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Procedure:
The following procedure may help to solve a "notchie" gear issue. As long as you haven't heard any gear clash (grinding) when shifting to the problematic gear. If you have already experienced gear clash when shifting into the affected gear then it is unlikely that any oil brand will remedy the problem of notchiness even after correcting a faulty clutch hydraulic system. Once the dogteeth start developing clash related burrs on them, replacement of the gear and ring usually is required along with replacement of the synchronizer.

Leak Down Test Of The Clutch Master And Slave Hydraulic Cylinders

How to check out the operational condition of the hydraulic clutch system:
  1. Thoroughly bleed the system with GM P/M 12345347 hydraulic clutch fluid.
  2. Simulate the vehicle being parked on an incline for 12-24 hours with the front end at least 1 foot higher than the rear.
  3. Lower the vehicle and start the engine.
  4. Do not cycle (pump) the clutch several times before the next step.
  5. Try to put it into reverse when it first gets started (cold).
    Note: Difficulty getting into reverse when in this condition indicates that the clutch master cylinder is not at 100%.
  6. Repeat steps 1-5 of this procedure except with rear end raised.
Note: This will be the leak down test for the slave cylinder.
It is possible that the slave cylinder can developed a build up of deposits in the barrel at the fully depressed piston seal position causing a hang up of the actuation seal under mild depression rate cycling of the cylinder. Rapid and complete depression of the clutch will cause the seal to stretch momentarily at the fully depressed end of the cycle causing the actuation stroke to cycle slightly deeper than when cycling the clutch at a slower rate. This condition may be corrected with a good bleeding of the system.
I have rebuilt many ZF S6-40s which had badly burred and worn reverse gear synchronizer dog teeth. Reverse is usually the first gear to be used after cold startup and tells the most accurate story of the clutch hydraulic system's condition.
If either cylinder is below standard, I highly recommend that the master and slave cylinder are replaced as a set.

The key to optimal shifting performance for your ZF:

When servicing the clutch master cylinder, be sure to flush clean the reservoir with clean new fluid.
Pay EXTREME ATTENTION to getting the reservoir LEVEL BETWEEN the HIGH-LOW marks (ABSOLUTELY NO LOWER OR NO HIGHER) WHEN the weather barrier BLADDER-SEAL is SEATED and the reservoir screw-on cap is in place.
Author: Bill Boudreau (ZFDoc)
 
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