Primary or Secondary Brake Lines?

I read somewhere that the primary outlet of the master cylinder is supposed to be connected to the front brake lines and the secondary to the rear. If that is correct, is the following picture showing an incorrect setup? Looks like it is reversed to me. Does it really matter?
http://flickr.com/photos/74334622@N00/15163813/

 

Rear chamber being the one toward the rear of the vehicle or opposite end? My spare master cylinder has two chambers the same size.

 

That's my understanding too. The FSM even shows a larger chamber toward the rear and states that the primary outlet should be connected to the front brakes. Mystery being, the FPM shows the rear chamber (primary) connected to the rear brake lines. And then there is that picture referenced above that seems to support it. If all of this isn't confusing enough, the pre-bent brake lines that I purchased from Classic Tube align the rear port to the front brake lines, the opposite of the pic and FPM. Confusing huh?

 

Sparky's on the right track just look at any late model car with discs on the front the master has a bigger resivoir at the rear of the master

 

Sounds like it. Wish I could understand the discrepancies though.

 

I would think the weight distribution would be about 60 / 40 with most of the weight on the front of our jeeps. If this is true, I could see why more system braking force would need to be up front. And the m/c would send more braking pressure to the front.

Have you ever noticed when braking hard (as in a panic stop) how the rear wheels lock up first? I guess this because it gets pretty light back there when doing a nose dive during serious stops at highway speeds. I don’t know about the rest of you, but some of my past jeeps will, without warning, sometimes decide to dart to one side or the other during a panic stop. Just enough extra thrill to increase you stress and adrenaline level. If it is on a wet highway and your doing 65 or more and have to slam on the brakes, you can get plenty busy using that steering wheel. I bet most of us have been there a time or two.

That’s the reason my 56 CJ5 does not get over 55 on the highway.

 

I'm taking a guess here, but it may be due to the fact that the front calipers may have more movement in their design(requiring more fluid, especially over time to compensate for pad wear) than the rear drums do.
The size of the reservoir has nothing to do with the pressure or force generated, that is determined by the master cylinder size. The reservoir just holds fluid.

 

Actually it's the ratio of the piston areas that is important. Brake fluid is incompressible, and piston volume equals area (pi * diameter) times height (distance of movement). So, a slave piston with 4 times the area of the master piston will move 1/4 as far. This results in a 4:1 mechanical advantage.

You can guesstimate what kind of force balance you need from the piston sizes on the original cars.

 

We had a conversation on this the other day. I agree that it makes the most sense to have the rear-most chamber hooked to the front brakes. However, apparently, Jeep did not see it the same way. Lulu has what I assume to be the original lines - the rear brakes are hooked to the rear port on them MC. Mike Ruffin's were the same way. We called in an opinion from an original owner of a '70 CJ5. Sure enough, his were hooked the same as everyone else, and we know for 100% sure that his are original.

Go figure...

 

How about this for a possible explanation? Because we are talking about a tandem master cylinder where the two pistons (primary and secondary) share the same cylinder bore, the pressure to both line segments is the same. And given that the front wheel cylinders are larger in diameter than the rears, the force transferred to the front brake shoes is larger. This is consistent with those of you that have mentioned a bias with the front brakes. Now, assuming that the distance that both the individual front and rear wheel cylinder pistons travel are the same, more fluid is consequently displaced in the larger front cylinders. I suspect that the separate chambers making up the master cylinder reservoir are designed to account for this difference. In other words, if the goal is to design a master cylinder using the smallest possible overall reservoir, one compartment must be smaller or larger than the other. Or forget economy and size them both the same based on the largest size needed, which is probably the case with my spare master cylinder. Make sense?

 

Yeah, if the MC bore has a single diameter, you would expect the larger reservoir would be assigned to the larger wheel cylinders - just so that you could bleed the brakes more easily, if nothing else.

I think the notion of equal pressure only affects the system until the shoes contact the disk or drum. There are plenty of drum systems out there that have a dual master cylinder with a single bore size, and different sized cylinders front and rear. If everything were adjusted properly, we would expect these systems would push the shoes into the axle with smaller wheel cylinders and stop. We know this doesn't happen - there's enough tolerance in the system so that, properly adjusted, all the shoes will contact their drums. Once the shoes contact the drums, all that matters is the amount of force pushing the friction material into the drum - no more displacement is needed. The same pressure over differnt sized cylinders makes a differnt force. More force = more friction, and more of the forward energy of the car is spent as heat. The force is proportional to the mechanical advantage (areas ratio) of the cylinders.

 

We've created a monster with this thread. From looking at the picture above, but not knowing about the rest of the brake system, I'd say it's a disc front/drum rear set up, and that means that the thing the MC's lines go to is a metering valve. If one of the chambers in the reservoirs is larger than the other, it (the larger one) should be hooked to the disc braked axle. If both chambers were equal sized, then it would not matter how it was hooked up, assuming there is no metering or proportioning feature built in to the MC being used. Brake systems are simple yet complicated, no? If the system in the photo is a diagonally split type, that would further complicate things.
Read HPbooks Brake Handbook sometime. I've had it in my library for years, and it's helped me out many a time. To quote them:
"disc brake pistons displace more fluid, so they usually require more reservoir capacity than drum brakes. Reservoir volume must exceed the displacement of all output pistons, plus allow for lining wear."

SO to answer the original question"does it really matter"...... it depends on other things in the system. Maybe yes and maybe no, but if you've got discs up front and drums out back, connect the larger res to the disc side.

 

The pressure in the brake lines are the same because of a common master cylinder. Doubling the cross sectional area of the wheel cylinder will double the force.

 

I made the assumption that in a properly adjusted system, the distance each wheel cylinder piston must travel is the same before contact between the brake shoe and drum. The combination of springs and the brake adjusting mechanism seem to do a fairly good job in setting up a consistent equilibrium before the hydraulic force is applied. Why would not all of the pistons in the wheel cylinders have to move the same distance? Agreed about after shoe contact is made. I was just trying to come up with an explanation for the different reservoir volumes.