Front Master cylinder locked up

Went for a ride yesterday and left my '73 850 parked in the hot Alabama sun outside for an hour + and when i came out, the front brake was locked up. Couldn't move the bike at all. Went and got a handfull of ice and applied it to the m/c and it started to release.
A similar thing happened last year at the national in NC, I pushed my bike out from under my easy up and let it sit for just a couple minutes while it was warming up, and it was locked up. I rocked it back and forth and it released.
I'm sure it's the m/c because the lever is locked and won't push in, then with a good strong pull, it will go in, but the piston won't return and there's a ton of free play in the lever.
I've got one of the miles resleeved m/c on it, been on there at least 10 years, no problems till last year.
I'm guessing the sleeve expands in the sun and locks the piston.
Anyone else experienced this and have a solution, other than a replacement m/c?
I'd like to retain my stock look and switch cluster. I've got rebuilt caliper, DBR drilled disc, and SS line that has served me well for many years.

It's not something as simple as the breather hole in the m/cyl cap blocked is it?

For the time it takes, strip and investigate, it sounds like either the piston is seizing or the cupped washer not collapsing the front seal when the lever is at rest or with the lever being released.

When the brake lever is released, the master cylinder piston should retract sufficiently so the piston seal (B) uncovers the 'vent' port (C) so any residual pressure within the system can bleed off into the reservoir.

'Car' master cylinder unit but the principal is the same:



The Lockheed Service Manual instructions stress how important it is to adjust the master cylinder correctly so the vent remains open when the brake is fully off however, this applies to all master cylinders although sleeved cylinders can't be adjusted in this way some have an adjustable 'pusher' between the lever and the piston.

Slackening the caliper bleed nipple momentarily (with a tissue or cloth covering it to prevent brake fluid squirting in your face) will often miraculously cure a locked brake in the short term. If either air or fluid will not easily pass backwards through the master cylinder when released then it is a sign the vent is not opening or is blocked.

http://www.classicbike.biz/Triumph/Main ... Norton.pdf

I had the exact same trouble with my resleeved stock master cylinder, posted here and learned that it is a fairly common problem. FWIW I eventally replaced it with a $150 asian MC from "Madass", works great and matches up perfectly to the stock switchgear, mirror, and hard brake line.
(I added the adjustable lever shown but wasn't happy with it and went back to the conventional lever supplied with the kit)

Same thing happened to mine. I sent it back to Al Miles and he rebuilt it - though it cost me, since it was long out of warrantee.