fitting by master cylinder and pressure releases.

Used a "1 man" vacuum pistol to bleed brakes, keeping the calipers kicked back (wedged flat-headed screwdrivers to keep the calipers back).

Why won’t the master cylinder take a rebuild? Do I need to buy a new master cylinder?