I have before just removed 1 of the coolant pipes to the throttle body and pushed in a valve secured with a jubilee clip to make airtight then applied pressure from a hand pump...really any way of getting air under pressure into the coolant space will work but dont exceed 12/14psi.
As high as that? I feared that might blow off or burst a hose. The old metal caps had a blow off pressure stamped upon them, but the yellow plastic caps on the Vectra and Astra are less helpful.
The old metal caps were stamped around 5 PSI I recall, which I could arrange by hanging a funnel full of coolant high in the garage roof. 14 PSI is nearly atmospheric pressure of 15 PSI, a head of 32 feet of water or 30 inches of mercury.
all the cooling systems I've worked on run at about 1bar - about 14psi for those stuck in the 19th century -, which includes several cars designed(and a couple built) in the fifties.
Drill a hole in your 'radiator' cap to fit an entire tyre valve, fit the valve into the hole in the same way it fits a wheel, and seal it with your favourite flexible gunge. Pressurise the system with a manual pump; a bike pump will work well. Check for leaks by listening, and spraying suspect areas with soapy water. When you've fixed all the leaks, fit a new radiator cap and store the Cooling System Pressure Test Adapter Thingy(CSPTAT
) with all your other car junk. A battered cardboard box kicked under the bench is the traditional way.
You can test radiators off the car by cutting a mountain bike inner tub cut opposite the valve, and clamping it to the radiator stubs with hose clamps