Rear Brakes Lock Up - 1993 C2500 Suburban

I have a used 1993 3/4-ton 2wd Suburban (w/abs and hydraboost) that I recently bought. It has a problem, usually when it's humid and/or in the morning. If you step on the brakes (even very very lightly) then the brakes come on full-force as if you had stood on them hard. It's enough that the back brakes lock up (I don't know if the front lock up - the backs chirp as they lock up though).
The linings in back are new and the systems has been adjusted and bled with no effect.
Any thoughts on what to look for next?

Thanks!

You did not put the shoes on backwards did you??
There is a primary and secoundary shoe

You did not put the shoes on backwards did you??
There is a primary and secoundary shoe

Well, they were new brakes when I bought the truck a week ago. The brakes usually do work right and the mechanic says that everything looks ok. With that said, I will also re-check my mechanic when I bring the truck home this evening. I can't rule out anything yet.

Also make sure the return springs are good.
A quick check is to drop them 1 at a time on the hard surface and see what kind of sound they make. If it is a tinny ting sound they are bad because they should make a thud type sound

Just wanted to post a follow up on this...
The drums on this truck were out of spec. It sounds like the shoes would be over-extended, causing them to grab as the end of the shoes contacted the drums instead of the whole shoe.
Replaced the drums and did a full brake job (new shoes and hardware). The brakes are great now.
Thanks for all your help!