That happens frequently when folks replace their pads and don't open the bleed valve when the pistons are retracted.

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But would that back up the fluid so that it goes all the way to the top of the reservoir? Mine has always been at the full mark but now it is just about to overflow when I remove the cap.

The fluid has to go somewhere when the pistons are retracted. Unless there is an alternative, it's going back into the reservoir. (That's why it's a good practice to open the bleed valves. That avoids forcing contaminants from the caliper back up to the master cylinder. Or worse yet, the ABS unit, in vehicles so equipped.)

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.