See the below (and attached) files for images of PPC control and isolation valves. The valves can be cleaned with a water/alcohol mix or a non-aggressive solvent. The last image is a close up of the sealing pad on the spring and has contaminants on it that can cause a leak. Scrape them off with your fingernail and/or with a wet lint-free towel.
See this PPC3 article for a schematic and tips for PPC3. Note that the valve numbers are the same for these models, PPC3 Control Module Valves
See this PPC4 article for a schematic with valve numbers, PPC4 - Schematic with valve numbers
See this article for contamination prevention tips and options, Contamination Prevention of Gas Operated Pressure Controllers / Calibrators