Low is a function of the manual valve and the manual valve only. If the manual valve is in low, and the band is capable of applying it has to. The low shifter position is there for 1 reason- low braking. There are no spool valves, springs, checkballs (well one checkball but reverse wouldn't work without it.)
If there was a massive leak at the servo cover, reverse could work and low not because of the apply area difference.
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Starnes Performance Automatics
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