GMC Repair: rear brake lockup with damp or bumps, rear drum brakes, brake lockup

Question
I have a 93 GM 3/4 ton 4x4 6.5 D pickup.  It has 5 speed tran.  It has rear drum brakes that are ABS in rear only.  I've had it since new.  In wet weather or hitting bumps when raining the rears lock up and then the brakes let go and the pressure on the front seems drop as well.  You have to let off and the re apply to get them back.  This has got me in 2 accidents.  I've never touched the rears till now and I'd like to know if there is a fix for this.

Answer
Hi
I really hate to gusss at this. I have never run across this problem before.
I know that I would check and repair ALL the base brake system first. Make sure there is no leaking rear wheel cylinders. Fluid soaked shoes will cause a grab.
Check all that out first.
Check the Master cylinder. It could be internally bleeding out.
I would suspect the RWAL system lastly.
I would then, for your safety take it to a repair facility in your area that YOUR familiar with.
Have them really check it out good.
I Hope this helps.
Just advice from experience.
Good Luck
Pawl