Sticky brake -- only when it gets hot outside??

Hey guys, long time lurker, but don't post much. Usually can find an archive on any question, so no need. Either that or throwing obscene amounts of money at the old girl (or both) gets the problem fixed.

I have an 84 GPZ 750 for reference.

Here's the problem. When it is cool in the morning, my front brakes seem to be fine, operation normal. But let 75-80 degree sun bake on it for a while (like 9 or 10 hours at work) or just sitting in the driveway, the front brakes seem to lock up -- like I am pushing the front tire to get it off of the centerstand. That is, until I squeeze the brake lever hard, then it releases and I am somewhat good to go. Have noticed alot more brake dust lately, so it is probably wearing the pads, but even on 2+ hour mountain rides, I don't get any fade.

Also when hot it seems that I don't have as much lever travel, and that makes sense.

Question: would it make sense for caliper rebuild or master cylinder rebuild, and why? Anyone had a problem similar? It just doesn't make sense to me that a hydraulic system would be affected by ambient temperature as much as it is.

Changed and bled the brake system last year, and worked fine for the summer/fall/winter previous.

Thanks in advance.

Louie

just a guess here...maybe some air in the caliper. when it gets warm and expands its pushing the fluid and activating the brake

Last night looked at it closer: looks like the master cylinder at least needs rebuilt. Actually I think I will go with one of those new ones off of e-bay.