On 23/06/2019 09:42, Graeme wrote:
In message , Dave W
writes

I could try running the pump at 12v and restricting
the flow out of the pipe, but worry that could overload the motor?

It seems to pump air not water, so instead of restricting it, perhaps
you could add a hole in the tube to let out unwanted air?

It does indeed pump air, and I assume, although haven't looked, it is
just a basic 12v DC motor driving an impeller.

They are usually diaphragm pumps. The Brrrrrr sound is the give-away.
May be a motor and crank, may be like a door-bell, i.e. a solenoid with
a breaker switch on the arm. Screen washers are often impeller, note the
higher frequency sound.