Hi 327 .The cast iron unit has a port hole in each side of the reservoir which are clear through to the bore. I checked the outlets of each with a piece of wire and they are clear as well. The only valve in the system is a "2psi blue willwood" non return valve fitted directly in line out of the master cylinder on the rear part or secondary side only. I am going to try and bench bleed the thing with plugged outlets as i saw some guy demonstrating on you tube. He depressed the pistons slowly and waited for air to rise up.
Its a puzzle.