Quote Originally Posted by L696ULO View Post
Now onto my findings. When the car is parked in garage the brake pedal remains solid however with my foot on the pedal as soon as the engine starts the brake pedal immediately 'sinks' and becomes firm again at a lower point in the stroke. The braking point remains at the bottom end of the pedal movement whilst the engine is running and once the turned off after a couple of pumps it returns to being solid almost as your foot touches the pedal.
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.