Martin were those pumps you were using in your above described scenario positive displacement pumps? If so those should operate much differently than a mag drive pump with a fixed rpm, if you cut off the flow in a lcs with a mag drive the impeller keeps spinning and the pump will eventually overheat due to lack of circulation but a positive displacement pump (mechanically linked) will either burst a tube or blow a breaker (if it's appropriately sized) when you stop the flow. Both pumps are designed to operate against a load but unless the positive displacement type has some kind of rpm limiter it will just keep spinning up until something fails if it has too little of a load.