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The most common cause for the failure of Toyota 4runner rear main axle seal is a plugged differential breather. Internal pressure that builds up inside the engine at the time of running is vented out through this breather. If the breather fails to do the job, the pressure is released through the axle seal. Hence, this is the foremost part that should be inspected in case of any leakage from the rear end. Other causes for this problem include a damaged wheel bearing or a bent housing or a problem in the axle shaft. A damaged axle seal could be easily identified through gear oil leakage over the rear wheel. If unnoticed, the oil might even enter the brake drum causing a slipping rear brake. Hence, it is always better to replace a leaking seal as soon as it is noticed.
How to replace rear main seal? The rear axle should be securely placed using jack stands. The tire is removed carefully. Any leaking or dripping gear oil could be collected by placing a drain pan under the end of the tube. Now, the retaining pin from the emergency brake lever mechanism is removed. Axle shaft is attached to the housing through four 14mm hex-head bolts. These bolts should be unfastened so as to remove the axle shaft. After removing the bolts, the backing plate is grasped and the entire brake and axle assemble is carefully pulled out from the housing. The rear main seal is present at the housing end. This seal is removed using a seal puller and is replaced with a new one. However, after removing the seal, it is necessary to clean the seal surface so that nothing goes into the axle tube. A grease coating is applied on the outer surface of the new seal and is pushed carefully into the housing end. Another grease coating is applied on the inner surface once the seal is fixed.
Before putting the brake and axle assembly in place, all the traces of gear oil that occurred in the assembly due to leakage should be thoroughly cleaned using a brake cleaner. Care should be taken while cleaning parts associated with brake actuation mechanism including brake shoes and drum. It is always better to replace the gasket on the axle flange. Once everything is cleaned up, all the parts could be carefully fitted back to original position with the emergency brake cable attached and the brake line threaded into the backing plate. The repair is completed once the rear brakes are bled for checking any differential oil levels.
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