tachometric relay by the master cylinder ? you should hear it click when rpm gets near 2000 rpm when you listen under bonnet .
Vacuum leak from the plastic lines which go through the chassis , welding tends to melt them!
The rubber Diaphragm in one side or other of the primary unit drums can perforate .
The seals which seal the rotating union which let air in and out of the drums in the centre of the Primary unit drums can wear .
the vacuum 4 way valve may well click , but the head of the rubber valve inside tends to fall off ! easy to dismantle and bond back with superglue , it gives you another 18 months of life from it ..
Broken diaphragm springs in secondary unit ..
etc etc . there are a LOT of things which can cause your problem ...
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 My comments are only based on my opinions and vast experience .
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