As others have said, I'd bet you don't have the shoes adjusted correctly. All the fluid is filling the slave cylinders before they engage the drum. Tighten them up till the wheel won't turn, then back them off a little at a time.

Also, you might consider a 10lb residual pressure valve between your MC and brake lines. This will maintain 10 pounds of pressure in the line which should prevent the slave cylinder from totally emptying between uses even if your shoes aren't adjusted just right.