Interesting drum brake problem

If the problem manifests on one side of the car only (I.e. left side locks up while right side doesn't), then I would also double check the brake lines into the proportioning block to ensure the front brakes are both on the rear reservoir of the master cylinder (since the lines were replaced by a previous owner and may have been done incorrectly). Of course there would be a problem with whichever rear side locked up too as both rears share the same hard line.