What diameter brake line will work?

[porkchop]

I bought a bunch of line to hook up my in-line brake lock. But it was the wrong size. It is the size of the front line. The back is a little larger. Can I use the smaller line for the rear? With a smaller line you get more pressure right? Thanks.

[tomd]

porkchop, i can't help ya on size, but let me know how your line lock works out.

[Crazy_Jeepman]

I would not down size the brakelines. I would keep them as they are. I would think reduceing the size would increase line pressure but effect Proportioning valve, which I doubt would be good. I also think reducing brakeline size, while increasing line pressure but decreasing volume to the wheel cylinders would actually decrease brakeing power. Just my thoughts on it. ;)

[TexasJ10]

What size are they? I thought all brake line was 3/16.

[porkchop]

So did I that is why I bought the 3/16 line. When I got home I found out the rear line after the propotioning valve is about 5/16(?). I am thinking it is larger because it has to supply both the rear lines. Oh well, hi ho hi ho it's off to checkers I go........I will just pick up the same size in the AM.

[gsmikie]

rear line is 5/16

[Midnightwagon]

ok this goes along with some old guys theory id spill his name if i could spell it :confused: bernulli? but anyways smaller diameter=more pressure=less volume in the brake line you will have a pressure difference for the rear and it will affect the porportioning(?)(sorry cant spell) valve you still need to move the same amount of fluid to fill the wheel cylinder w/ smaller tubing it will take longer with the less flow and will take more pedal pressure to move it especially if you are bottlenecking it ( going from a large tube to a smaller size then back to the original size) so in my opinion bad idea (but what do i know by brakes suck anyways)

[kong]

midnightwagon I think is right on the brake line thingy.
Bernoulli (I'm not sure that's the right spelling) was a purdy smart guy.
This is my question now: do these changes make that big of a difference since brake calipers are self adjusting?
In this forum I have read about people putting Caddy master cylinders and boosters and such. That would also change the amount of pressure in the rest of the of the system. Others move to bigger calipers, which means the piston diameter is increased (which means you gotta generate more psi to move the piston the same distance).
So now my question is how much modification can you make before it starts making a difference?
I think that might have been the original question.... ;)
BTW, I'm purdy brain dead when it comes to brakes, so don't start quotin' me or nuthin' :D
I'm mighty curious to how this one works!

Kong
81Wag401

[ August 04, 2002, 10:27 AM: Message edited by: kong ]

[porkchop]

Ok I have started on the in-line brake and this is what I have come up with. The rear line after the proportioning valve is 1/4" and then it goes to the rear axle which then reduces to 3/16". So it is reduced from the factory. I used all the brake lines from the 79 when I put them in my wag. So I belive that the 3/16" line would have worked just fine. I got the 1/4 inch line any way. I will just use the 3/16 and redo the front lines after Ouray.

[TexasJ10]

Can one of the engineers on this board give me the rule of thumb concerning the amount of volume a round tube can carry as it gets larger? I guess that an increase of 1/16" in diameter doubles the capacity of the brake line to the rear.

[porkchop]

I got the brake lock in, just need to bleed the lines. It is going to take awhile to get the fluid back there. If my hand vac was working I would use that to pull it back but.....Oh well, I will just use the old fashion method :D . I should have a report on the in line brake lock tomorrow evening.

[lanierboy]

Speaking as an engineer, albeit electric, going to the smaller line will introduce more lag time from when you press the peddle to when the rear brakes engage. The amount of fluid that will flow through the line is directly related to the cross sectional area of the tube. If you weren't asleep in High School, you'll remember the the area of a circle is A = pi r squared. Therefore, going from 1/4 down to 3/16 almost halves (44% smaller) the cross sectional area and the amount of flow you get through the lines. Will it still work, Yes but the rear brakes will be almost twice as sluggish as before.

A smaller tube will increase the pressure differential from the front to end of the line only when fluid is actually flowing. Once the brakes are engaged and fluid is not flowing, the pressure is the same no matter what the diameter of the line.

Personnally, I wouldn't use smaller line. The brakes are sluggish enough.

[Kenall]

remember:

drum shoes need to move farther to contact the drums than do discs. therefore they need more volume. also, being drums, u dont necessarily need lots pressure cuz drum brakes have 'servo action' which imparts a naturally tendency of the shoes to be pulled into rotating drums. (this is y u have forward and rear shoes with different length lining material) something that caliper pistons dont have.

calipers need more pressure, not volume. cuz by the nature of their design, the pads are much closer to the rotors and less pedal travel puts the linings into the rotor.

since on PCs rig, like my own and most post 74s, the line from the proportining valve (PV) is large, then is reduced at the junction of the xmember that the upper shocks mounts are on.

think of the area of the large tube acting against the area of the small tube. the large tube fluid can move the small tube fluid under higher pressure than the other way around. this makes up for the loss of pressure that happens when using the larger tube from the PV to the xmember.

ur thots?

[ August 05, 2002, 11:02 AM: Message edited by: Kenall ]

[porkchop]

I think you have it on the money Ken.

I tried the brake lock tonight and it works great! I just tried it in the driveway since I still don't have a rear shaft in yet :( . When you push it it doesn't seem like it is pushed in but when you take your foot off the gas the wag gos no where :D . Sweet. Further testing will follow.

[Hump]

Don't know how I missed this one, because I just re-did all of my hard lines and I am an engineer! :D

Sounds like you have it all figured out though. The sizes on your rig are the same as on mine. Rear is 1/4" to the hose then it is 3/16" from the splitter to the drums. Both front lines are 3/16", and the banjo bolt is 7/16" which I am still waiting on Rusty to provide the correct banjo fitting for the stainless lines that I bought from him.

As far as the whole pressure thing goes, lanierboy was right about the volume. Pressure neither increases nor decreases just because there is a change in diameter. The velocity (speed) of the fluid is what changes because of change in diameter. The change in pressure would be a result of a change in volume. But since in this case the change in volume is directly related to the change in diameter, there would be a pressure difference. Not that you guys care, but just thought I would add that in there for future reference. :D