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My 2004 Sierra, 4.8 engine and 4l60e, is slow shifting into reverse when cold. All forward gears work great, reverse works fine after I drive it long enough to warm up the transmission, about 10 miles if the outside temp is -20 c. There is no slippage in any gears. I can put the shift lever in reverse cold and nothing. I have heard about a piston that controls reverse that may be worn or cracked, can anyone elaborate? Where in the transmission would I find the "piston" or valve that controls reverse, and how big a job is it for a dyfer?
 

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True Hotrodder
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I would search for a manual on that transmission and usually there is a list of problems/solutions. My guess is that either a valve is sticking or a seal is leaking due to age. Once fluid warms up, the seal could swell enough to allow reverse to operate. You didn't say how many miles on the unit or how often the fluid has been changed out but if it's under 100k I would take a chance on a fluid change. There are also "snake oil" solutions available that temporarily swell the seals but it might be better to live with it until you can R&R the trans.
 
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