HI and welcome to JA. THe first thing that needs to be done is the forward clutch needs to be pressure tested. and if the pressure is slow to come up it can be the modulation valve or a failed piston on the clutch pack. if the pressure comes up quick then you have a failed clutch pack. The pressure should be 160-180 psi at idle.
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THe forward pressure port should be located on top of the reverser. test your pressure and also see if your solenoid on the reverser is getting a full 12 volts with the lever in the forward position.