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Discussion Starter #1
can someone confirm which outlet does front brakes, and which does the rear? I'm told rear does rear, but need to know for sure before I start plumbing, as I know some are the other way round.
761593
 

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1966 GT Fastback, 289, TKO 5-spd, EFI, 4-discs, TCP coilovers
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can someone confirm which outlet does front brakes, and which does the rear? I'm told rear does rear, but need to know for sure before I start plumbing, as I know some are the other way round. View attachment 761593
Open the cover, look inside and check the bowl sizes. Typically, the smaller reservoir is in the front of the M/C, which goes to the rear brakes. The bigger one, typically in the back of the M/C, is for the front brakes. Also, the brake lines are the same OD, but the nuts are two different sizes, with the larger one (1/2") belonging to the front brakes lines. It looks like that is what you have. Each front brake gets its own connection in the proportioning valve, while the rear brakes share a port on the proportioning valve, which then goes to a tee branch somewhere near the differential.
 

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Discussion Starter #3
I thought the bowl size would give it away, but they are the same. The rear outlet is larger, but the seller said the rear is for the rear brakes, not the front.
 

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I thought the bowl size would give it away, but they are the same. The rear outlet is larger, but the seller said the rear is for the rear brakes, not the front.
OK - I may have misspoke in my first message. I have attached some pictures of my setup. Below is the master and booster, with the smaller bowl in the front, but also with the large outlet fitting as well. I think I said the larger one went with the front, but that is wrong - sorry. Your larger flare nut fitting is in the back, so it may actually be for the rear brakes.

Master Cyl.jpg

Here is a view from underneath of the distribution of the lines from the master to the proportioning valve. Clearly, the small bowl / large outlet fitting services the rear brakes.

Brake Line From Prop Valve.jpg

Given that a) the bowls on yours are the same size, b) the larger flare nut is in the back, and c) your seller says the back is for the rear, that may be correct. My only concern (because I think I might have read it somewhere on VMF sometime maybe) is that the master push rod is designed to slightly delay delivery to the front bowl to give preferential braking to the front brakes. But this could be wrong, or dependent on the manufacturer of master cylinder.
 

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Discussion Starter #5
Thanks for that. So the consensus so far is larger outlet is for rear, whether it is at the front or rear?
 

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The rear piston in the MC is activated directly by the pushrod from the pedal. The front piston is activated by pressure from the rear piston. I would want my front brakes activated directly by the pushrod and the rear brakes activated indirectly.
 

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The rear piston in the MC is activated directly by the pushrod from the pedal. The front piston is activated by pressure from the rear piston. I would want my front brakes activated directly by the pushrod and the rear brakes activated indirectly.
That is probably more important than the size of the flare nut fitting, which would say that Billet's rear bowl should go to the front brakes.
 

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Discussion Starter #8
If that were the case, and the seal on the rear piston failed, both brake circuits would fail. I assumed it was one piston, with two staggered seals, or the rear piston directly pressed the front piston, pressure or not.
 

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If that were the case, and the seal on the rear piston failed, both brake circuits would fail. I assumed it was one piston, with two staggered seals, or the rear piston directly pressed the front piston, pressure or not.
The dual piston MCs that I have taken apart have 2 individual pistons and there is a coil spring between them. If the rear piston's seal fails the piston will move forward until the spring is compressed and then it will physically move the forward piston.
 

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Discussion Starter #10
So the rear (larger) outlet would do the front brakes. Makes sense, just the opposite to what the seller said.
 

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So the rear (larger) outlet would do the front brakes. Makes sense, just the opposite to what the seller said.
That seems to be the case, and it makes sense. The only thing that would counter that would be if the seller was also the manufacturer and had a logical explanation as to why the rear bowl should be the rear brakes. Not likely.
 

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That would look right at home next to an LS swap.
 
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