Just catching up with this thread, interesting read.
I did confirm the clutch-brake was not stuck, it was releasing correctly, it wasnt overheating, the motor board is driving the motor forwards and backwards when there is no load on it. Its just got no torque. I can grab hold of the pulley and stop it moving so when its all bolted in it just cant shift the chair even though its trying.
I had an interesting conversation with a guy who rebuilds magnetos, he said they have problems with weak alnico magnets. The problem is caused by taking the devices apart without using a 'keeper' to keep the magnets strong while the armature its not installed. It can weaken the magnets considerable in a very short period (minutes). Add to that the natural decay after 30-odd years and its convincing. Especially as I know I saw that chair move when I first picked up the machine.
Unfortunately - due to not having a clue - I did exactly that, took the motor apart, cleaned 30 years of crap out of it and reassembled. It was apart for a few hours without a keeper.
Sounds like an old-wives-tale but it seems to check out. The gist of it is that the magnetic flux has a very easy time travelling from pole to pole through the big metal motor armature. when its removed its primary route from pole to pole becomes the air, something that it finds very hard by comparison and this resistance causes it to weaken rapidly until a new balance is established.
The solution to rebuilding is to just clamp the motor case in a metal vice while its apart, it provides an alternative easy route for the magnetic flux to travel down and acts as a keeper.
It does depend on the material the magnets are made from, more modern 'hard' magnets are not affected nearly as badly as the manufacturing process and material make its harder for the magnetic charge to get randomised.
Note to Tin - dont take you motor apart unless you own a big metal vice!