That is a pretty normal behaviour. So the brake booster is working properly.
When the car passed MOT did the also put the car on a brake dyno checking if all brake are working? A bad master can be detected by pressing the brake pedal for a minute or so. If it's rock-solid and doesn't sink any more during the test-time the master is ok. But after 25 years it doesn't hurt to replace it. I guess that part of your problem is in the calipers or maybe the ABS. If the tech said that one bleeder is slow that's maybe a hint to give them a full overhaul.