Oh yes and on my w123 Vickers Type 24 pump (top mounted on the engine so different from yours) the relief valve consists of a simple plunger and a spring. There is only a single O ring sealing it. I guess all pressure relief valves will be about the same.

Anyway you could measure the stiffness of this spring by applying a weights to the top of it and measuring the deflection. The deflection should be linear until the spirals start to touch. To calculate stiffness you need to divide the applied force (weight X force due to gravity {g}) by deflection. In SI units stiffness k = F / x

It is best to calculate the stiffness (k) because that is how stiffer or softer springs will be sold. This is good information to have...

You could then measure the radius (or diameter) of the surface of the pressure relief valve to calculate the area. You now need to estimate the distance required for the spring to travel for the valve to crack open. It probably won't be much. Plugging this estimation into the equation given above. The opening Force acting against the spring when the valve is cracked open is k / x.

From this you can calculate the pressure at which the valve will open.

Pressure (P) = Force (F => found from k/x see above) / area of piston / relief valve.

If there is a way of measuring the pressure at which the relief valve opens then it would be good to compare this with your theory!