In a disc/drum cylinder, the front section [nearest the radiator] is for the rear brakes & these usually have a residual pressure check valve in them which is designed to maintain some pressure in the line. This could have the effect of causing the front brakes to drag. Have you checked this out?
Have a look under the bonnet of several cars & check out which way the pipes are plumbed. In Australia, the rear chamber is for the front brakes & I have not yet seen an exception.

If the cylinder is a 'used' example that has not been dismantled, cleaned & overhauled, then it's possible that one of the tiny ports at the top of the bore that allow brake fluid to return to the resevoir, might be blocked. This could have the result of 'pumping up' the brake circuit & locking on the brakes.