I think hydraulic parts like brake master cylinders, clutch master/slave cylinders etc should be expected to be either serviced or replaced at 100k miles. They last longer if you flush your brake fluid regularly , however who does that? Dot3/4 fluid attracts and absorbs moisture, and that can lead to corrosion. Dot 5 I hear doesnt absorb but lets the moisture pool.

 

I sure do, and I can not remember the last time I had to replace a hydraulic brake part on one of my cars.

 

Most people ignore the recommended every 2 year brake fluid flush. If you keep a car long enough, you will see the effects of ignoring this. Perhaps/probably that contributed here. With a power or vacuum bleeder, flushing is a one-person, one-hour (most of that time spent on removing the wheels), one-quart job.

 

I do mine annually just because it's easy, part of an annual fluids/maintenance protocol. Wheels are off for cleaning pretty regularly so no issues with that. The winter session this year was the first time with the lift, prior was on stands. Similar to GregN's experience, no hydraulic components have failed on my cars with this level of attention.

To the question on single circuit failure, a rear of MC fluid loss means the front piston must be pushed by the rear, and the check disk must work perfectly to have any pedal. It's a tall order. Usually, by the time a circuit is dry, there's other junk in there. The dual diagonal system is the best compromise for sure, short of dual masters with a rigid pushbar. But the rigid pushbar doesn't self-balance the pedal pressure. Use a balanced bar and you are back to the original problem again.