Ues, most folk think the valve core is the thing that stops the air leaking out, which it does at slow speed. But with the capability of modern machinery, the valve can open under wotsit force as the wheel rotates and if you don't have valve caps...
I happen to have a wheel with two valves fitted (don't ask) and took the core out of one and put the normal plastic cap back on and it held 3 bar with ease.