thanks for vacuum help, now....

I've got more vacumm than Hoover now. I appretiate the help. New problem....My 89 Sub' has a cooling issue. When I bought it last march, It had plenty of coolent and the temp gauge stayed at 180 when warm with very little fluctuation. No problems or changes all summer or fall...untill I had to run the heater a month ago. One day I was driving at 55mph, and I saw the water temp shoot up to 260(red zone). It stayed there a minute then fell to the normal temp of about 180-200 deg. Found this pretty alarming, and was home a few minutes later, checked under the hood and the coolent resivoir was empty. the rad still had soe in it, then added a gallon for pr-mixed coolent. Now when I drive at speeds of 30 up, it shoots up to 260 then drops then up again. It does this when I'm driving but at idle or low speeds the temp is perfect. I have overflow of coolent after a drive, and I have to add more mix regularly. Help!