I have a 73 valiant,auto trans 904 and I was buried in the snow and had to rock the car back and forth,drive,reverse,etc. and my reverse stopped working! How bad is this?Any help would be appreciated! thanks!

If the forward gears, still work OK, you probably broke the first/reverse band, or strut.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Sounds like it is time to drop the trans pan and have a look inside.
It could be that a simple band adjustment will fix the problem or as Charrlie_S suggested, the rear band is broken and it will need to be replaced.
One other possibility is that the rear band servo piston seals are worn / torn or leaking. These can be replaced without removing the trans.
DD

DD. I'm thinking, since he was rocking the car, between forward and reverse, the band broke. The other possibility is , if the rear band was loose, the strut might have popped out. If it is the strut, it might be possible to get it back in, with a lot of patience. Then readjust it correctly. But I agree, time to drop the pan and have a look.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

I vote for a broken reverse band. Back when I was young and stupid I wondered what would happen if I put my Valiant in reverse when I was going down the road forware at a slow speed. Well.... I just killed the engine that day. A few weeks later as I was backing up I hear a BANG !! and I had no more reverse. Yes it was a broken reverse band.

Look for a dent in the bottom of the transmission pan.... from the inside out........ when the band broke, some lever inside was free to dent the pan from the inside.

If you try driving downhill in 1st. Does the engine try to hold the car?(engine brake) or does it just roll like in neutral?
Anyway I think you have to open it up and have a look.

definitely pull the pan.

Check for three things:

Low/reverse band looseness or breakage

Band apply strut damage or mis-location

Sticking low/reverse servo assembly (you may need a C-clamp to check this, space permitting)