I agree with Moparman. That servo is used for both manual low and reverse. But in manual low, the sprag clutch is operating as well, so you might not notice a slow servo.
However the low pressure in reverse is a tell-tale of other issues. When you put a Torqueflite into reverse, it kills all other circuits and maxes out the line pressure to whatever the pump can put out, usually better than 220 psi@1000 rpm.(the spec is 230 to 260 @1600rpm) You didn't quote a number as to reverse pressure, but these pumps hardly wear, so even a high-mileage unit will often have a good pump. So my guess is that there is a problem in delivering that pressure to the reverse servo when the oil is cold.This also is backwards from the usual way of things.
The way reverse works, it also engages the hi-clutch. But this clutch is also used for 3rd grear. To separate these two circuits they put a little check ball in the apply line. If that ball was left out, pressure would creat havoc in other circuits which you didn't mention.
But more likely is missing the L/R checkball. When this happens,the oil goes into the Manual low circuit which If the selector is in reverse, dumps at the manual valve straight into the oilpan. Since the pump is operating at maximum, it would still apply the L/R band, just slowly. In Manual low, the oil would again continue past the L/R servo and back to the manual valve to be dumped. But now the sprag is the back-up, and you might not notice a slow-applying forward clutch.
So long story short, I'm guessing ball #4 got left out, or you put the wrong one in that location. There are 6 balls; 5 smalls, 5/16 IIRC and one large 3/8, for the hi-pressure relief, which is typically set to 275 psi.
So back to the pressure test, looking for say 260 to 275psi in reverse at some rpm higher than 1600. If you haven't got it by 2400(my estimate) , then something is wrong. If the pressure is slow to build, then something is wrong. If the band is set even remotely close,the pressure should hammer up to max pressure almost instantly. If it fails at both of these, I would be taking the VB off for a look-see. While that VB is off, I would repeat the air pressure test, and reset the band. Then go find that ball #4.
If I am right,and you leave it like this, chances are good that both the band and the drum will eventually be destroyed, Also since manual low is also weak, ever time you use it, those same parts will be wearing.And this puts a lot of extra strain on the sprag as you will be thinking the band has got a grip on things, when in fact the sprag will be doing all the work. Also since the forward clutch will be slightly slow to engage, those clutches are also subject to extra wear.
If I am right.
If I am wrong, the air-pressure check will ease your mind that the tranny can stay up there, and the band adjust will make Moparman happy,lol.
But I still think, baring a failure in the air-pressure test, that the problem is in the VB.
Best of luck to ya!
