I called Z Therapy, but they are 11 months out on rebuild exchanges.

Then texted Paltech http://paltech1.com/how-to-order/ No response.

Then contacted https://www.sucarburetors.com/services They don't work on Hitachi's

Next on my search is this company: https://www.applehydraulicsonline.com/collections/datsun/products/copy-of-datsun-tokico-master-cylinder-tokico-sleeved-and-re-built-1   Looks to be an option.  5 stars on google reviews.  Anyone else here use them?

I've not heard of them but there's other options too.

https://www.thezstore.com/page/TZS/PROD/11-3001

Zcardepot.com has some but I can't find them for some reason.

I rebuilt mine with the kit from ztherapy and just with basic knowledge it was easy and they turned out great. Took them apart and cleaned the exterior with aluminum polishing wad rebuilt the insides and am extremely happy with the results. The rebuild kit comes with a DVD that helped a ton, made it really easy with the rewind. I watched it a few times before I ever turned a screw.

Another suggestion would be look in the classified section on here for some. If there's not any start a new thread about needing a good set. They're out there for sale ready to bolt on. Check ebay too.

Goodluck!