all of them out.. makes it easier to turn
.. plus that way you can isolate which cilynder is leaking ..
It would take time for the bubles to come around out.. so i say just keep it preasurized at X preasure and watch the gauge while you are doing the compression testing.. both gauges that is...
and your engine compression should be at 160+ or whatever...
but your coolant preasure should be the same, not raised or droped..
-E