727 sometimes not going in reverse

HeeHee,Here we go again..... What are we working on? Oh wait, 727; I see it now.
Well then the first thing to do is prove the tranny is in reverse when the cable says it is. This is called synchronizing the shifter.
Then you need to understand how the tranny works.
To make Reverse; the L/R band is applied plus the Hi-drum.
To make any forward gear only, the Forward clutch is engaged.
To make Manual Low; the L/R band is again applied, and with the F-clutch.
To make Drive-Low; the L/Rband is not used. Instead the roller clutch is used.And again the F-clutch.
To make Drive-High, both clutches are applied.
What does all this mean? It means we have a built-in diagnostic system.
If you study this. you will see that Since you do NOT complain about any other gears, that tends to mean that the both the clutches and all the bands are working. But hold on there. In Manual Low, L/R band could be Not working, and the low could still be working cuz the roller clutch would be doing the work instead. The result is that you cannot know the band is applying; until you find no reverse. Then you know.
All this leads up to me saying;something about the L/R apply circuit is not working.
It could be the Band adjustment. It could be the servo. Could be a broken band.Or the case, or the VB, or a gasket.Since the rear servo has an external port,the easiest thing to do is is an oil-pressure test.
-If the servo pressure is good, next would be the band adjust.If the band is adjusted right, next would be to drop the VB and do an air test.While the VB is off you can check the gasket.If the air test is ok, and the oil-press is good, then it's time to inspect the band.
-But if the servo pressure is bad, then you will need to check the line pressure as well. If that guy is good, while the servo pressure was bad, then the VB would be suspect. Since you have already swapped VBs, I would then suspect the case, or the servo piston, or again, the gasket.
So don't drop the pan until the oil-pressure results are in.
You will need a service manual to check the pressures
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