1985 Chrysler 5th Avenue Car will not go in reverse!

No idea what "slippery" means. A common cause of no reverse is a broken low / reverse band, however, given the history, it sounds like the fluid is low. If you didn't drain the torque converter, it should have taken about four quarts to refill after removing the pan. Inspect the bottom and rear of the transmission for leaks. You won't have a puddle if it doesn't leak until the engine is running. If you can't find a reason for the low fluid level, check the coolant in the radiator. If the fluid looks like brown mud, the internal transmission cooler is leaking.

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Did it take about 4 quarts to fill the fluid? Double-check the level after the engine is warmed up, you run the gear shift lever through all the gears, and end up in park or neutral. Fluid will splash up onto the dipstick, so wipe it off before putting it back in to check the level. The fluid should be bright red. If it is pink and foamy, air is being sucked up around the filter. Air compresses and will prevent the clutch packs from fully applying. Slippage under load or upshifts will result.

To drain the torque converter requires removing the front inspection cover to get to the drain plug. Since the mid to late '70s most converters no longer have drain plugs. If you had drained it, it would have taken nine quarts to refill the transmission.

Besides low fluid, engine runaway during upshifts can be caused by a missing filter gasket or the filter is mispositioned or loose, or the kickdown (throttle pressure) linkage became disconnected. A big clue that the linkage is disconnected is it will always upshift at exactly the same speed regardless whether you're slowly speeding up or are under heavy acceleration. Watch that lever above the gear shift lever on the left side of the transmission. It should move rearward when a helper presses the gas pedal down.

Some fluid can leak from the pan gasket when the engine is off because fluid will run down from the clutches and fill the pan higher than the lip. Aerated fluid can make its way up to the vent and leak out, but that will only happen when the engine is running. The fluid will appear in the front of the transmission. Fluid can also leak from the rear of the tail shaft where the drive shaft comes out, but it's not under pressure there. Only a little fluid will leak out of that seal after the engine is turned off. It will just leave a small spot on the ground, but once the engine is started, fluid will work its way back there and start to leak again. You should see wetness there too if that's the cause of a leak.

Did this trouble start after you replaced the filter, or were you trying to solve an existing problem?

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Front pump seal should leak in park too, anytime the engine is running. Fluid will be dripping from the front inspection cover. You might look at the two cooler lines going to the radiator too. There's not much pressure in them; it's lowest in park. The rear seal will also leak when driving, but not as fast when you're standing still. Fluid isn't under pressure there; it has to splash up there as you drive. Even if it only leaks when driving, you should be able to see the evidence when you crawl underneath. The alternative is to strap yourself under the car while someone else drives it. I recommend someone you can trust to avoid pot holes!

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