Brake callipers are very basic devices, and usually consist of little more
than a piston, an o-ring and a dust seal.  At the least you're likely to
find that pulling apart the callipers, cleaning out the gunk and fitting new
o-rings & seals will restore it to perfect working condition, and at the
worst you may find that it has suffered from some sort of corrosion damage
and requires new hard parts (which would be unusual in the main).

Most likely you would get away with just a "kit", but finding that kit for
your car in this country might be difficult.

If you can't find anyone who lists parts for your car directly, the  get a
calliper apart and take the bits to your local brake specialist as they may
be able to match parts up according to physical dimensions.

--
Regards,
Noddy.

May well be an internal hose failure.

I have experienced precisely what you described.  The brake hose lining had
failed and the pressure could get to the caliper but not back out when the
brakes were released.  The residual pressure needed to keep the brake on is
far less than the application pressure.

Quick test.  When it grabs, crack the bleed nipple on the caliper and see
if the brake releases.  If it does, it's the hose.  If it doesn't, it's
probably elsewhere.

You should be able to make it grab simply by pumping the pedal repeatedly.
Jack it up, take the wheel off, get someone to pump the brakes while you
check to see if it has grabbed yet.

If you have to ask about overhauling a caliper, you probably should not
attempt it yourself.

If you find that it is a hose fault, I'd recommend replacing all of the
rubber hoses (probably 2 front + 1 rear), preferably with plastic coated
stainless braided lines.  The braided lines without plastic coating are
hard enough to cut through the chassis if they rub on it!

Sounds like hose collapse , make sure it isn't before rebuilding those
calipers or replacement with late model alloy units ( needs some metal
work but is easy)