@Phil: are you sure you are measuring AC voltage? If you are using a multimeter, make sure you are using the AC level, and not DC, otherwise your measurement will come close to 0 as you observed.
Dan is right that the master zone is electrically the same as any other zone: it outputs 24V AC (or whatever voltage your transformer outputs) when it’s turned on. Normally if you want to use it to turn on a pump you can get a pump start relay, which takes 24V AC as input, and can switch a pump at powerline voltage.
You mentioned ‘AC/DC Converter to convert what I thought would be AC25V to DC12V which is the input for the next component in the sequence a solid state relay’. Could it be that the AC/DC converter is doing something strange that makes the voltage show up as only 1.5V?