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Im pretty sure I know the answer to this but thought I would check here before I pull the transpan for the 3rd time (the first time was because Im an idiot).
I just went through and completely rebuilt the trans. I had no extra parts on the bench (thats a good thing). I used the ATSG manual and a 2 1/2 hour long Youtube video to do it. I had to rebuild it because I torched the direct clutches, Im thinking because whoever had it last didnt install the belleville washer under the power pack in the drum. Anyways, the first time I tried to drive I got nothing. Turned out, not enough fluid. It wasnt actuating hydraulically so no internal damage done. 2nd time I try and Ive got forwards but no reverse, only a whining noise. Pull the transpan and remove the valve body and attempt to remove the reverse servo but the pin is stuck up in the trans body. I had to pull and wiggle really hard to get that pin out. I assume that was the problem with NO reverse at all the first time. To fix that I polishing up the pin and lightly filed a burr off the pin bore. Seems to slide in and out pretty nicely.
I reinstall everything and now I barely get any reverse. It'll move the car but it's slipping bad. Trans pressure on the Terminator X reads 34% at idle and down to 5% under hard throttle. Everything seems fine there.
So I believe this to be a too short reverse servo pin. Can anyone confirm that? Or have any other ideas there?
Did you check band clearance?
Did you do a pressure test yet?
Did you check fluid level again?
Does it have a strong 3rd gear? The direct clutches are used for reverse
For DIY guys that don't have a tool...or haven't made a tool. Here's an easy way to check.
Shove take the L/R servo piston apart so that there is no spring or accumulator piston etc...just the servo piston and the pin.
Put that in the bore. (remember the bore is not perpendicular to the case...it sits at a slight angle) and when the seal is JUST BARELY inside the bore, consider that your starting point. Then push down by hand until it can't move anymore...look at how much of a gap there is between the servo piston seal and the top of the machined bore. If it looks like more than 100 thou (need to have a trained eye here) you have too much clearance
Fml. I pushed the pin up there and it just bottomed out on nothing deep in the hole. Tried to take a pic but it's not clear. Anyway, no lug to be found. Guess I'll pull the trans again and not fubar the reassembly.
My first thought was a broken band and you have more problems than you think, like high pressure.
BUT it does sound like a first time rebuild, so theres no telling. The band pin was jammed in there thats not a problem people normally have. Was the band positioned before assembly? You surely didn't air check it because you would have found the problem before you put the rest of the internals in if you did.
From my own experience if your reverse servo was bottoming out, check for hairline cracks around the piston where it slammed the case.
I clearly remember putting the band in the lugs before placing the drum in there. But it's obvious it came out of position during assembly. Whatever happened, it was me that did it. Part of the learning process? Idk. I fooked it up. I'll admit it.
I did read that sometimes the lugs can back out of the case a bit on older 80es. I can't tell. Out it comes.
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4L80E Reverse Problem 
