Those of us who have the Blake LAVAC Vacuum Head systems with the manual pump should take the following precaution.
Most pump out stations are blind uncaring monsters, when they work correctly, and the people who run them are ignorant of their (the equipment's) brutality. These pumps have tremendous suction, which can collapse your holding tank and crack the head seat cover, under certain conditions. The operators are unable or unwilling to moderate this, so you must protect your system.
Open the head cover and pump it down before they start to pump out your tank.
If your holding tank vent is restricted, the vacuum will be relieved through the open head.
I try to restrict the suction by partially closing the valve on the pump out line, but the operators have a fit about this usually and insist that it be left wide open.
One day I will install a larger vent on our holding tank. In the mean time I make absolutely sure that this vent is not clogged which it could easily be.