1989 CHEVROLET CELEBRITY User Comment / Complaint

Following replacement of the rear drums, shoes and wheel cylinders, only moderate brake pedal pressure will cause the rear wheels to lock up and skid. in wet conditions the rear end can fishtail leading to potential disaster. problem was never noticed in previous 9 years of ownership. the front disc brakes appear to be operating and the pedal feel does not indicate air in the system. the garage has looked at the brakes on two occasions and made minor adjustments and replaced the shoes once (less than a year), but could find nothing else. please advise.