February 21, 2013
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Vehicles with GM 4L80-E and 4L85-E transmissions commonly exhibit wear in the #7 Low/Reverse checkball seat location. This issue is caused by normal movement of the checkball and slight movement of the seat within the bore over time. As wear progresses, sealing function of the checkball seat becomes compromised and results in loss of rear band apply (RBA) pressure. This can result in a burnt rear band, delayed or no Reverse and loss of engine braking in manual low.
Installing Sonnax #7 Low/Reverse checkball seat 34200-52 restores the hydraulic integrity of this circuit to re-establish normal pressure and performance.
Vacuum testing at the port(s) indicated fails to hold the recommended minimum in-Hg, or if wear is visually detected.