Gilera Runner SP70 bigger cooler, water pump not flowing in the tank

Hi all,
I've recently changed my 2005 SP50 cooler for a bigger one with a fan, maybe from the SP125.
I also have a flow cooler in line.
Now when I give it throttle there is no waterflow at the top of the expansion tank.
The waterpump is rebuilt some 400km ago
While driving the cooler and hoses get hot, the temperature on the gauge stays down (sensor and thermostat are new).
I'm not using the termoswitch on the new rad only manual switch.
Could it be that I got the hoses on wrong.