Lynn,
   I thought the general consensus was that the bottles were after the
   radiator
   and before the engine,  where the pressure was less.
   George ( down under)

That would have the bottle sampling the lowest pressure in the system.
Between the high restriction radiator and the suction side of the pump.
So the true system pressure could be higher than the cap pressure, and
so long as the engine is at cruise revs the cap would not be exposed to
the actual pressure.

I thought about having the bottle tap at the top of the outlet, before
the restrictor to get a more dynamic cycling effect, but then the cap
would be exposed to block pressures that are probably higher than any
pressure you would want to expose the radiator to. So then you might
need a 22 pound cap to prevent over boarding coolant at cruise, and then
at rest have less than 16 pounds in the system, and again have the cap
pressure disconnected from the actual loop pressure. My wife had a
Renault she loved. Had the best seats ever. It had no pressure cap but a
solid fill cap on the bottle, with a bright red line to indicate the
fill level,
about 1/3 full. The bottle was large in volume and system pressure was
whatever it might be. Controlled only by thermostat controlled
temperature. She had the oil changed one day and the crack maintainers
topped off that bottle. Then 4 miles up the freeway, the end of the
aluminum radiator with the plastic end caps blew off
lubricating the freeway and fogging all of the windows in the Renault.
Even a simple cooling system can have some traps built into it. Murphy
never sleeps.

Lynn E. Hanover