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This could probably be moved to Newb...but here's my $.02.

It's a generalization that disc conversion require more volume and like most generalizations, it might be correct as often as it isn't. The obvious difference between discs and drums is the bore size difference between typical calipers and typical wheel cylinders. The stroke of the wheel cylinder, adjustment of the drum brakes, health of the residual valve, etc., all play a part in how much volume is actually needed. The same goes for calipers as not all calipers are created equally. Some have low-drag seals that retract the piston further to reduce brake drag and improve mileage. These require more initial volume than calipers without this feature in order to egage the pads and hence are best used with stepped-bore master cylinders (quick take-up).

I think it's fairly well accepted that disc brakes require more pressure than drums to work effectively. This is due in part to the smaller pad area relative to shoes and the fact that calipers are not self-energizing in the way that drum brakes are once engaged. Most systems are designed around 600 psi for drums and 1000 psi for discs. Higher performance disc systems with more rigid caliper designs approach 1500 psi.

So that's a gross oversimplification...and I didn't really answer your question...but this is Pirate :flipoff2:

If you're really interested in learning all about brake systems...keep researching. There's a lot of good info out there. :D
 

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onetonbb74 said:
That is why i change the factory 1/4" rear drum brake line to a 3/16 line for the disc brake conversion. On the same master, the 3/16" line will have more pressure and the 1/4" line will have more fluid volume. I never thought about it much as i just bolted everything on, and replaced all brake lines and parts and never had any problems after the first bleed.
FYI - Line size has nothing to do with pressure. You can run 2" DOM for your brake lines if you want to...fluid is incompressible and therefore the volume in always equals the volume out. The only thing line size effects is fluid velocity as based on the volumetric flow rate of the system. Therefore, brake fluid in a larger brake line will flow proportionally slower than fluid in smaller brake line as related by the cross-sectional area of the tubing (A=pi*r^2) but everything else is the same for a given system.

As within any system, there are losses due to friction and other factors. Larger lines can reduce those and also due to the slower velocities, can minimize cavitation through fittings which can create miniscule air bubbles in the lines (very similar to propellers, impellers, pumps, etc). While I'm sure that there were reasons to run 1/4" line in some OE applications, I've never had a problem running 3/16" pretty much everywhere.

6869704x4 - I didn't mean to suggest that I thought it was a worthless topic when I suggested that you move it to the Noob section...just that it is fairly basic general brake info and that's where I would search for it if I were looking. And you're right...there's a ton of misinformation out there on brakes, but there are also a few members and vendors on this board that really know their stuff.

As for the low-drag calipers, they've been around for decades but never really became prominent on the American iron until the 80's when the manufacturers were being pushed to decrease their mileage averages (CAFE). If you search Pirate under QT, quick take-up, stepped bore, dual bore...there are actually several threads discussing the potential benefits of such a setup, whether you're running standard calipers or ones with low drag seals. Common sense tells you that a large primary bore will help to keep a high pedal and that a smaller secondary bore will aid in developing higher pressures. It's a simple concept that minimizes the drawbacks associated with straight bore master cylinders.
 
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