This is the second time in 7 years that the holding tank has imploded on me while pumping it out. Am I the only one to whom this has happened?

Thanks for your help gentlemen. I gather that the vent isn't sufficient when the holding tank is pumped out. Indeed, by directing the head diverter to the holding tank, everything will sucked out and eventualy even what's in the head, therefore able to suck thrue an 1.5 inches hose at all times. Now, let's remember to properly direct the diverter.