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


Unregulated EPC pressure forces pressure regulator into high line position.


This self-regulating Reverse boost valve regulates EPC pressure to 95–105 psi with the Sonnax drop-in encapsulated relief valve.

4L80-E, 4L85-E

Boost Valve Kit
Part No. 34200-10K

With encapsulated relief valve. O-Ring style

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

Common problems in vehicles with GM 4L80-E and 4L85-E transmissions include uncontrollable line rise (upward of 500 psi), high line pressure in Reverse, broken direct clutch drums and/or broken cases. These problems can be caused by excessive EPC torque signal pressure. Sonnax self-regulating boost valve kit 34200-10K eliminates these problems by limiting EPC pressure.

  • Hardened steel boost valve has an encapsulated relief valve that will allow EPC pressure to exhaust if it exceeds 95-105 psi
  • 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
  • Closely toleranced sleeve restores the hydraulic integrity between the valve and sleeve
  • O-Rings have been added to the sleeve to provide a positive seal that compensates for pump bore wear
  • Drop-in Zip Valve parts install quickly and easily
Learn More About Zip Valve™ Drop-In Repairs
  • Valve
  • Sleeve
  • O-Rings (2)

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 34200-10K
  • 4L80-E, 4L85-E Boost Valve Kit 34200-10K
  • 4L80-E, 4L85-E Boost Valve Kit 34200-10K details

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