The second
head is on its own vacuum
generator.
First step is to examine the hoses from the head to the tank. You might be able to hear the leak.
Second is to replace the duckbill valves in the vacuum
generator. There are 4 and they are
sold in kits of 2. (BTW the pump-out
pump only uses 2 duckbill valves)
You might as well replace all 6 duckbill valves in your system while you are at it.
Do place some disposable pads under the generators when you open them up..... And wear clothes that you do not mind getting soiled.
Just did this on my system. Expect $22 for a set of 2 valves - Shop around for the best
price.