What should I do
 

I currently did the rear suspension on my 68 firebird and while I was back there I put a set of wilwood dynapro on the back. It's originally a manual drum car on all 4 corners. My question is what would be a good master cylinder to go with until I get disc brakes up front. I do want to drive it some till I get front brakes. I probably should of done disc up front first but o well.

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Not optimum, but will work...
 

You can leave your existing MC in place and then install an inline adjustable proportioning valve somewhere between the MC and the rear brakes (which you will eventually have to do anyway) like this one:

http://www.hydratechbraking.com/mast...7DE11142513E51

This way you can adjust what the rear brakes will be doing by comparison to the front drum brakes. It will be drivable, but not optimal, suggesting you get the front brakes updated ASAP of course...