"I followed Bentley's description which said tighten it to 1.5Nm."

I think you mis-read the Bentley. )n page 770-4, the 1.5nm reference is Step 4 of the procedure.

Step 1 says to tighten the nut to 55nm (41 ft lb) while spinning the wheel by hand (assuming you can, with a stuck piston).

Step 2 says to loosen the nut 1/2 turn.

Step 3 refers to pre-'81 240s (you don't say what year your's is)

Step 4 is where you tighten slightly (1.5nm), then turn as needed to fit the cotter pin in the closest set of holes (vertical or horizontal).

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When you replace that bad caliper, you can prevent brake fluid from draining from the open brake lines if you prop the brake pedal down about 1.5". The MC piston seals will block off the reservoir ports so no fluid will be lost. Pull the correct fuse to keep the brake lights off.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.