While instructing my younger son, who'll soon be licensed, to change
the rear pads on the 93 940 wagon this past Sunday, I found the
passenger side caliper had a piston stuck.  I have him install it
anyway so the car can be back on the road.  The new pads won't go in by
the way because of the stuck piston.  I have him put in a used pad I
happen to save in a box.

I presume a rebuild kit won't do because the caliper probably needs to
be replaced.  So getting a rebuild caliper is the way to go.

Now the question.

Is there a way to tell which caliper the car has?  The mail order
outfit says there are three different types of calipers for my car,
36mm piston, 38mm and 40mm.  I really hate to take the wheel off to
check again.