[Sirrotica]

You could eliminate the hold off valve, then replace it with an adjustable proportioning valve.

https://www.speedwaymotors.com/Speedway-Black-Adjustable-Brake-Proportioning-Valve,1977.html?sku=91031355&utm_matchtype={match_ type}&msclkid=307685afb47810d98feee3897c016650

I have also had this happen on a 1969 GP many years ago. I blew compressed air from one of the wheel cylinder bleed screws back towards the master cylinder with the cover removed, after I emptied the reservoir. I was able to gravity bleed the rear after doing this. The problem arose when trying to pressure bleed the rear, as long as you just let it drip, the rear bled fine.

It was decades ago, 1980 ish, so details are kinda hard to recollect, but I spent hours trying to get fluid, before applying air to the bleeders. Could have been sediment, or a sticky reset button, but the air going backwards got it open.