brake hose caps for servicing; are they size 10AN?
 

1984 300D turbo diesel; I would like to find a cap to screw onto the end of a brake hose to stop brake fluid leakage. I have located some for sale that are 10AN size. Would this be the correct size for capping off the end of a brake hose?

No, AN fittings are not metric, but the hose ends are metric.

Placing a few layers of plastic cling wrap between the vented cap and the brake fluid reservoir should stop any leaks at the brake hose end.
If not, also cover the brake hose end with a few layers of plastic wrap, and secure the plastic film with a rubber band.

So....what is the metric size of the brake hose end fitting?

Should be M10x1.0, IIRC.
You can get a free pitch gauge from b e l l metric dot com.

I agree - m10x1.0. Not sure if a female plug exists in that thread, or if one would need to machine one themselves. Seems like it would be easy enough to do from a piece of soft metal rod.

If it's just to stop the fluid from leaking during service use a golf tee. An old Triumph manual I had recommended putting cellophane under the reservoir cap. That might keep the system from completely draining but never worked very well for me.