My rear brakes grab hard in the wet or cold weather. I bought the car in october of last year and i first notice it then. so i pulled the drum and sure enough i had a leaking wheel cylinder so i replaced it. Now I notice it again.

 

If you has a wheel cylinder fail, it is best to replace them in pairs, because the other is usually not far behind.

If the shoes are contaminated with brake fluid replace them, and the hardware as well.

Make sure the system is properly bled.