Final answer:
The gas cap pressure relief valve's main function is to balance pressure and prevent tank rupture from overpressure by allowing the tank to vent, similar to how tire pressure increases when air is continuously pumped into a full tire.
Step-by-step explanation:
The primary function of the gas cap pressure relief valve is to maintain a proper balance of pressure within a fuel tank. If the fuel tank were sealed too tightly, the expanding gases could cause a leak or even burst the tank due to high pressure. Both liquids and solids resist compression with significant force. To account for changes like expansion and contraction due to temperature fluctuations without creating stress on rigid containers, air gaps are included in these designs.
For instance, in the context of propane tanks used with barbeque grills, pressure gauges are available to determine the quantity of gas remaining. These gauges measure internal pressure, which can vary with temperature - indicating higher pressure on hot days due to gas expansion and lower pressure on cold days due to gas contraction. Gas cap pressure relief valves take these factors into account much like when air is pumped into a tire, initially increasing in volume then later increasing in pressure once the tire is full and further pumping introduces more air into the limited space. Similarly, pressure relief valves prevent excessive buildup of pressure that might otherwise lead to tank rupture or damage.