Thanks, after bleeding the brakes I don't have any leaks but the rear reservoir turned dark brown while the front stayed clean. So I am pretty certain the master is corroded internally.
I see that Cruiser Corp lists their model as "for drum brakes" but it's a match to my model: Brake Master...
I recently replaced my rear drum brakes on my '75 FJ40. The front has disks and the PO recently had new shoes fitted. After flushing the whole system I kept it on the jack stands and ran the engine in 2WD. The wheels turn, but the brake pedal goes straight to the floor. Looking at the master...