1994 Oldsmobile Achieva rear brake lockup

Brakes problem
1994 Oldsmobile Achieva 6 cyl Front Wheel Drive Automatic

I had the 2 rear brake lines replaced with new wheel brake cylinders and pads, followed the proper bleed procedures for abs.I now have a left rear brake that continues the lock up under med. To heavy breaking.
my machanic thinks that when my rear line failed it jamed the abs system. Is there a way to reset the abs proportioning valve. Thank you