Coolant Leak

I went over tonight and looked in the riser to find... no coolant! Searching for a leak I noticed that the area around the expansion tank, near the hot water heater was damp. So, I refilled the riser with perhaps a quart or so of coolant to top it off, and started the engine. When the temperature got to about 150, the cap on the expansion tank began to leak. The cap was old and rusty, so it looks like the fix for our problem is just a new radiator cap.