I understand these brakes use a 3.5mm Allen wrench (hex key, if you prefer). I also understand that these specific wrenches were (a) nonstandard at time of the Delta brake's introduction, (b) were supplied with the brakes, and (c) were for a long time almost impossible to replace if lost.
Well, today apparently there are alternate sources for said wrench:
https://www.amazon.com/Eklind-18607-...dp/B07FP3QXH4/
Haven't ever used Eklind tools, so I can't comment on their quality. FWIW, the company website indicates their tools are US made and that the company has been around for over 100 years.
There are also some other 3.5mm Allen wrenches listed on eBay, with varying prices.
Disclaimer: I have
no commercial interest in Eklind tools, Amazon, or eBay (other than having made purchases from the latter two). Just found this and decided it was worth posting the info here.