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Discussion Starter #1
I've searched a bunch of earlier posts and have been finding some conflicting information, so I'm looking for more opinions. I'm using a manual Disc/Drum set-up on my 69 Camaro street/strip car and plan to install an adjustable proportioning valve on the rear line. The one I purchased has 3/16 fittings. A lot of the posts mention using 1/4" line to the rear brakes, and my new rear flex line is 1/4" (splits to two 3/16").

Should I just run 3/16" line all the way and use an adapter at the flex line?

Would I be giving up any braking performance by not running 1/4" to the rear?

If so, would anything be gained by adapting down to 3/16 and back up to 1/4 at the prop block? Seems like the restriction down to 3/16 would negate any benefits of the 1/4 line.

Is there even enough difference to worry about between the two sizes?
 

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Jeff,

As far as I know, drums in most cases use a 1/4 line to the rear.
I believe it's due to the volume requirement of the rear wheel cylinders. They must travel a fair bit before the shoes contact the drums. Both 3/16 and 1/4 line both carry a high pressure, but at a lower volume with the smaller line. The drums would simply take too long to actuate in relation to the front with a small line.

Disc rears can use a 3/16 line because the pistion only must travel just slightly for the pad to pick up the rotor. The volume is not required but high pressure is. For any given pressure at the master, a larger line will see less pressure (but more volume)at the wheel cylinder than a small one. Hence the small line size requrement for discs. Disc calipers need higher pressure than wheel cylinders due the the abilty of drums to "Self energize". A good bit of drum line pressure is simply used to overcome the spring resistance.

I'm surprised a Disc/Drum prop valve has a 3/16 outlet. But I'm fairly new to this. I think you'd need a 1/4 outlet. I don't think adapting up would give you the volume required, the restriction present already negates that.

All in all though, I've only ever heard the cardinal rule of sizing down to 3/16 from 1/4 when converting the rears to disc. Never heard of anyone going back to drums on a 3/16 line! Any street car I've ever seen with drums had a 1/4 line. Perhaps it will work. More expert advice needed!

Cheers, Mark.
 

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Discussion Starter #3
I did some searching and most adjustable prop valves have 3/16" fittings. I thought maybe I'd get some different fittings for the valve to allow use of the 1/4" line (if they are available), but the orifice size in the valve doesn't appear to be much if any larger than the orifice in the 3/16 fitting. So it appears that the valve would still be a restriction, even while using 1/4" line. Does anyone have any opinions on if this will be a problem and if I'd be wasting my time running 1/4" line to the rear because of the valve restriction? I'm thinking it would reduce the rear brakes to some degree, even when set to full-open.

Maybe it'll be fine, I was just hoping that someone else might have run into this and had some advice. It has me wondering if I should skip the valve and see how it behaves without it. The car won't be running for a couple months yet, so I won't know anything for a while. I'd just like to get the brake system finished up ASAP.
 

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The majority of the rear disc conversions I have seen use the factory size drum rear lines with a wilwood style adjustable prop valve right off the master cyl. Just get the proper size fittings and you're good to go...
 

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when i bought my front disk conversion kit the adjustable porpurtioning valve has the bigger fitting but the line they gave me was the thinner line. so you just need an adapter to go from 1 to another. running a bigger line to back wont change nothing since it will go to small line. amigine running a water line 5 inch thick to a garden hose going down to 1/2 inch same pressure once you go to smaller line. thats wht valve is adjustable. you want a 60 on front 40 on back. slam on brakes til all brakes skid the same marks on pavement all set than
 

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Discussion Starter #7
Thanks for the input everybody. I talked to a hydraulic engineer at work today and explained my situation. He too thought I would be OK using 1/4" fittings in and out of the prop valve to run the 1/4" line and suggested the 1/4" line would probably be better than using 3/16" to the rear. He thought it would help make up for added distance away from the master cylinder, compared to the front brakes. It just seemed strange at first to step it down so much at the valve. I guess I'm more used to thinking in terms of flow, such as with a fuel system, not pressure as in the brake system.

Sounds like I'm good to go. I just need to finish bending up a 1/4" line and find a place to mount the valve.

Thanks again

Jeff
 
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