It's not the ABS.We have had the right rear brakes cleaned. But man these things skid and lock up if the rear drums are wet. Could the shoes be bad in the drums? Or is it the ABS system? And btw I am new here.
all j bodies will do this but always moreso if you dont use gm brake shoes,aftermarket shoes are notorious for causing your problem but like I said j bodies will grab on the first few brake applies if vehicle has sat for awhile.I've got a similar problem.
To me it seems like the drum very quickly gets a thin film of dirt/rust on it that bites when the brakes are applied.
It usually happens to me when the car sits for 2 or 3 days without being used. Once I make it out of my driveway - get up to about 30 mph and gently use the brakes - it seems to shake off the rust and work good for the rest of the day.