• High line pressure in Reverse
  • Low line rise
  • Gear ratio codes
  • TCC slip
  • Flare shifts


The boost sleeve increases line pressure for shift quality, engagement and proper TCC apply. As the sleeve wears, torque signal oil will leak to exhaust and line pressure will not increase as commanded.


Sonnax boost valve kits restore proper clearance with upgraded design and material to prevent EPC and Reverse boost loss. Increased performance and OE ratio kits are available.

5L40-E, 5L50-E

Boost Valve Kit
Part No. 55209-04K

OE ratio with .474" & .441" dia. valve spools.

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

Vehicles with GM 5L40-E and 5L50-E transmissions are prone to poor line rise or high Reverse pressure problems causing poor shift quality and clutch failure due to leakage at the boost valve assembly. Sonnax offers two boost valve kits: the 55209-01K performance ratio (higher line rise and firmer shifts) and the OE ratio 55209-04K for factory performance.

  • Hardcoat anodized aluminum valves combat premature wear
  • Boost sleeves are made from alloy aluminum specifically designed to resist wear
  • Increased performance ratio 55209-01K or OE ratio 55209-04K assemblies are available
  • Drop-in Zip Valve parts install quickly and easily
Learn More About Zip Valve™ Drop-In Repairs
  • Valve
  • Sleeve
  • O-Ring
  • Dia.: 0.474"
  • Dia.: 0.441"

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 5L40-E, 5L50-E Valve Body Layout

GM 5L40-E, 5L50-E Valve Body Layout
  • 5L40-E, 5L50-E Boost Valve Kit 55209-04K
  • 5L40-E, 5L50-E Boost Valve Kit 55209-04K
  • 5L40-E, 5L50-E Boost Valve Kit 55209-04K 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.

5L40-E, 5L50-E Boost Valve Kit Vacuum Test Locations