How long has it been since the duck-bill valves were serviced in the vacuum generator pump? #1 cause for this problem.
Yes, probably the duck bills - 2 upstream and 2 downstream. Lube w/ Vaseline, make sure you have them oriented correctly, and don't tighten the connections too tight. If that dosen't fix it, you may have a leak at the vaccum pump- you should be able to hear it. Replacing the vaccum pressure switch is easy. Just remove the wires, unscrew the old one, lube the new one w/ Vaseline, and screw it in - again, not too tight.
Use superlube grease instead of Vaseline. Vaseline is petroleum based and can effect the rubber seals and duct bills over time.
