O' Reallys POS

The wire/bale clamp over a stamped cover was a poor design in my opinion. Slosh or parking on a hill could cause seepage. That rubber sandwiched between cast and stamped sheet metal might conform and create an air tight seal eventually but I wouldn't bet on it. I've never seen but 1 that sealed so tight that pushing the pistons back into the calipers actually made pressure inside the reservoir instead of leaking the excess fluid. That example was a 73 Ford Torino.
Modern plastic reservoirs are not as prone to leaking but... those have a "fill to" marking well below their cap.