91 Rodeo Brake FailureS

We have had the brakes fail twice now due to hydraulic line failure. ( First time was left front line rust out in a very small hidden spot, second was an
odd ball fatigue?? crack inside of rear coupler on axle.) Since the late 60's all cars have had 'dual master cylinders' so in the event of line failure there would be some braking, but in our case both times the brakes failed totally & the proportioning (combination) valve has been replaced as well as master cylinder. I think there is a design safety flaw issue here. Anyone else had any brake failure experience like these? (By the way, even though in the mountains, both incidents ended in no injury or damage.) Thanks BC in BC
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