• Delayed Reverse
  • Flare shifts
  • Clutch distress
  • Low line rise


Boost valve and sleeve wear causes EPC to exhaust or cross leak.


The Sonnax drop-in main boost valve assembly has upgraded materials to prevent wear and restore proper hydraulic control.


Boost Valve Kit
Part No. 94987-09K

This kit fits '98-later models: Isuzu NPR, GMC Forward Tiltmaster, Chevrolet Forward, Nissan UD and Mitsubishi Fuso trucks.

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

The boost valve assembly is designed to increase line pressure in Drive and Reverse. When the Aisin AW 450-43LE sleeve is worn on the inside or loose within the valve body, boost or EPC fluids can leak to exhaust or cross-leak and increase forces on the valve. Leakage to exhaust reduces line rise, while the cross-leaks can result in broken clutch drums or cases. Sonnax main boost valve kit 94987-09K has upgraded materials to prevent wear and restore proper hydraulic control.

  • Sleeve is manufactured from highly wear-resistant billet aluminum
  • Hardcoat anodized aluminum valve combats premature wear
  • Restores proper hydraulic sealing and OE pressure control
  • Drop-in Zip Valve™ parts install quickly and easily
Learn More About Zip Valve™ Drop-In Repairs
  • Sleeve
  • Valve

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.

Aisin AW 450-43LE Valve Body Layout

Aisin AW 450-43LE Valve Body Layout
  • 450-43LE Boost Valve Kit 94987-09K
  • 450-43LE Boost Valve Kit 94987-09K

You need this if...

Vacuum tests at the ports indicated (Figure 1) fail to hold at least 16 in-Hg of vacuum. Using a small vacuum tip (found in Sonnax vacuum test stand kit [VACTEST-01K]) attached to the vacuum line, test the three feed holes in the OE main boost valve sleeve.

Note: The pressure regulator bore also is subject to extreme wear and should be vacuum tested at the port indicated (Figure 2). If the port fails to hold at least 14 in-Hg of vacuum, do not install this kit. Order and install Sonnax oversized PR valve and boost valve and sleeve kit [94987-11K] to repair this valve body.

450-43LE Boost Valve Kit Vacuum Test Locations