It's rubbish yes, but it still should be easy to lock the wheels even in dry. It just needs much more force on the pedal than modern cars. Any of the newer servos are much better; Mondeo, focus, mk6 Escort, ...

Best would be a Cosworth/Granada type system that doesn't rely on vacuum. Personally I hate the RST servo beause when on throttle, you can brake once. The second time there's no vacuum left before you lift the throttle.