[stangfreak]

agreed. check the radiator cap first. make sure to by one that is the correct pressure (since boosted you may want one a few pounds more.) if you are still leaking then get your system pressure checked. as for leak down test that is for the cylinders .... don't really think it's related at this point. You said you took it to the shop and they ran a vaccum and it still over heats? did it take a bit longer or was it immediately overheating? is it flowing or seeping? more details pls.