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You might want to drop the pan and check to ensure the filter is in tact, look for debris in the pan and check the valve body bolts. Otherwise, sounds like it gave up the ghost from what you describe. Time to upgrade.
Was your original transmission rebuilt or did you replace it with a different rebuilt unit. If the original was rebuilt it probably has an inherent pressure loss. It could be a crack in a clutch piston that could have missed when rebuilt. These transmissions had a very narrow bushing in the front drum which typically would not support the drum well and cause it to wear the surface of the stator support causing a pressure loss and burnt clutches. Later units had a wider bushing and an added support ring to keep this from happing. If the original was rebuilt I would have them take it apart and find out if the forward clutch is burnt. If it is I would have them look for a pressure loss. If they can’t find a reason I would get a different core and have it rebuilt.
Was there anything in the pan? Do you have a tachometer? Have you performed a stall speed test?