I had sent a private message to a gentleman who had posted some info on the J72 brake on another thread. He was very kind to reply to me. I don't think he will mind if I post his reply here. His ID is mrV8outboard.
I am surprised you are able to find parts. Good for you. Your brake unit has one JIC hose fitting and no others on it correct? If it is new OEM it should be capped and full of fluid. If not I have prefilled mine using a piece of clear tubing pushed on the JIC fitting with the unit upright and slowly filled it one inch at a time up the hose until all air is removed. Then capped it. Now once installed it is a two person job, you have to get the pump system full to the unit by cycling it out of park. Wheels chocked person on the main brake. Aim the hose end up with another section of clear hose in a loop so the fluid will go up the clear section a few inches before looping over (have a drain pan ready). Once you have clear fluid flowing stop the system, pull the clear section off and hook it up (trying not to loose fluid) to the brake unit. Just snug. Now operate the system and crack the fitting slightly to purge the last little bit of air (ever so slightly as there is 1400 PS
I in the system) and it should happen quick. Clean the area of spilled and sprayed fluid, operate the system several times then check for leaks. Now test for holding power. The last tiny bit will make its way to the pumps reservoir. I have heard of newer units with bleeders but have not seen it in person. Hope this helps.