Why would car run hot after putting new resivior and coolant disappears also?

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My car was running hot, found out the reservoir had a pin hole in it. Replaced reservoir, refilled with antifreeze, now the antifreeze keeps disappearing, Now the car is running hot again. HELP Don't know what is wrong or what I am doing wrong? Can't see any leaks under the car have put 21/2 gal of antifreeze in new reservoir and it keeps disappearing and car runs hot again.