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.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

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.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

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.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

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.