I assume you're talking about the pitman arm shaft (pitman arm comes off the box, connecting it to the centerlink/steering linkage). There is a seal, appropriately called a pitman shaft seal. It should be held in by a snap ring. You might want to mark the position of the arm to the shaft, so you remember where it was. Take off the pitman arm (you'll need a pitman arm puller), then take out the snap ring. Place a drain pan under it now, and start up the car. Turn the steering wheel all the way in one direction (to build up pressure behind the seal), and it will blow the seal out into the drain pan (along with most of the power steering fluid - it can be a mess). The seal should be available at your local parts store, and there will be directions on putting it back in inside the box - on some cars it's actually a pair of seals with a washer or two. It's pretty simple.
The Chevrolet shop manual gives a few more tips on how to replace your pitman shaft seals with the gear still in the car.
1. Remove the pitman nut and remove the pitman arm with a puller.
2. Thoroughly clean the end of the pitman shaft and gear housing.
3. Remove the snap ring.
4. Place a container under the pitman shaft, start the engine, and turn the steering wheel all the way to full left lock so that the oil pressure in the gear will force the pitman shaft seals out. This method of removing the pitman shaft seals is recommended.
5. Inspect the seals for damage to the rubber covering on the O.D. If the O.D. appears scored, inspect the housing for burrs and remove them. Check the sealing surface of the pitman shaft for roughness or pitting. If it is pitted replacement of the pitman shaft is recommended.
6. Clean the end of the housing thoroughly so that dirt will not enter the housing with the installation of the new seals.
7. Tape the splines on the end of the pitman shaft to insure that the seals will not be cut by the spines during assembly. NOTE: Only one layer of tape should be used. An excessive amount of tape will not allow the seals to pass over it, due to the close tolerance between the seals and the pitman shaft.
8. Lubricate the seals thoroughly with power steering fluid before pressing them into the housing bore. Install the single lip seal first, then a back-up washer. Drive the seal in far enough to provide clearance for the outer seal, back-up washer and snap ring. Make sure that the inner seal does not bottom on the counterbore. Install the outer double lip seal and the second back-up washer in only far enough to provide clearance for the retaining ring. Install the retaining ring.
9. Fill the pump reservoir with new GM power steering fluid to the Cold mark on the stick. Start engine and allow engine to idle for at least three minutes without turning the steering wheel. Turn the wheel to the left and check for leaks. Add additional power steering fluid as required.
10. Remove the tape and reconnect the pitman arm and nut. Torque the nut to 180 ft-lbs.
The shop manual shows a tube that is used to press the seals into the housing. You want to press evenly on the seals. (You do not want to tear the rubber covering on the O.D. of the seals and you want the seals to be square to the housing bore.)
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