On some Master cyl. the cover mounting screws are long and hold the res. on also If so just try to turn the res back and forth. It should have a o-ring on it.
I know on my 79 1000 I had to get to the chamber under the res. There were two holes that go through to the cylinder bore.
One was about 1/8" and cleaned out easy the other was about the size of a needle. It took a torch tip cleaner and some time to get it open no flushing would touch it.
Go to a site
www.partnmore.com and check the brake parts. They have rebuild kits, new master cylinders, rotors. brakd pistons and shoes and more.