Quote:
Originally Posted by et cetera
The pistons retract and move freely during use. When pressure is relived from the hydraulic system they all retract without issue.
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To me, this says that pressure is building up in the hydraulic portion of the system (of course). The only way this can happen is if the fill/return holes are being covered by the master cylinder umbrella seals.
As the master cylinder is pushed in to apply the brakes the pistons immediately cover the fill/return hole and only then can pressure start to build.
Something is causing the master cylinder to be slightly activated at all times. So it could be something "off" with the booster, no play in the linkage, etc.
With the bleeders closed you should be able to push the pistons in and fluid will freely flow back up into the reservoir. If this isn't happening there's only one thing it can be - the holes are covered.
Maybe next time you get the system "pressurized" you could try loosening the nuts that hold the master cylinder to the booster. If that releases the pressure you know there's something wrong with the clearances between the booster and cylinder or somewhere else in the linkage.
Instead of driving and trying to heat things up I think you might be able to push a piston against the fluid pressure in one of the calipers and it will have the same effect as heating up the brakes. If the fill/return hole is indeed covered the other brake calipers will apply pressure on their pads so make sure there is a rotor between the pads!
Perhaps a safer way of going about this!