Are you lubricating the upper and lower ways (or pins) with high-temperature brake grease, and possibly lubricating the anti-rattle springs as well? If it's eating brake pads, something is catching, dragging, or not releasing.
I've seen a lot of rebuilt brake calipers with badly damaged ways, right out of the box. Several were critically unsafe. The ways should be finely machined, quite straight & flat surfaces. I've also seen the ways almost universally be painted on rebuilds, which is probably not a good idea... Bare greased metal won't gum up like paint at high temperatures, then act like glue once cooled.
If the calipers are in good order and have been lubricated properly, there's a possibility that the rubber flex brake lines may have broken down internally. A situation can arise where they look fine on their outside, but the inside collapses, acting as a one-way check valve, preventing the brake fluid from flowing back to the reservoir in a timely manner once you release the brakes. That keeps the pads engaged for far too long, which burns them up quickly. Typically this doesn't affect both sides simultaneously though.
Once in a while a proportioning valve may malfunction too, though they're generally extremely reliable.
There are different formulations of pads. Not all are compatible with each other. Best policy would be to sand the rotor surfaces to remove deposits from the previous pads, then follow the pad manufacturers recommended break in procedure, which usually consists of a series of near stops at low speeds, followed by a series of near stops at moderate speeds. That heats up the new pads and mates them properly to the rotors.
-It's been such a LONG TIME... BlueShift>>
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