Issues bleeding a T4 Non-ABS System after replacing rear brake lines

 

First off, understanding? This is a non-ABS system which simplifies things a fair bit. You press the pedal, which activates a plunger in the master cylinder. This sends hydraulic fluid to each front calliper and two lines back to the balancing valve located under the bed. As the back squats on the van, this opens the valve, controlling the flow to each calliper, increasing to maximum braking once the bed is carrying a certain load. So far, so good?

 

Observations. Went down there again this morning just to look and the reservoir, which was full last night, is now empty. Looking all over the van at every pipe connection and component, including around the piston connected to the brake pedal, there is no sign of any leaking. Now, I have been struggling to get and fluid to run through the system to the rear, could this vanishing level in the reservoir be the system gravity bleeding slowly to the balancing valve at the rear? Another observation is that no matter what I do with the pedal, I cannot get it to build any pressure in the system. It’s this which is making me lean towards a problem with the master cylinder as well.

 

I’ve got a new pressure bleeding system arriving tomorrow which connects to my compressor to give a decent draw through the pipes. hopefully that’ll be man enough to suck any air lock out of that balancing valve. But, am I right to be questioning the entire system and whether I have a second issue?