Morris Minor Owners Club

I have 1965 minor convertible which I had booked in to a garage who "specialise" in classic cars for a service. On driving car to the garage it was running great until after about 10 miles when it started to lose power and seemed to be running on 3 cylinders. It finally cut out and would not start. Called garage who came and trailered it for investigation. So far the problem has been diagnosed by the garage as bad coil, fuel pump, contaminated fuel, then finally the dizzy which he exchanged for an electronic one. Called to say car was ready and he had road tested for about 15-20 miles all OK. Collected car and it ran ok to our house. Put it in garage then left it as we were going on holiday. Upon returning home lifted bonnet only to discover he had disconnected the servo hose from the servo and blanked the end with a bolt and jubilee clip. I reconnected to the servo unit and behold it was running rough again no power. I do not want to talk to the garage again as consider them incompetent.
The problem obviously is connected to the servo unit are there any tests I can do to isolate the problem tia

Well there are some basic tests you can do.

See this website: https://mgaguru.com/mgtech/brakes/brakes.htm

Scroll down to :
BT-201K - POWER BOOSTER, Remote, How It Works
BT-201L - POWER BOOSTER, Remote, LOCK-UP
BT-201M - POWER BOOSTER, Remote, FAILURE MODES

A lot of good info, explaining how it operates and failure modes.

I suspect the Air Valve diaphragm has a leak.

The garage claimed that four different - and separate - components all went wrong at the same time?!
And replacing them still didn't fix the fault!
I don't believe it!
Do you know a good solicitor?

Giving your car back with disabled brake servo and not informing you is negligence. You should seek legal advice. This is far more serious than their poor diagnostic skill.

Is the repairing garage a member of any of the recognised motor trade bodies?