4L80E 4L85E Transmission Sonnax Boost Valve Kit Symptoms: Broken parts Buzzing noise High line pressure High line pressure Uncontrollable line rise Cause: Worn boost valve sleeve allows reverse oil leakage and worn end plug causes imbalance of pressure regulator valve Correction: Ensure proper main regulator valve control with this boost valve and sleeve kit with O-ringed balance oil plug. Replaces 1991 - current OE boost valves. Factory style sleeve, no O-rings Common problems in vehicles with a 4L80-E transmission are uncontrollable line rise (upward of 500-600 psi), high line pressure in reverse, buzzing noise, broken direct clutch drums and/or broken cases. These problems are usually caused by reverse boost oil that enters the torque signal circuit due to a wear-created cross leak at the sleeve. High line pressure and buzz is created by oil leakage past the aluminum balance plug. The Sonnax replacement assembly 34200-03K eliminates these problems. Hardened steel boost valve with over 20% wider spools to provide better durability and increased sealing contact area. Boost valve spools have annular grooves to reduce side loading that causes wear. Valve and sleeve are manufactured to close tolerances to restore proper clearance and hydraulic integrity. Kit includes a replacement aluminum balance oil plug with an O-ring to ensure proper sealing and adequate back pressure on the pressure regulator valve. Kit contains instructions to retrofit parts dating back to 1989. NOTE: If you detect leakage between the sleeve and pump bore, use 34200-01K, which includes an O-ringed sleeve to compensate for pump bore wear and prevent pump scrappage. Kit includes instructions to retrofit parts dating back to 89.