I bought a 90 740 wagon that needs new rear (ATE) calipers, and all the parts houses that I have checked, say that the pistons are 36, 38, or 40mm--- Mine are 34mm.
Does anyone know of a source, or is it possible to use one of the other sizes.
I am also going to need parking brake shoes.

Thanks in advance for any advice.

 

Are you changing the calipers or replacing pistons in the present calipers?
If replacing the whole caliper why do you need the exact same piston?
Have you read the FAQ file on calipers?