My thery is that in the era fording a stream was not unusual, the design is such that if water got in it would go out quick, if there was a seal there and no dust cap on the outer end it would retain anything that got inside ---the service interval was short also. --perhaps it was just cheaper to make --there were many other cars made, I just took apart the brakes on a Chrylser ( plymouth, dodge), it has a seal, but how many of them do you see? ---the Mopars also have hydraulic brakes, that didn't make them survive better either.