The pre filter bubble I consider a constant. The small bubbles transiting through the pre filter are an abnormality.

When you park the car with the front higher it is reasonable that more air gets pulled into the system. Simply because the suction force is higher.

This car is old and the rubber hose at the back near the tank is suspect. As well as some line leakage at a clamped hose joint or the line itself.

The pre filter is before the lift pump and the primer pump so they are not where the bubbles are being created by air entering the system. Sometimes it is even a cracked prefilter or a clamped hose line entering it is allowing air in.