• Uncontrollable line rise
  • High line pressure
  • Broken parts
  • Broken clutch piston


Worn boost valve sleeve allows Reverse oil leakage and worn end plug causes imbalance of pressure regulator valve


Ensure proper main regulator valve control with Sonnax drop-in boost valve kit with O-ringed balance oil plug.

4L80-E, 4L85-E

Boost Valve Kit
Part No. 34200-03K

Fits '91-later units with factory-style sleeve, no O-rings. Can be retrofitted to '89–91 units. See instructions for details.

You need this if... View Instructions Find a Distributor

Common problems in vehicles with GM 4L80-E and 4L85-E transmissions 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. Sonnax boost vavle kit 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
  • Drop-in Zip Valve parts install quickly and easily
Learn More About Zip Valve™ Drop-In Repairs

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.

  • Valve
  • Sleeve
  • Balance Plug With O-Ring

Sonnax valve body layouts provide a detailed overview of individual units making it quick and easy to determine what’s available for the specific valve body you’re working on. See all Sonnax valve body layouts.

GM 4L80-E, 4L85-E, TH400 Valve Body Layout

GM 4L80-E, 4L85-E, TH400 Valve Body Layout
  • 4L80-E, 4L85-E Boost Valve Kit 34200-03K
  • 4L80-E, 4L85-E Boost Valve Kit 34200-03K

You need this if...

Vacuum testing at the port(s) indicated fails to hold the recommended minimum in-Hg, or if wear is visually detected.

4L80-E, 4L85-E Boost Valve Kit Vacuum Test Locations