Rebuilt Master cylinder? wrong one..

[bruce1972c10]

Hello!
I have a 1972 chevy c-10 350, power brakes, front disk brakes.

This weekend I found out my MC was bad, I had no back brakes.

I got a rebuilt unit, it was a different but I was not sure if the one that was on was the right one or not.

I was looking through a parts book and saw the new one should be for Manual Brakes, it has a larger front reservoir then the rear, the one I took off had them botth the same size, and that one looks like it is the correct one for Power Brakes.

I assume I should get the correct one, it does work, is this a bad thing?

What will happen if I do not change it?

Thanks for any help
Bruce