A common problem for users of these instruments is to have a leak without any evidence of a puddle of fluid -- the leak is across the reservoir shutoff valve. The simple fix is to apply more torque to the valve stem.
Note that this article does not apply to the P5514B comparator. The model P5514B comparator has an improved valve that seals with an o-ring. See this article for more information on it, P5514 Replacement Parts
The problem arises from the type of metal-to-metal seal made between the valve stem and the seat. This is a common spot for operator-induced leaks, especially for new units. The seat has a squared off lip which is designed to bite into the tapered nose of the stem, making the seal. Doing this requires a higher degree of torque on the knob than most of us are used to applying. Most lab technicians have been trained to not use excessive torque on needle valves (and pressure fittings) because it will lead to premature failure. Therefore, in my experience, most operators of the P5514 / 15 do not apply enough torque to adequately seal off the pressure. This leads to pressurized fluid leaking back into the reservoir. The image below shows the contact points of the valve seat with the stem.
Therefore, when this happens a two-step solution should be applied:
a) remove the valve stem from the reservoir and inspect both the tapered nose and the seat for evidence of particulate contamination. For example, a metal shaving, a grain of "grit", or any other type of material that would prevent the valve from fully sealing. NOTE: it is normal to have a groove around the tip of the valve stem as this is where it seals with the seat. You don't want to see an intersecting cross-hatch type of dimple or scratch as this represents a flow path for a leak.
b) reinsert the valve stem and apply more torque when closing the valve and retest. If you think that you are applying sufficient torque and the leak remains then apply more torque. It is difficult to damage this type of valve seal from torque alone.