coolant res. is leaking around the bottom somewhere. Is it a hose or does the tank get cracks?

 

Usually it's the tank, but better check first. The hose is cheaper.

Tank, cap and level sensor can be had for about $150 if that turns out to be the problem. It is an easy DIY fix.

 

Most tanks have leaked by now,mine certainly did.Sneaky leak at the back where you can't see it too..took ages to track it down,as the coolant ran down the firewall and evaporated.
Easy to fix..just add a tank. (brought one on ebay) and found a cheap good s/h one just in case.
Take it off and test the sucker is easy too. Good Luck.

 

Cooling system pressure test equipment is generally available for 'rent' at your local auto parts stores. Pump it up to 15 lbs and the leak source will be obvious. On two cars I have now had 5 reservoirs so it is very common problem. I tried a used one but that didn't work out well.

 

When you do this test do you get it up to 15 lbs and it should hold there, not go down a lb then stay there? How long should it hold pressure, until you take it off?

 

The pressure cap may be 14psi but it is off and you are pressurizing the system at that point. If you pump it up to 20psi it should hold there. A leak between the test equipment to reservoir connection or a leak in the cooling system would be the reason for the meter to loose pressure.