Here's a general outline of the process. Specifics may vary slightly depending on your exact transmission and truck model. Always consult a repair manual specific to your vehicle for detailed instructions and torque specifications.
1. Preparation:
* Safety First: Disconnect the negative battery terminal. Support the vehicle securely on jack stands. Never work under a vehicle supported only by a jack.
* Gather Tools and Materials: You'll need a transmission jack or sturdy jack stands to support the transmission, various sockets and wrenches (metric and/or standard), a driveshaft support, a seal installer/remover tool (essential for a clean install), a new rear transmission seal (get the correct one for your transmission!), penetrating oil, rags, and possibly a new crush sleeve (depending on the design). A helper is highly recommended.
* Drain the Transmission Fluid: This is crucial to prevent a mess.
2. Accessing the Seal:
* Disconnect the Driveshaft: This usually involves unbolting the driveshaft from the transmission yoke and supporting it securely to prevent it from falling.
* Remove the Transmission Support: If your truck has a transmission support crossmember, you may need to remove or loosen it to gain sufficient access.
* Inspect the area: Thoroughly clean the area around the seal to assess the condition of the surrounding components, such as the output shaft.
3. Seal Removal:
* Remove the old seal: This is where the seal installer/remover tool comes into play. Carefully pry out the old seal, working from the outside edge and using the right tools to avoid damaging the output shaft.
4. Seal Installation:
* Inspect the Output Shaft: Check the output shaft for damage, scoring, or excessive wear. If damaged, it must be addressed before installing a new seal.
* Install the new seal: Use the seal installer tool to carefully press the new seal in straight and evenly. Avoid damaging the new seal's lip during installation. A properly installed seal will be flush with the transmission case. If the seal is installed incorrectly, you'll have leaks.
* Install New Crush Sleeve (if necessary): Some transmissions use a crush sleeve to create the correct seal pressure. If your truck uses a crush sleeve, it needs to be replaced with a new one during the reassembly.
5. Reassembly:
* Reinstall the driveshaft: Carefully align and bolt the driveshaft back to the transmission yoke, ensuring it is properly seated.
* Reinstall the transmission support (if removed): Tighten all bolts to the manufacturer's specifications.
* Refill the transmission: Add the correct type and amount of transmission fluid.
6. Testing:
* Check for leaks: Run the engine for a few minutes, checking carefully for any leaks around the new seal. If a leak is present, recheck all bolts and the seal installation. A small weep is sometimes normal for a few minutes but should stop.
* Road Test: Take the truck for a short drive to ensure there are no further problems.
Important Considerations:
* Transmission Jack: Using a transmission jack makes the job significantly easier and safer. Improper support of the transmission can lead to damage.
* Torque Specifications: Always follow the torque specifications in your repair manual to ensure proper tightening of all bolts. Over-tightening can cause damage.
* Repair Manual: A factory service manual specific to your 1978 Chevy truck is invaluable for this repair. It will provide detailed diagrams, torque specifications, and troubleshooting information.
This is a general guide. Improper installation can lead to transmission damage. If you are unsure about any step, consult a qualified mechanic. It's better to pay a professional than to risk damaging your truck's transmission.
