The Brits love to build brakes that only work sometimes... I used to drive BMC stuff, and the pistons were always seizing in their bores. Eventually, brakes became as regular a maintenance item as oil changes: take the wheels/drums off, remove pistons, clean out the bore, coat bore and pistons with anti-seize, new cups, reassemble. After a few times, the whole car could be done in just over 1/2 an hour, and the brakes were completely reliable.
Whether this would work on your units I cannot say for lack of first-hand experience, but it might be worth a try...
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