antilock brakes

On rainy days, when rear drums get wet, the rear brakes lockup. I get no "service engine" lite. Whats up?

Interesting,,, Usually when drum brakes get wet, they don't stop very good at all
Bob B

Yes it is strange. Its a GMC Sierra K2500. If I am very easy on pedal pressure applied, it wont lock up. But normal braking to a stop, they lock up.