Just wondering if the leading edges of the shoes have enough chamfer, maybe water getting into the drums is causing the shoes to grab and after that the water layer on the drum is swiped-off and no longer causes an issue from there onwards.
Sounds to like there is just a lot of crap in the drum. I would take the drum off, give the inside a good clean and refit, and readjust the brakes. The damp is probably causing the surface of the drum to rust slightly, at which point the rubbish has enough material to gunge up the pads and cause them to bind. Applying the brakes the first time will clean off the rust on the drum surface, so you don't get it again until it's been wet and standing for a while.
It is definitely worth having the drums off, and giving them a good wire brush round the inside anyway.
Check the return springs are ok as well, not stretched or broken.
I developed the habit when starting off in morning of riding the brakes while accelerating in 1st gear before changing to 2nd.
Flush the brake fluid at the callipers for the whole system. Keep adding new fluid into the master to be sure to flush it all and check to make sure you're using the same DOT spec (3 or 4). Water expands when it gets hot, possibly keeping positive brake pressure at non-braking. Brake fluid absorbs water and the only place to get water is at the seals in the calipers.
Bad flex hoses would be an all-the-time problem, not just at higher temperatures.
I may have cut and paste this from other websites. 
