George,
I had written up the timer fix and was about ready to post it when I remembered the reverse polarity issue, so don't think that you are alone regarding getting old :-)
I had posted info about cleaning the brake which fixed my issue with an intermittent slide operation problem. In the end, mine just stopped working which forced the issue. If your slide motor has to work harder it will draw more current and drop more voltage so you may want to lube any mechanical points where that might be a help.
Beyond that success on other peoples part have been with improving the connections by soldering or better crimping. If the slide works with the generator and inverter on it would say you need about another volt. I would clean the brake, lube the slide mechanism where I can and see if it helps. Beyond that I might see if I could order up another brake if it was relatively inexpensive and see if I got lucky and the new one released more repeatedly.
One last thought on an electrical solution.... If you got hold of an automotive relay (Bosch Type) without a built in suppression diode I think the relay would close using either polarity on the coil given current creates the field. You would have to test that on a bench. These say they have no suppression diode in the Q+A.
http://www.amazon.com/Relay-Harness-Bosch-Style-40AMP-HRNS/dp/B005HFYE1O/ref=sr_1_1?ie=UTF8&qid=1447339613&sr=8-1&keywords=relaysIf polarity on the coil is not an issue, you could connect the relay coil to the motor leads. Then bring a separate 12V suitable gauged fused wire (14 gauge would be fine) from a good power source like the battery compartment to the relay normally open contact. When the motor gets triggered the relay would then supply the full 12V source to the brake. You would also need a good separate ground to the brake coil lead given you would not be reversing polarity anymore. There may be metal frame in the bed area that is grounded well, not sure. If reversing polarity causes the normally open contact to become the normally closed contact you could parallel two relays so that no matter which polarity fires a coil you always get a set of contacts that close.
You may find a polarity on the brake leads that work better regarding the release so you could try both polarities and see if any difference exists. If the relay polarity on the coil is a non issue then one relay would work, if not then two. Hope this helps.
Later Ed