Automatic transmissions use planetary gears to function. They have a sun gear that the planet gears revolve around. They also have an outer ring gear to contain the planet gears. By driving different gears and having others remain stationary, different gear ratios and outputs are achieved. The 700 R-4 does this by using a servo to apply a band to shift from first to second gear and second to third gear. By modifying the servo, firmer and faster shifts can be achieved. A popular modification is installing a Corvette servo to firm up the shifts. Sluggish shifts can also be revived--if the apply band is in good shape--by installing new servo seals.
Jack up the car and install jack stands under the frame. Lower the car and make sure it is stable. On the passenger side of the transmission, just above the front of the transmission pan, there is a large, round cover. This cover is held in place by a lock ring. Place a drip pan underneath this cover.
Depress the cover with a large screwdriver. Push directly inward with some force and have an assistant help by pushing and holding the cover in place while you remove the lock ring with a small, flat-head screwdriver. The ring will pop out with a little effort. Use lock pliers to grab the center protrusion on the cover and pull the cover off. This will take some effort.
Remove the servo piston. This piston will also need to be pulled out with lock pliers since it is tightly held in place by seals. The piston also has a long rod attached that fits into a guide hole. This rod is the part that actually pushes against the apply band. After the piston is removed, pull the return spring out that is the last part of the servo assembly.
Inspect the servo piston and spring for cracks or damage. The pistons and springs do develop cracks, however, it is much more likely that the servo seals are in poor condition. Either replace the lip seals or install a new servo piston and spring assembly. Install the new spring, lubricate the servo seals with fresh transmission fluid, and press it into place. Install a new O-ring seal on the cover and twist the cover while pressing inward to properly position it in place.
Press firmly against the cover with a large screwdriver. Have an assistant hold pressure against the screwdriver while you install the lock ring. Once the lock ring is located in the retaining groove, have the assistant release the servo.
Jack up the rear of the car and remove the jack stands. Lower the car and top off the transmission with the estimated amount of recovered transmission fluid that is in the drip pan. Start the car and check the transmission for leaks.
