Brand new master cylinder leaking from the lid

[76JimmyBlue]

Hi everybody, having a really frustrating problem here. I replaced the original master cylinder on my '76 K25 with a new AC/Delco one and it leaks from the lid at the gasket. I swapped the bales from the old one and it seems to hold the lid tighter, but still leaks. Either the gasket isn't thick enough or the lid is bent or something. Worst case is the machining of the casting was done poorly. Doesn't look as good as the original M/C, but a straight edge doesn't show any gaps that would cause a leak. Getting REALLY angry at this truck so please, any help is appreciated!