Based on your suggestions, I think I need to put a 2000 psi brake pressure gauge on a wheel cylinder and press down progressively on the pedal with the engine running. If I see sudden or erratic changes in pressure, the Hydrovac unit is acting up. If pressure ramps up smoothly, it's likely the shoes, as drums are in excellent condition.
This M8 is in great condition but it rarely gets driven. Maybe the first thing I should do is thrash it round the track for half an hour with lots of braking to see if some bedding in helps.
As usual, thanks for making me think outside the box.
Marco, I'm not sure what you mean by dual line. It has one line coming in from the master cylinder and one line coming out for all 6 wheels. This is the old style cylindrical hydrovac that was used from the 40s to the 60s.
Malcolm
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