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.

I have an old Silca floor pump that's used 3 or 4 times a week for at least 10 years and is only on it's third chuck washer.