If air in the line the oil would compress it possibly giving a false reading. It seems to me any time I've taken the line off the back of a gauge there's oil in it right up to the gauge. Probably over time the line bleeds itself.
Early square Chev gauges were capillary but the think the fitting on the back of a round gauge is 1/4"
Quote:
Originally Posted by Grant Bowker
Does it matter? The relative densities should keep the oil below any air. As the engine oil develops pressure, any air above it will develop an equal pressure and indicate the pressure on the gauge.
Regarding "proper" size, most (all?) civilian pattern Chevrolet gauges I've seen used a small/capillary line. I'm sure I've seen the round gauges with larger (3/16" or 1/4") OD line (the fitting on the back of your gauge may help on sizing) but I'm not sure if I remember capillary line being used as well.
Parts book C15A-04 section 1.519 lists 1/8" tubing (tubing is normally specified by OD as opposed to pipe which is by nominal ID) for use as "Oil Pressure Gauge Pipe" which is a bit larger than the capillary tube I'm thinking of.
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