Rear brakes hesitate, then THUNK when humid

Hi guys. After it rains, the first stop sign I'll pull up to in the morning will give me some braking problems. I'll brake for about a second, and all of a sudden there will be a large THUNK or POP. A brief shudder jolts through the chassis. Then, even though I haven't budged my foot, my braking force increases.

It feels like the first stop after a rain or in high humidity, the rear brakes hesitate for a second and then slam on all at once. I suppost it could be the fronts, but I sort of doubt it (rears alone would lock too easily on wet roads). After the first stop of the day they seem to be fine for the rest of the drive.

I've tried standing on the pedal as hard as I can before I even leave the driveway on rainy mornings, but that doesn't seem to help.

Any ideas?

Mine did that same thing, wet or humid they would stop like that for the first or second push of the pedal. Then they were fine all day. A quick fix (doesn't stop it forever) is the remove the drum and sand lightly the braking surface with a light grade sand paper. The long term fix is what I did, new drums, shoes, hardware and wheel cylinders. Juice