Pressure testing reground valves

[Toko_Jo]

CB250/350K Shop Manual tells you to test reground valves by putting fresh engine oil in combustion chambers and pressuring ports up to 2 bar and look for bubbles in the oil. I always just looked for a continuous full grey line on the faces. But tried the pressure test on 500/4 head and four of freshly re-ground valves bubbled very slightly so after much effort got them all bubble free. Seen pro test rigs using vacuum but not positive pressure testing.  Any thoughts? Even though pressure is acting against the valve seats tending to open them, it seems a reasonable test to me. The vacuum one looks a bit iffy..as how do you know its leaking slightly....they have a pump and vacuum gauge which gives a % sealing figure. Anyway, I know my valves are totally sealed now as zero bubbles.   

https://www.dropbox.com/s/8i51vjqqdvllazj/CB350ShopManual.pdf?dl=0

[Oddjob]

I used to hold the valve stem with a chuck type tap wrench, I used to same tap wrench to grind the valves in BTW, I'd then just pull against the valve whilst filling the inlet or exhaust port up with petrol, nothing finds leaks like petrol, just using hand pressure if I could spot a weep in the combustion chamber I'd regrind it until I didn't. With petrol the weep looked like vapour in the air, very easy to spot.

[Toko_Jo]

That's exactly what I use to hold the valve Ken ..i.e. a tap wrench. With the valves is question I filled the combustion chambers with isoproponol with the spark plugs fitted but this did not drain or show any signs of leakage past the valves when left for a couple of hours. Probably filling the other way  with solvent/petrol in the ports would be a better way, as you suggest though. Now that I have the air pump and adapter setup, I think I will always use that method. Unless anyone  knows otherwise. I have not studied other Honda Shop manuals carefully but never  seen that pressure method described other than in the CB250/350K manual

[Toko_Jo]

 I have been using a rubber adapter to actually pressurised the port but in the 250K manual they describe 'blasting with air' at 2 bar ...am I misinterpreting the technique. Should I be blasting with an airline into the port?

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[McCabe-Thiele (Ted)]

Never seen this type of valve seat leak testing during my time in the car trade though it might be that it's more critical on smaller engines that rev much higher.
My late father who did his apprenticeship at RR/Bentley in Derby & Crewe never mentioned it - he taught me the pencil line method of testing that the valve seats have been fully seated. I suspect it would have been Engineers Blue originally.

[Oddjob]

Yeah, I'd say it's just blasting with an air line and seeing if you can spot bubbles. I prefer my method, it's never failed me yet.