it seems that on my 97 3500 Dually, the rears just lock up like crazy when braking lightly. Is this normal or does it means that something wrong with my brakes?

 

definitely not normal that sounds like an accident waiting to happen. are they drum brakes? have you driven through water lately?

 

Does your truck have a load sensing valve on the rear axle?
My friends 98.5 2500 has it and it was set wrong and it used to lock up the rear brakes.

 

Honestly i Don't know. its a 16 valve dually. SLT Laramie though. how can i check?

 

On my friends truck it is on the left frame rail over the rear axle. ( If it has one) you will have to look at the rear axle to see if there is a linkage going from it up to the valve. To be certain just check does the flex line join straight onto the steel brake line on the frame rail. My brothers 2000 dually does't have the load sensing valve. If there is no valve, maybe it is your master cylinder?

 

proportioning valve maybe.....or stuck pistons on the front brakes