Parking brake locked

this is second time it's happened both times after rain. Previous time I just reversed and it broke free however this morning after I had driven in the rain and let truck sit 2 days the drivers side rear wheel was dragging and passenger tires was spinning. I looked this problem up on TW and one post said to release parking brake and at the same time push on brake pedal and it immediately freed itself. My question is what is most likely happening. Tia

 

I don’t use my parking brake much anymore just for this reason. Usually if you press on the brake it’ll release it.

 

The semi-metallic brake shoes may be corroding to the drums. I have learned not to set the parking brake after I have hosed out the under-body.