Coolant Leak?

Brett - from my automotive experience;

Get a friend(s) to assist - this way  you can have eyes on the cooling system, while one person does the pressurising.

Clamp off your overflow line befog you start

You don't need much pressure - a repurposed plastic cap, that just fits inside/outside the expansion tank top, with a hole that just fits your air gun, can be placed/held (hand pressure) over the "expansion tank" while gently pressurising the system. The cap only needs to make a partial seal ie it does not have to be a good fit. Remove a little of the coolant before pressurising. 

The additional eyes can be searching for any leaks, while the pressure person concentres on their activity.