Need some help with Brake issues

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Here is a video of the issue, first burp pushing pedal down, once its bottomed releasing causes the 2nd burp. No air in the system.
 
Anyone know what the issue was with the master or what might be causing it to do that?

I sent the master back today, ordered one from Classic Ind, along with a Combination valve.
 
C2ndLTpigeon said:
Something had to be causing the fluid to go low in the front to higher in the back reservoir, that cant be normal
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The 2 cylinders are internally tied together via passages. If the pistons are at the wrong places they could pump fluid from one resivour to the other.
 
Dana67Dart said:
The 2 cylinders are internally tied together via passages. If the pistons are at the wrong places they could pump fluid from one resivour to the other.
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Changed the master with a new one today same issue but not as fast. I bought a new combination valve but tried the master first. Not sure what else it could be? This is the same master cylinder that is on my buddies 75 duster with power brakes and his works fine.
20210922_174907.jpg
 
C2ndLTpigeon said:
Changed the master with a new one today same issue but not as fast. I bought a new combination valve but tried the master first. Not sure what else it could be? This is the same master cylinder that is on my buddies 75 duster with power brakes and his works fine.
View attachment 1715794512
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Did you ever solve your issue? If so, how? Inquiring minds want to know!!
 
Demonx2 said:
Did you ever solve your issue? If so, how? Inquiring minds want to know!!
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Yes I did!
Ended up being the proportioning valve. I purchased a new one from classic industries. When I got it I disassembled it to make sure it had no burrs and give it a clean. I then pulled the old one off and it had markings like it was rebuilt. Took it apart and most of the internal valves and sprigs were gone! and on top of that it was filthy. So this was the reason it was not functioning correctly. Kinda surprised it my brakes even functioned for the year I drove the car.
 
Interesting! I'm in the middle of doing all new brakes/lines/etc on my car hence why I asked. I'm not changing the prop valve...but maybe I should. We'll see. My pedal goes too low before it gets firm so I've got an adjustable pushrod coming for it. We'll see if that helps anything. It's an original 4-piston K-H power disc up front w/ 10" x 1-3/4" rear drums but the power booster is removed. Putting new rear wheel cyl's on it - trying to decide if I should stay with the 15/16" bore like original or drop down to the 13/16" like some people suggest to reduce the propensity to lock up the rears. It has a new 1-1/32" master cyl on it.
 
Sure was, but I learned something new. At minimum I would take yours apart, very simple design. Clean and inspect it.
Good luck with your project!
 
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