Adjust the brakes. The shoes are moving far enough before they contact the drums that the pushpins between the wheel cylinder piston and the shoe is getting angled. Driving forward tips it back into position. Do you have Bendix or Huck brakes? A Bendix brake needs to be tightened to a firm drag on the drum, then backed off 10 notches from that position. Don't tighten the adjuster enough to lock the wheel, just go for a very firm drag.
Jerry
Last edited by Hotrod Lincoln; 01/27/2011 12:40 AM.