Something to consider when working on any brake system, and particularly any disk brake system.
ALL Brake Fluid is hydrophilic, meaning it attracts and absorbs moisture. This includes atmospheric humidity, not just droplet water. A small amount is tolerated under normal circumstances, without even being noticed.
The problems begin showing when the brakes begin to heat up. (long downhill braking, heavy traffic braking, anything that starts heating up the brakes, and can be made worse by hot weather) when this happens the moisture in brake fluid suspension begins to turn to vapor (steam), this causes the brake calipers to apply pressure of their own accord, exponentially making the heat problem worse. And more complex. AS the vapor finds it easier to move the caliper pistons, pressure is applied to the pads and discs, rather than pushing the vapor and brake fluid back up through the master cylinder into the brake fluid reservoir. The other thing this does, is the excess moisture that has not yet turned to vapor, when pressure is applied, returns again to free liquid form, and settles to the bottom of the slave cylinders / calipers, which prevents it from being flushed out with a normal brake fluid bleeding, as the bleeders are at the top of the units. Once this happens, the problem tends to return again and again whenever the particular brake heats up. Once enough heat energy is dissipated, the steam returns to liquid, settles to the bottom again, and the brakes release.
I learned of this years a go when a Grumman-GM commercial van kept having front brake locking issues I couldn't duplicate in testing, so I went for a ride with the normal delivery driver. On a long downhill with a full load, suddenly the front brakes began to lock up and he lost solid brake pedal control. By the time he got it pulled over and stopped, both front brakes were locked and even the tires were too hot to touch. I opened the brake bleeder too far and ended up with first/second degree burns from the brake fluid blasting out propelled by steam. We spend the next 2+ hours sitting on the shoulder with the bleeders barely cracked while the water boiled and the steam slowly escaped.
From that time whenever I do any brake work, it includes removing calipers, cylinders, whatever is involved, draining ALL fluid, and replacing with new. I have only had the same thing happen one time since, when I bought a vehicle that had been stored at the beach for 3 years, and drove it home. After changing all the fluid and draining / cleaning every caliper, it never had another such problem to the best of my knowledge.
I'm not saying this IS the problem you are dealing with, but it sounds as if it could be.
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