The MPV is a spring-loaded device just downline form the AOS. The MPV is factory set usually at around 30-40 psi. The system will use this minimum pressure to push the hydraulic oil through the compressor lubricating and cooling system. At minimum pressure, there is no air available to mix with the solution in the hose. To make CAF, the pressure must be above the minimum.
The MPV REQUIRES regular operation to keep the spring and plunger from seizing. It will generally seize in the minimum pressure position. Enough air will pass to push the hydraulic oil, but the seized MPV will not allow any additional air to pass to make CAF. The most visible symptom of this issue is noticing no air/solution mix in the hose. Solution only will flow, yet all the pressure gauges are functioning properly, including showing pressure above the minimum. It is imperative that the system be operated regularly (according to manufacturer’s instructions). USE IT OR LOSE IT!!! A seized MPV is not fixable in the field and will require your F-7 Certified EVT mechanic.
