Did you remove the residual valve from the master cylinder that feeds the front end brake line(rear reservoir) and disc brakes ? If not do that first and see how that helps Normally the standard drum brale master cylinders have two residaul valve in them, the standard disc brake master cylinders have one for the rear drum brakes only, which is noramlly the smaller front reservior on the master cylinders. The stock disc brake master cylinders have two different size reservoirs, small front for the rear dums with the residual valve and larger rear reservoir with no residual vavle for the disc brakes on the front