The cap has two valves in it. One is the rubber faced spring loaded metal cup that holds the 13psi radiator pressure. In the middle of the wet part of the main part of the cap is a small dime sized metal disc. This is a one way check valve that holds radiator pressure with the main valve, and under very light spring loading, allows the partial vac in the cooling system to pull water back in from the recovery tank. Remove the cap and make sure the little disc can be lifted with a fingernail. Make by blasting with a hose that the line from recovery tank to radiator neck is clear and not obstructed by the cap seal. There are two seals for the cap. One seals the top of the neck with a positive clamp seal as the cap is tightened, and the other seals the lower radiator neck with spring pressure up to the cap pressure rating. The lower seal is broken by pressure as the radiator vents to the recovery tank in a temp overheat or hot soak, and the little central valve lets the water back into the radiator as the system cools and the partial vacuum builds.

I bet the bottom hose is getting weak. 10yrs is way too old.