Probably a bad hose. When they deteriorate inside the master can push fluid pressure through but the restriction acts like a check valve and won't let it release.

 

This occurred to me as a possibility too, one that I'm hoping is not the case.

Anyway, I never had the problem until we moved across the country with the bike in the back of a uhaul truck, hopefully I get it sorted soon. Thanks for all the tips, I'll try and lift the bike with my floor jack and set it on some wood planks when I get a spare weekend.