Hi Toxophilite and welcome to the forum.
Take a look at this thread over on BenzWorld for the vacuum information
DIY W123 Transmission Diagnose and Adjustment 722.xx OM 616-7 - Benzworld.org - Mercedes-Benz Discussion Forum
I'm no expert in the variations and workings of that yet. You should be able to find the good "vacuum values" for your car there.
It seems like you've done most of the things that can be done without seriously taking the transmission to bits...
Here's a picture showing the relationships between the elements of the transmission for a 722.1 transmission - I think you might have a 722.3 though - can you confirm?
Most people scream B2 piston for virtually any problem with these transmissions but according to the information above it stays on for the first three gears - so I don't think it is that. When shifting up the release of the B1 piston and the application of the K1 clutch could be an area worth investigating.