Car is UK Voyager; I'm looking for help on a.

First off, apologies for any confusion, my keyboard and this website have problems communicating; when I used the sign for "and" (uppercase 7), and posted, it added "amp;", I was trying to write "and bled brakes" and, "is high and firm". So, I didn't mean to write amp, I just couldn't edit it out of the post.

Yes, by slave I mean master cylinder.
Yes, with engine off, brake pedal is high and firm but, with engine running, it travels almost to the floor; yes it has ABS and no, both secondhand master cylinder and original master cylinder, when re-fitted, resulted in the same outcome, pedal still travelling, nearly to the carpet.

Hi, my mechanic (bro-in-law Tommy) has not ruled out a fault on the servo (booster), but, he is doubtful; he has plugged off the vacuum hose and the pedal still sinks, whilst that is happening there is no audible leak or hiss from the engine compartment or under the dash. We are doing the work on my driveway and don't have a scanner available to use. Tommy is going to ask around his mechanic pals for anyone with info but, there aren't many Chrysler enthusiasts around here; the last thing he came up with is, as you suggest, an emergency brake test to engage the ABS and to then re-check the system. By the way, I don't think I mentioned that it is a manual gearbox, don't know if that might alter anything. We are only able to work on the car at the weekends so I won't be able to give any feedback or update 'til Saturday evening, thanks for your input so far.

Happy New Year; the issue with my brakes appears to be resolved but I can't give any insight into the solution as I had to put it back with a garage to finally sort out.
I took the car down a quiet road and tried to engage the ABS, I don't know if I managed to as I've never had to use it! The system has been bled three times, the last under pressure and nothing has changed. Running short on time and ideas, I went back to the garage and asked them to investigate. Their assessment was that the brake pistons had become hooked(?) And they were not moving in and out but rotating in their seats; they dismantled, lubed and reassembled and charged me 1 hour labour and issued my certificate so, I'm on the road legal again. Still a bit wary but had a 1000 mile round trip to Scotland with a couple of hard braking occasions and everything stopped OK so I guess it's sorted. Thanks for your help.