Two toilets flush normally but pump runs sporadically even though they aren't  being flushed.  we replaced the pump the clear box and the O-ring fittings.     no one seems to be able to figure out why the pump runs off and on.   Just had freightliner check for air leaks and all good there.  Any ideas?  

Thanks for your reply. We always keep water in the bowls.  Our last guess is that maybe one of the toilets need replaced?   

Click the Members tab on this website.  Select EDUCATION 

then select the VacuFlush instruction links.  

all that you need is there, including video tutoring, for a DIY repair.  

I have used this info myself and have saved hundreds of dollars in fixing the system myself.