It is frequently hard to find a pressure tester that will fit the Discovery II expansion tank. An adequate tester can be created with a bicycle pump with a guage, a presta valve, & some hose and clamps.
1. Obtain a presta valve stem. They are common on road bikes. A longer one is better. See your bike shop for an old tube.
2. Bike pump with guage. The guage is important, need to keep the pressure down.
3. Short hose and clamps.

The throttle body cooler is a good place to attach the tester.
You may want to wrap the pesta valve stem with tape to get a good seal. Remove the hose from the thottle body. Replace hose with a short length of plugged hose. (a bolt works good.)

Clamp the valve stem in the other end of the hose. Attach the pump. Pump carefully-15 psi should be enough.

 

Believe me or not, I did a similar tool to test my cooling system, but I have a Disco 1 and I will use the radiator cap to install the valve. I also used an old bike tube but I will use a tire pump with gauge.

I have not had time to test the system, then I do not know if it will work.

Did it work for you?

 

The bike pump method is not good for holding pressure over time, but it does a nice job of locating leaks, especially if you have two people, one to search the other to maintain pressure....