Look at the back of the trans , see the two Allen head plugs going into the case ?
Now the big one is the main pressure and the small is reverse .. Should get over 40 lbs at that port .. Witch u saying forward is Working then ur front pump is pumping good , but the valve body could be caput ..
Most the time it will be just the clutch plates that are worn down and the springs that hold the pressure plate to activate the
Friction plates don't have but a 1/8 " of compression to hold that kind of pressure required to make the plates stick ...
I ran my boat I'n reverse into a sand bank and it broke my plates and shredded my springs .. Best case I would check the fluid for bronze shavings .. The tel tel sign of the plate going caput .. But if u check Ur pressure first at the port I described this will tell You either bad plates or bad pressure .... Good luck
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