VacuFlush cycles

[chesapeakedan]

I have a SeaLand 848 toilet. It functions fine except that every 10-15 minutes the vacuum generator cycles indicating to me that it is slowly losing vacuum somehow. I have no problem with the toilet; it does not lose water. I believe the problem is with the vacuum generator/tank. I removed the unit and disassembled and cleaned it out. The bellows does not appear to have any holes. I suspect the duckbill valves, although I don't see anything blatantly wrong with them. My question is this. Should the duckbill valves be completely closed or is a slight opening in the valves (just enough to see a sliver of daylight) normal? At $20+ for a set of two new valves, I'm hesitant to spend $40-50 to replace 4 valves. I appreciate any advice.

Thanks
Dan