There is no low/rev drum lip seal... There is no low/rev clutch.... This isn't a Chevy.... It has a band for low/rev..
The fact it suddenly stopped after a cold start and dropped into rev points to a band failure... The end where the band attaches to the strut has been knowm to snap off....
The component terminology is probably causing confusion here. I have built 30 or so 727's and each one got a new low- rev lip seal. That is the part that seals the L-R piston in the L-R drum which applies the rear clutch. In other words, the rear clutch lip seal.
And when that seal fails, you'll have forward gears but no, or minimal, reverse.
As an example of the terminology,
Mopar invented the high drum, designed it, produced it, and named it. TECHNICALLY the
ONLY correct name for this drum is
"Front Clutch Retainer, "which is what Mopar named it.
Some of the CORRECT names for this drum are:
High Drum, High/Reverse Drum, Front Drum(in front part of the transmission),
Forward Drum(in the forward part of the transmission, not to be confused with the next drum which controls the forward MOTION of the vehicle),
Kickdown Drum, Direct Drum, Reverse Drum(not to be confused with the Reverse Brake drum in the rear of the transmission), all of the above with
"Clutch "inserted before
"Drum, "and all of the above with
"Drum "replaced by
"Retainer "or "Cylinder. "These are just SOME of the American English names for this drum.
Don't you just love the terminology in the transmission business?