Hah, I have a '77 XS 750 with a leaky brake reservoir, and I swapped it out for this one: https://www.amazon.com/gp/product/B01B1BNQTS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

It's currently unavailable, but it worked with zero issues, managed to swap it with minimal effort. Anything similar should work just fine.

Edit: Swapped it 2 1/2 years ago, plenty of adventures, no issues. Check your brake lines and swap those too if they don't look any good.