I think possibly because you changed the shoes it caused a change in how far the push rods in both the booster and master cylinder travel-possibly causing the rubber cups to scrape into a dirty/pitted area of the cylinder bore and possibly damage the piston cup= internal leakage. Also by bleeding this Vacu-Ease unit, if it did have scale or sludge in it possibly did cause a port to become partially blocked. Is yours the internal or external type "Vacu-Ease unit? I have sevearl cars that use this unit. If one of the little vacumn valves which is rubber splits/ cracks the engine can suck all the brake fluid out in a few seconds and you lose your brakes! Ask me how I know! Had it happen in my 50 Royal wagon and a Dodge 3 tonner. Smokes out a 1/2 block of road!