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I am in the process of building FFR#3298. It has IRS. I know there have been comments on the rear brakes not having full braking ability. Some solutions mentioned have been power brakes, 84 Jeep manual brake master cylinder, high quality brake pads, etc. I am building the car with the 84 master cylinder and have the CarboTech Panther Plus brake pads all around. I have also installed a proportioning valve in the rear brake line.

While laying out the rear brake tubing, I noticed that the original SuperCoupe calipers had 1/4" OD brake line tubing. The build manual calls for 3/16" OD tubing.

Do you think there is a correlation between not having enough braking pressure to the rear brakes and using 3/16" OD line in lieu of the stock 1/4" line?

Could it be possible that running 1/4" OD tubing for the rear brakes could improve the braking at the rear?

Although I have already installed the 3/16" tubing, I would be willing to install the larger 1/4" if it would make a difference.

Let me know what you think.

Thanks,
Tim
 

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Tim,

I went through the same thing with 3641. I decided to stay with the 3/16" tubing. If anything I want less braking from the rears not more. Also you will need to move more fluid with the 1/4" tube which means your MC may or may not have all it takes. I went with the Powermaster with a Wilwood prop valve and rotated the calipers to the front of the rotors to make it easier to put in a Vintage sway bar.

Ben
 

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By using the 1/4" tubing in the rear you will reduce the amount of piston travel to the rear calipers over that of the 3/16" hard lines.

Saleen did this years ago on the Mustangs they converted using the Lincoln LSC rearends. Those cars came with an adapter and 1/4" lines. The Master Cylinder was larger to accomodate the increase in front caliper piston size from 52mm to 73mm in design. The MC with a greater piston size has more fluid displacement. When using this in conjunction with a much lighter Mustang versus the Lincoln LSC, the rear brakes would lock-up causing the car to spin.

This necessitated the use of larger rear lines and an adjustable bias valve to get the front to rear differential correct. Now I have swapped the rear IRS from a Cobra and used 3/16" lines with -3 Stainless Steel hoses, I have a much higher clamping load on the rear brakes. When i upgrade to the larger Brembos up front, my bias will be perfect.

I would suggest using the 3/16" line and having an adjustable bias valve to make the system perfect for your driving. By going large initially you reduce the overall rear braking effectiveness you could achieve and also reduce the effectiveness that adding a bias valve would give you.

Hope this helps.
 

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Great question on the lin diameters.

Guys, Assuming brake fluid to be incompressable, the diameter of the line between the master cylinder and caliper has no effect on the relative pressures or the displacement. It is strictly areas of the cylinder and caliper bores.

That said, there must be a reason why Ford went to the larger ID line to the rear of the T-Bird. My guess is that it has to do with the ability of the ABS system to cycle the pressure at the front of the car quickly, and have it result in pressure cycling at the back of the car - the larger line reduces flow restriction.

So if it is for the ABS, and you've defeated ABS -probably no good reason to run the large ID line, other than it fits the fittings better in the rear.

BTW Mustang Cobra rear calipers are the same part as the 89 - 93 T-bird, and the Cobra used the 3/16" line.

Mark Reynolds
Breeze Automotive
508 533 6455
 

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I am using the same system you are planning to install. I can lock up my rear brakes just fine. I can then adjust the Prop. value and reduce the braking force on the rear for even braking on all four. I read alot of post on this subject and wonder why? I think the guys that have the most experience with braking are the Spec series drivers. I talked with one driver this weekend and he runs the standard Mustang 5.0 braking system, no boost. He stops just fine and has to lots of opportunities to use his brakes.

San Rafael Mike
 

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Mark is 100% right, (in general) line diameter has no impact on braking ability - you'll just move fluid more slowly through the 1/4" line vs. the 3/16" line. The piston size and relationship between MC and caliper diameters determine braking feel, pedal travel and effort.

Larger diameter brake line has the effect of better pulse damping and more fluid capacity for power brake and ABS systems. There's a good writeup on this topic on one of the racing brake mfr's Web sites - Brembo or Alcon, I think (but not sure).
 

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Mark and Barry are correct.

For what it's worth, I built my IRS car with 1/4" rear lines, but changed them to 3/16" when I upgraded to stainless braided brake hoses.

Mike, Spec Racers have drum rear brakes, no?

Forrest
 
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