Similar problem. I replaced the rear cable as the previous one was absolutely rusted solid on the driver's side, so much so the cable frayed and chafed as i tried to get it off the caliper lever. I ended up cutting it off. New cable on and adjusted up it holds much better but still not fanastic. Ill try out that footbrake tip and see what happens. If not much better I'll consider replacement calipers as I need to sort the fronts out as well (nearside disc is slightly warped) so I might consider doing the lot and have something that will stop in a straight line.