I just purchased a 79 vette with soft brakes. I replaced all the brake calipers and pads. There are no signs of any leaks. I pressured bleeded the brakes. I know there's no air in the lines. All the pictures i've seen of vette master cylinders shows a cover with two round domes and two clips mine has a rectangle dome and a round dome with one clip. could it be I have the wrong master cylinder?

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I'm pretty sure you have the wrong MC. You should have the one with 2 domes. The one you have looks like it has a smaller rear reservoir. That could be the problem, but I'm not too sure about that. What shape is the power booster in??