[Bill Hanlon]

The problem isn't with the bands (yet).

The front unit clutches are always engaged or it is physically locked up due to some kind of internal failure.

When the transmission is in 1st, 3rd or reverse the front unit clutches should be disengaged and the front unit band should be applied, causing engine power to flow through the front unit planetary and provide a reduction gear ratio. With your failure, the front unit is trying to provide both reduction because the band is applied as it should be and direct drive because the front unit is "locked up" as it should not be. Something has to "give".

In your case the amount of torque supplied by the engine through the "locked up" front unit is strong enough to overcome the band's grip on the front unit in 1st, 3rd or reverse gears. In 2nd and 4th gear the front unit is supposed to be "locked up" with the band released, so these gears work as they are supposed to.

I'll bet reverse on your car is operating at a much faster speed (because the front unit is locked up) than it should be. But if it is operating at normal speed that would mean that the front unit clutches and springs are operating as they are supposed (disengaged) and would point to problems in the control unit of the transmission. Pray for this as it wouldn't require a complete tear down of the transmission.

There are "air check" tests in the GMC HydraMatic Maintenance Manual that involve removing the side cover and the control unit under the cover. Then using compressed air through ports to activate various functions (such as front unit clutch engage) in the transmission and listening for the transmission to react. These could probably be done with the transmission mounted in Jack's "ladder frame" GMC pickup truck, but doing them on an "X frame" '55 Pontiac would most likely require removing the transmission to have enough room to get the side cover and control unit off.

Me? I'm saying 3:1 odds that the problem is inside the front unit. That would be verified by listening to the "thunk" when the front clutches are applied and released and comparing those sounds to apply/release sounds in the rear unit.

Back to the "yet" in the first line of this post. Because the front unit band is (and SHOULD BE) applied during 1st, 3rd and reverse gear and the front unit clutch is (and SHOULD NOT BE) applied the bands are slipping and subject to more than normal wear during 1st, 3rd and reverse.