Some background - '70 1600-2, new waterpump, some hoses replaced. Car was running ok but temp would climb on long drives before water pump was replaced. No previous coolant loss.

Now I am losing coolant but can't figure out where its going. No puddles under the car and I haven't seen any obvious dribbles on the engine. Have not noticed any white smoke coming from the tailpipe or unusual smells. I had a chance to start it up and watch the coolant level yesterday and here's what happened. Car started and idled fine and coolant guage stayed pegged at cold for 3-4 minutes, then shot up to about 2/3 on the guage in 8-10 seconds. The coolant level was still ok at this point, but as the guage temp came down to around 1/2 the coolant level started going down. (Or something close to that... I was trying to watch the guage, the coolant level and my 3-year old son at the same time!) I shut the car off when the top tank on the radiator was empty. Still no signs of leaks or smoke.

No sign of coolant in the oil and oil is clean and level stays the same. Also, the carpets are dry so I don't think the heater core is leaking. I feel like I'm missing the obvious here. I thought it was probably a head gasket leak but I'm not so sure. Any suggestions on what to check?

TIA,

Karl