Many vehicles with either a ZF4HP14 or ZF4HP18 transmission experience poor line pressure control due to a worn pressure regulator bore. This bore wear can be verified by testing line pressure and/or performing a Wet Air Test on the valve body. The past fix was to replace the valve body, which is expensive and hard to find. These problems are often caused by wear from the oscillation of the aluminum valve in the valve body bore. Line/balance, modulated TV, and converter/cooler pressures then leak past the valve and exhaust to the sump or cross leak into other circuits. Excessive leakage can be eliminated by installing the oversized Sonnax replacement pressure regulator valve 85991-01.