K seal is marvelous stuff if used correctly, there are two types, one which you have to add to the coolant with the existing antifreeze and the other you have to completely drain and flush your system and then add the solution to just a water based system only. Drive around for a while and then empty the system, fill with correct antifreeze/water mixture. If you dont do this then K-seal and antifreeze will go globular and block rads etc. This is just not for old engines, I use it in my RX7 engine as a pre-cautionary measure as they are prone for water seal failures due to corrosion. There is lots of info on this stuff and it can last for a very long time and not just a short term fix if you have had an internaL leak of some sort. Personally if your fault is down to a head gasket then i would endeavour to get that fixed.