The check valve holds about 2psi in the system to keep the pistons from returning when the lever is released but it's not enough pressure to cause brake drag. Over on chinariders, there was a member that had a similar problem. He removed the master cylinder and did an old fashion bench bleed. During the bleed, some hardened chunky brake fluid debris came out of the hole that allows fluid to travel down through to the piston. You should be able to disassemble and clean the master cylinder without dmaging any seals. I have done it on my Jap bikes with no problems.