When you rebuild frt or rear calipers, know there are two places for leaks, one of which the seal kits will not fix. All Girling calipers built '60s-'70s used mild steel pistons with chrome or nickel plating, and they rust under the plating from old wet brake fluid. This produces a row of pits that, if they're in certain places, produce a leak with brand new seals. Some pits will tear up new seals as well.
Supposedly, someone sells stainless pistons. I made my own out of 312-stainless bar stock. Porsche guys strip the plating off old pistons, braze/weld the pits, remachine and re-plate. NOT cheap! PanteraParts.com in Carson City sells reconditioned front & rear calipers on an exchange basis, and he has a pickup truck full of pitted pistons. Good luck.