little help please

[GREASEMONKEY72]

i have a 72 custom/10 with a stock 350sb and a stock radiator and ill fill the rad up and the will truck sit for a day and i check the level it will be like 1in or more down and the engine didnt run or anything it just sits so what i want to know is if anybody has ever had this problem with the level in the rad dropping before and what it is or may be?????? oh and it does it if the engine runs or not

-steven