I just did my master cylinder on my GL500 last night - same problem - sticky plunger. The metal is in poor shape after 31 years of brake fluid so I used 'Brake Parts Cleaner Wash' in a spray can. Worked great with an air gun to blow the crap out. The instructions did say coat the parts with brake fluid after cleaning. Master cylinder works smooth and pumps again. Time to ride.The problem I had on a similar system, different make bike, was that the caliper piston was hanging up in the cylinder. The more I rode the hotter the calipers got the harder the brakes held. I cleaned both calipers that fixed the problem. Use soapy water or brake fluid for cleaning solvent. Don't use petroleum products for cleaning the piston and cylinder.