Hi Bill,
Does it work when you put it down to 2 or 1? If not, then I suspect you have a leakage in the rear clutch area. Here is the test that you need to follow. More than likely you will find low pressure. The fact that Reverse works shows the pump is working so you have a pressure leak internal to the transmission somewhere. Unfortunately, the only way to find it past the pressure test is to take it apart.
Take a look at this and let me know if you have any other issues.
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Sunday, May 12th, 2019 AT 4:16 PM