An improved manifold assembly useful in patient ventilation/aspiration systems includes a stop cock valve with a hollow stem and a transverse port arranged to permit the stem to be rotated between open and closed positions. In both positions, a ventilating pathway is maintained through the stem between ventilator and patient connection ports of the manifold. In the open position the stem provides a travel pathway for a catheter between the patient connection port and an access port of the manifold. In the closed condition, the stem seals this travel pathway. The improvement provides an arrangement of structure operable to resist imparting shear damage to an inserted catheter, and to provide enhanced feedback to an attendant, as the stem is manually rotated into contact with the catheter.