The front servo is a pressure apply/pressure release type with unequal piston areas.
It is engaged together with the front clutch to provide second gear and acts on a lined spring steel band to lock the secondary sun gear to the casing.
The apply force from the piston is transferred through its central stem and is multiplied by the rocker arm, which has a leverage ratio of approximately 3:1.
This applies the band firmly when second gear is engaged.
The apply fluid acts on the smaller piston area surrounding the hollow portion which accommodates the return spring.
If road speed is reduced, the 1-2 shift valve will move to the first gear position and this apply fluid will be exhausted at the 1-2 shift valve allowing the spring to return the band to the off position.
If road speed increases, the transmission will eventually upshift to third gear and this requires engagement of the rear clutch together with the already engaged front clutch.
This means the front band must be released, however the apply fluid is not exhausted to provide this change.
The apply fluid is retained and when the 2-3 shift valve moves to supply fluid to the rear clutch, the same fluid is supplied to the release side of the front servo.
Although the line pressure acting on both sides of the front servo piston is the same value, the larger area on the release side generates a greater force and moves the piston to the released position.
Retaining the apply fluid has the advantage of providing a "snap" band application to reengage second gear if the accelerator is depressed for "kickdown" or when road speed drops off.
