The rear brake pistons came out of their bores

The rear calipers on this truck carry the parking brake. There is no cable: a small electric motor on the back of each caliper turns a threaded spindle, and a nut riding on that spindle pushes the piston. With pads and a rotor in front of it, the piston stops when the pads clamp. With the caliper hanging off the axle during the swap, there was nothing in front of it.

With the truck running, the brake pedal was pressed to shift into neutral, and both rear pistons came out past their seals, with brake fluid running out on both sides. Whether the electric parking brake also played a part is not certain. It can apply on its own, and with no rotor in front of the pads it would do the same thing.

In hindsight, the workshop manual covers one of the two. On trucks with the electric parking brake it requires a Brake Service Mode to be entered before any work on the rear brake components, and exited afterwards. The other needs no manual: nothing should press the pedal while a caliper is off its rotor. Neither was followed here. Service mode was not on, and the pedal was pressed.

The repair was the kind the manual does not describe. Each caliper was cleaned, the pistons were worked back into their bores with a plastic trim tool rather than anything metal, and the dust boots went back on. Ford has no rebuild procedure for these calipers, and calls for a new caliper and motor if brake fluid gets into or around the parking brake motor. These are being reused anyway.

So it is unproven. A piston that has been all the way out of its bore has had its seal unloaded and exposed, and there is air in the system. The test is fresh fluid, a proper bleed and a watch for leaks. If a caliper weeps or will not hold pressure, this entry gets a follow-up.

Update, 14 October: the calipers are back on the truck. The bleed has not been done yet, so whether they hold is still open.