Anyone else had problems with gilera runner cooling?

I have an old gilera 180, liquid cooled. The cooling system always acts up and I've been having problems with it for a while. After giving it quite a bit of throttle coolant starts to drip from the cap. I've had it rebuilt at a local workshop, however, It doesn't seem they knew what they were doing, so i might need to rebuild it myself. What things should i check?

 

Don't overfill the expansion tank for a start.
If it's too full then the water can't get back into it and the pressure pushes it out the cap. It's the only escape for it.
As long as you can see the water flowing back into the tank through the nozzle about halfway down the filler neck, the pump is working properly. If the pump is working properly then it should be cooling the engine properly.