General Motors 375, 400, 425, & 4L80E Transmissions
Release oil for the rear servo in THM 400 and it's four speed successor, the 4L80E, also happens to be apply oil for the second clutch. Normally, the tight fit of the pin in it's bore seals the oil, allowing the transmission to use the servo's release as an accumulator. The problem is that as the servo applies and releases, the pin wears the case. This wear in the case creates a gap through which the intermediate clutch oil leaks, causing a lack of apply pressure to the clutch and release pressure on the servo. With this loss of pressure, the transmission's service life, even after a rebuild, is seriously shortened.
The 4L80 kit contains tooling necessary to repair the rear servo pin bore, and is available as a Master or Sub kit.