manual brake system in my 63 Ford

[GS455vert]

I need to tap into the vast knowledge base of this forum. I have a 63 Fairlane that I have done brake upgrades to. The front, atlreast for now is a 11" Granada swap, using a large bore manual brake master (may upgrade to a Willwood master), and plan to use a 11" Willwood rear disc kit. I was wondering if 3/16 line size is adequate, or should I go up a size? Any help would be great!!!!!!!!!!!

[Jetfixr320]

Larger brake lines would cause lower pressure from more area in my thinking.
I have manual brakes with Disc brakes front and rear on my 65 Falcon. Stock size brake lines.

[MrQuick]

3/16" line should be fine. We have had no problems running it.
The master cylinder match to caliper piston volume is more crucial.

[Apogee]

3/16" line should be fine. We have had no problems running it.
The master cylinder match to caliper piston volume is more crucial.

X2...line size is actually somewhat irrelevant to system pressure due to basic hydraulic principles, even though it may seem counterintuitive at first. Only the master cylinder bore area and caliper piston area is important when determining your hydraulic requirements. In general, most performance disc/disc applications are best plumbed with 3/16" tubing everywhere since it expands less under pressure than a larger diameter, and hence circumference, line size such as 1/4".

For those who want to extrapolate that thinking further and propose that if 3/16" is better than 1/4", than 1/8" or 1/16" would be even more so, right? Well not so fast. You can and will reach a point of diminishing returns where the frictional losses increase, as does the chance of spontaneous cavitation through the fittings due to the increased fluid velocity inherently caused by a reduced cross sectional area for flow. Needless to say, spontaneous bubbles are never a good thing in a brake system.

Tobin
KORE3

[ravege]

Correct me if I'm wrong, I thought the 1/4" vs 3/16" was out of the rear T. 1/4" in, two 1/4" out and you have half the pressure of the T input at the corners. 1/4" in and two 3/16" out and you have slightly less, but not near half, the pressure into the T.

[GS455vert]

Thanks guys. I kind of figured that it would be fine with the 3/16. I'm finishing off some of the final plumbing at the master/proportioning valve and wanted to cover my bases and not find out later that a better option existed. The master is a larger bore manual brake unit from another Ford product of the day. Any other stuff that I should be thinking of ?

[Apogee]

Correct me if I'm wrong, I thought the 1/4" vs 3/16" was out of the rear T. 1/4" in, two 1/4" out and you have half the pressure of the T input at the corners. 1/4" in and two 3/16" out and you have slightly less, but not near half, the pressure into the T.

Again, line size does not impact pressure. One 1/4" line at 500 psi will feed two 3/16" lines at exactly the same pressure, 500 psi. All fluid within a given continuous hydraulic circuit must all be at the same pressure as defined by Pascal's Law. The reason most GM cars used a 1/4" hard line front to rear was due to the fact that most GM cars came equipped with rear drums which operate at lower pressures and require more volume than most modern rear disc brake options. Line size really only impacts fluid velocity and system compliance, the amount the lines swell under pressure, since volumetric expansion is a function of the circumference of the tube (basically deflection measured as inch/inch).


Thanks guys. I kind of figured that it would be fine with the 3/16. I'm finishing off some of the final plumbing at the master/proportioning valve and wanted to cover my bases and not find out later that a better option existed. The master is a larger bore manual brake unit from another Ford product of the day. Any other stuff that I should be thinking of ?

I'm still a little confused...most manual brake master cylinders have smaller bores than their power brake counterparts. IIRC, the power brake MC bore size on the mid-70's Granada was only 15/16", small by most standards, however without more info regarding your pedal ratio, booster size, etc, I have to assume that it was appropriate for the application.

Tobin
KORE3

[ravege]

I was thinking of the formula F=pA. Increase area, and for a given amount of force, pressure must decrease. The T, with input and outputs of equal size, effectively doubles the area. Take a garden hose, turn it on and see how the water comes out. Now put your thumb over the end, decreasing the area of the nozzle, is the flow the same? Isn't this the principle behind using smaller master cylinders for manual brake systems, decrease the area and for the same amount of (foot) force, increase the pressure?

[Apogee]

I was thinking of the formula F=pA. Increase area, and for a given amount of force, pressure must decrease. The T, with input and outputs of equal size, effectively doubles the area. Take a garden hose, turn it on and see how the water comes out. Now put your thumb over the end, decreasing the area of the nozzle, is the flow the same? Isn't this the principle behind using smaller master cylinders for manual brake systems, decrease the area and for the same amount of (foot) force, increase the pressure?

You are correct that a smaller bore MC equals more pressure for a given pedal force. As for line size, the system will push as much fluid volume as is necessary based on the master/slave relationship so long as you don't run out of pedal travel, so line size is irrelevant. While F=p*A, that only applies to the master and slave of the system, not the lines connecting them since those don't impact the operating pressure of the system.

For instance, say you apply the brakes on your '69 Camaro with manual brakes, 6:1 pedal ratio, 7/8" bore MC and C6 Corvette brakes. Let's assume that the pedal travels 4 inches with the amount of effort applied, which would equate to 4/6 = .67" stroke at the MC taking the pedal ratio into account. This means that the 7/8 bore MC with ~.60 square inches of piston area pushed a fluid volume equal to (.60in^2)*(.67in) = .402 in^3. Now, whether you have a 3/16" hard line between the MC and calipers or a sewer pipe doesn't matter. You push .402 cubic inches of fluid in one side of a closed system, the same amount will exit the other side. This holds true for a front or rear tee-fitting, because while the effective outlet area may be double the inlet area, the effective outlet flow is half the inlet flow and the pressure remains constant.