Brake woes

[Jeff Walker]

   Brake temperatures at the pads routinely get past 200C, or 394F. During
   hard continuous braking, like going down a long hill at speed they can
   get to 1000F. 200C is easily hot enough to vaporize condensed water in
   brake fluid after heat transfer through the pad material and caliper
   pistons. All it would take is for a micro drop of water to vaporize to
   make the hydraulics feel spongy. Further, I've seen it happen on a
   poorly maintained car before. The water always condenses at the lowest
   point in the system, and that's also the hottest part of the
   hydraulics, at the piston calipers.
   Unless they've overheated routinely, it's far more likely for the seals
   in a master cylinder to fail than a caliper piston seals. The
   degradation mechanism comes from sliding wear, and the piston calipers
   hardly move at all. The good news is that the master cylinder also has
   the least contaminated fluid at it's seals, as the corrosion products
   migrate down to the calipers. The counterpoint is that the heat at the
   calipers causes hardening of the elastomers in the seals.
   However, that being said, sticking calipers almost always stick in the
   applied direction and fail to retract, vs sticking such that they
   prevent the hydraulic force from squeezing the pads on the rotor. Or if
   the seals are failing, brake fluid leakage is evident.
   Otherwise a master cylinder seals can leak by and fail to produce
   hydraulic pressure with no evidence of leakage. That's what happened to
   my rear master two years ago. Very minimal brake force applied at full
   stomp, then it would fail to retract when I took my foot off, and no
   leakage. Rebuilt it and it's back to perfect.
   In the last 130K+ miles on my GPz I've gone through several sets of
   pads, but I'm still on the original calipers. I always bleed them
   before and after pad replacement, as pressing the pistons back in to
   install the new pads always frees up more gunk to be flushed out.
   New brake lines are always challenging to bleed the first time. My
   metal braided lines both connect at the master cylinder, each having a
   straight run to their respective caliper and eliminating that "Y"
   cylinder above the fender. Much easier to bleed that way.
   Just goes to show the importance of preventative maintenance. I flush
   and bleed the brakes on all my vehicles every two years regardless of
   how often they're used. As a result I very rarely have any issues with
   the brakes outside of normal pad wear. The rate of degradation of the
   brake fluid is mostly time dependent, not mile dependent.
   Sorry for the long winded response, but as a mechanical engineer that
   specialized in maintenance, troubleshooting, and failure analysis of
   far more complex systems I get interested in topics like this.