Thanks very much for the replies, all very helpful. There is brake fluid in the machine, and it doesn't appear to be leaking anywhere. When I open the cap on the master cylinder, and depress the break pedal, there is no motion at all to be seen in the fluid. Does feel like the brake pedal isn't doing a thing.
Not surprisingly, the shuttle valve on this doesn't work either, so I would go the route of bypassing it. I have the parts list, so I think I'll order the cylinder repair kit from Clark, and take it form there. I'm not a mechanic by trade, but will see how far I can get. I'm fairly good with these things, but will no doubt be looking/begging for help once I start.
Thanks again