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The Master cylinder you are using has a 2to1 ratio for fluid delivery. Meaning that twice as much fluid goes to the front brakes as does the rear. Sounds like you have discovered that you need more fluid to the rear. Wilwood has some that I think are 1to1. they use them in late models. Below are some questions I took off a tech site that may help you. Look for a master that is for disc/disc applications.
Can I use a disc/drum master for four wheel disc brakes?
Usually not. For 4 wheel disc brakes to function properly you will need a master that has a longer stroke.
How does a disc brake master differ from a drum brake master?
A drum brake master will differ from a disc brake master in two ways. The amount of fluid that a drum brake master has to move is less than disc brake. Drum brake masters have 10 lb residual valves at the outlet to keep a residual pressure on the drums. If you use a drum brake master for disc brakes you would move an insufficient volume of fluid and the disc brakes would drag because of the residual valves.
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actually here is the link to the data sheet for my master,it is 50/50
http://www.wilwood.com/PDF/DataSheets/ds487.pdf
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50/50 is the pressure ratio, You have a volume issue, and again it is a 2/1 ratio. twice as much volume going to the front than the rear. That works ok on a disc/drum set-up. It takes more fluid volume to operate disc brakes than it does drum brakes.
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ok,so how do i fix it so we can use all 4 brakes?
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Have you tried swapping the lines on the master cylinder? If this gives you more rear brake vs front then it will prove the volume ratio is off.
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a disc/disc manual brake master cylinder. go to your local parts house. You should take the advice from the post and switch you brake lines and confirm first. If that gives you rear brake you know that is what you need.
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i did switch,got the same results
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Wow, I just picked up an unfinished mustang project roller that came with discs on the rear, looks like a factory mustang master. you guys have me thinking I should just put one of my Drum rears on.
Being I run on dirt I dont see any advantage to a disc setup.