Originally Posted by HillRider
Are you describing the Silca Floor pump? The Presta-Only chuck is a friction fit on the valve stem and relies on the tight rubber washer in the chuck to keep it in place. If the washer is in half decent shape, it will easily reach 120+ psi with no blow-off problems. When the washer is badly worn, it can be replaced for about $1 at nearly any bike shop.
There's actually a bit more to it than that.
That big brass "thingie" that holds the rubber washer in place is threaded. When the chuck is new, it's not threaded on very much. As it wears so that the chuck fits onto the valve stem too loosely you can tighten it up about 1/8 turn and make it work for a very long time.