Last summer I bought my son a used 110cc atv. It has been pretty solid for us. One issue it does have is that something in the hydraulic system of the brakes appears to not release correctly.

After using the atv for awhile the brakes get tighter and tighter and eventually will prevent the quad from moving at all. If I crack the fitting by the reservoir and squeeze the handle the pressure is relieved and the system will work normally again.

Any ideas if the pressure issues lies in the pressure cylinder or in the caliper? I hate to just throw parts at it.

 

I would bleed the brake system.
Sounds like there is air in the lines and as it heats up it expands and forces the brakes to activate.

Your pads may also be worn out and after some time the piston is pushed out too far and gets cocked a little preventing it from going back in.

Most of the brake systems for these things are sold as assemblies. IE the master, line and caliper. If you wind up needing to replace it you will end up with a whole new system. They usually run 35 to 55 bucks on ebay or less.