Hi mrow2, I had a similar problem with an early B790 arm too: sometimes it worked well, but sometimes it would go back up as soon as it touched the vinyl. I traced the problem to a (normal) safety behaviour of the arm control to an abnormal condition in which the arm went sideways as it went down. It seemed to be related to the screw-in lead counterweight that was off-axis, and I managed to fix it (mostly), but this particular arm was so finicky that I finally gave up on it as soon as I found an used B791 arm on the bay, which took its place on the TT and works great. I kept the old arm for parts.
If your arm is able to play a few seconds before retracting, it might be a servo problem: track following does not work, and when the limit is reached, the solenoid is deactivated in order to prevent the stylus from egressing the groove. Checking the signals on the arm control PCB according to the service manual should help you sort out the problem. Also, note that in those old decks the electrolytic caps and (especially) trimmers do not always age well and can very easily cause failures, so I would consider changing them if you have the equipment, and the time, to recalibrate the thing.
Adding weight on the rear end of the arm is not necessarily a good idea, as it tends to cause such stability problems in my experience. OTOH, Revox sometimes installed a brass weight in the front (visible through a hole), to adapt very light cartridges. If yours has this "counter-counterweight" installed, you might try to remove it (it is normally glued in place).
My other B790, with the "normal" arm (snap-in, not screw-in, counterweight), never had such problems.