My money would be on the pistons being stuck in the caliper through lack or use/movement, or the slider pins are seized solid, specially if the lever still feels taught. The brake fluid may have picked up some moisture during the lay up too, so for the sake of a few quids worth of brake fluid, a flush and bleed would be a good plan after you've freed up the pistons/sliders.

If you think the brakes were a bit dodgy before, it's quite likely that the pistons and sliders were partly corroded already?

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