I got trans problems with my newly acquired 86 420SEL. Reverse in all positions.

I have removed the valve body and cleaned it up a bit. Made sure everything is moving. Verified the shifter is actually working properly and engaging the valve body. I have no history on this car so I don't know any symptoms up to this.

In drive it is has both reverse and forward working against each other but in all other gears its reverse.
Trans is going to come out, because the park pawl is messed up but it would be really nice to know where to look to find this problem before I open it up. I have some parts left over from an earlier trans surgery.

There is a lug on the front of the reverse piston, It has broken off and now jams the piston in the apply position.
The reverse piston lives in the front cover.
Repeated attempts at running the engine will result in damage to the retainer and springs.
Mike Freeman

Not sure what front cover or reverse piston you talk of. Found the problem though.

The B3(reverse) clutch was worn out and one of the discs was friction welded to the steel. K1 was also burnt bad and worn out. Probably caused by being run in fwd and rev at the same time.
So I put some new friction discs in it and reassembled. Engaged in drive and reverse. Backed it out of the garage and let it run while I got the level just right and now I have no engagement at all.
Not sure whats up yet. Frustrated with it. Probably going to just do a complete rebuild now instead of just throwing a couple parts in.

Well I just checked the operating pressure and it got 100 at idle. Not sure whats normal but that should be enough atleast to make it work. At least my pump is working.

Got it all straightened out. Crap in the valve body. Cleaned it out real good and it works fine now.

Looks like I will be ordering a superior shift kit for it like I did with my SD.
Shifts much too soon.