Motorcycle Repair: Hydraulic clutch spongy on 1983 Honda Sabre 750, honda sabre, hydraulic clutch

Question
I have a 1983 Honda VF45 Sabre (750cc) that has a hydraulic clutch.  I had the master cylinder rebuilt by a local mechanic about 2 months ago (July), and the clutch has been working fine, until a week ago.  The scenario is I have been commuting about 40 miles each way to work, mainly freeway.  It's 4 miles from my house to the freeway, and I had a firm clutch (pull in the lever and a good progressive feel) all the way to the freeway, then after traveling about 30 miles at steady speed (70 mph) I got off at my exit, and the clutch lever was "spongy" with maybe the last 1/2" activating the clutch.  I parked at work, and checked it a few hours later, and had a firm solid clutch again.  I left work early, and no problem getting back to the freeway, traveled the 30 miles at steady speed, and when I got to my exit, the clutch was spongy again.  I barely got it home.  I opened a new can of brake fluid (DOT 3 as per spec) and bled about 1/3 of the bottle through the system (bleeder is by the slave cylinder).  I now had a firm clutch again.  The next day, I did a test drive of about 10 miles on the freeway, and the same thing again. So the pattern is cold/cool clutch - works fine, once warm/hot it's spongy, barely works.  I looked for leaks, and I'm not seeing anything obvious.  Also the fluid in the master cylinder didn't seem particularly low when I bled it.  I checked my shop manual and it diagnoses the problem as air in the line. I bled the system pretty well I thought, and wouldn't air in the line result in the clutch being spongy all the time?  The firm/spongy cold/hot thing has me stumped.  Could the mechanic have caused a problem that took awhile to surface?  Any thoughts you may have would be greatly appreciated!!

Answer
I'd bet the slave cylinder on the engine is sticking due to crud build up behind the seals.  When the bike is cold, the piston moves freely but when hot, it sticks in the bore.  Pull it apart and give it a good cleaning and I think the problem will go away.

Regards
Rich