[ToasterIQ2000]

I run large passive (fanless & silent, low flow -- no OC or award winning temperatures achieved) radiators. There is enough coolant in the loop that the thermal expansion and contraction can swell silicon hose.
To compensate for this, I put a soft hose ( a bicycle tube ) up from the bottom into my reservoir, to let the coolant expand and contract to air pressure. One side effect of this was that the coolant was (excuse my colloqual visualization here ) always trying to leak out this air / liquid hole in the bottom. If such an expansion / contraction membrane is at the bottom, a pin hole anywhere in the coolant loop would allow air into that hole & cause the expansion / contraction baloon to shrink.
Now I'll admit that 30-40 cm of negative head is awfully little compared to even a moderate pump and can be completely overpowered by the pump. ( I run an Eheim 1046 ) Make sure the pump pushes into the rigid radiator and draws from the softer hosed half of the loop.
I have caught leaks as little streams of bubbles through clear tubing & air buildup at high points & natural bubble traps without wetting my pcb s.