Ian,
maybe do the test the correct way, with the most relevant result being obtained when the engine is held at high rpm?
Note, 'held' not just 'blipped'...
It's the 'difference in level of the fluid' in the manometer legs you're looking for.
If the result shows a 'pressure', rather than a 'vacuum' inside the crankcase at high rpm, either the breather is u/s or there's excessive blowby between the pistons and the barrels.
Btw, best to use LHM as the manometer liquid, rather than water or Coca Cola; if LHM is sucked into the engine, it won't cause the sort of problems which those other liquids could produce.
DSC00079 by that is my real name!, on Flickr