TransGo’s JF015E replacement upper primary piston made from CNC machined billet steel is the quick and easy fix to eliminate belt slip.
Don’t overlook this very common failure. This piston MUST BE PRESS-FIT or it will leak. Slipping pulleys, with or without codes, get worse when hot. This is likely due to upper primary pulley ID piston damage. The upper primary piston’s inner seal is press-fit onto the shoulder of the pulley sheave. A worn or loose fitting piston leaks primary pulley-pressure, causing the belt to slip.