Primary or Secondary Brake Lines?

No, drums all around on a stock '70 CJ5. And no metering or proportioning valves.

Nope, not diagonally split either, just zoned front and back.

 

OK, so if it's an all drum system, what the heck is that big old junction block for? I see two lines coming from the MC to that block and two out. Why go to that trouble if there is no metering valve? :?

 

It's just a distribution block. Good question though about why it's there when it seems that routing each master cylinder outlet directly to the front and rear brakes would suffice. I suspect it's a leftover design when a proportioning valve was used. Jeep on the cheap again. They could use the same hard brake lines and brass block from the proportioning valve without its inards. At least that's my take on it.

 

No Beeser, I think you're exactly right in the in your assumption and in your conclusion. It's obvious how the system would work with a single reservoir master cylinder. The idea of a split system seems simple, yet it wasn't routinely used until 1967 or so - why not? The same principle that balances the different sized wheel cylinders in the single reservoir case should not work with double cylinders, yet it does. Maybe there's some bit of technology that we are missing? Or maybe it's simply that there's enough expansion in the hoses and lines to overcome the very small fluid volume differences between the two sizes of wheel cylinders? Don't know - all the explanations of dual MCs I've seen have ignored this question. I've taken dual MCs apart and never seen any fancy valving between the two halves... dunno ... was hoping someone would enlighten me.

 

Tandem master cylinders were required for new cars sold in the US starting in 1967. I don't know when the requirement started for trucks. There is one reason for the use of tandem master cylinders- safety. You will have brakes on half of your wheels in the event of a failure in one of the hydraulic circuits. With a non-tandem master cylinder, you'll lose all brakes with a hydraulic failure at any point in the system.

If you double the suface area of a wheel cylinder, you will double the amount of force generated with a given pressure. With that, it will take twice as much fluid to move the piston the same amount.

 

Disc brakes require a 2lb residual valve either inline or in the MC. Drum brakes require a 10lb residual valve. A MC that is designed to be used with discs in front and drums in the rear will have these in the MC. Thats why it matters which ports you use for the brake system. With a 4 wheel disc MC or a 4 wheel drum MC it shouldn't really matter whether you run front brakes off either the front or back port.

 

In the dual master cylinders if you have a larger reservior the larger one is for the disks because of the size of the cylinder pushing the pads out. Takes a lot of fluid to extend the caliper on disks. They make porsioner valves so you can adjust the pressure on the brake system to allow for less weight on the axles for more even braking. Mark

 

Where is the residual valve located on the master cylinder and what is the function of it? Is this the check valve located internally at each outlet?

 

Here is where the plot thickens. because the pistons share the same bore, the piston on the end (furthest from the pedal) has a greater surface area because you have to subtract the area of the rod between the two pistons from the piston closest to the pedal. therefore, the end piston would be able to move more fluid for a given length of stroke and apply more pressure to it at the same time.... $.02

now look at the picture again with this in mind...
http://flickr.com/photos/74334622@N00/15163813/

 

The residual pressure valve is indeed a check valve, the factory one was located in the side of the master cylinder. My understanding is that residual pressure valves are only required on a system with the master cylinder at the same level or lower level then the brakes. I have never seen any later factory master cylinders with these valves in them. Only the ones where the M/C was mounted under the floor.

This is from wilwoods site:

 

Yes Glenn your explanation is correct.

 

Spring loaded check valve then?

 

No, I don't think that's true. Here's a sketch I made:



When there's no leak in the system, piston #2 essentially floats in the bore between piston #1 and the end. When there's a leak, the fluid is pushed out - if the circuit in front of #2 leaks, #2 will bottom on the bore. If #1 leaks, the rod between the pistons will push on #2. In both cases you lose a lot of pedal distance, but the brakes still work - marginally.

Now, even though that rod is there, the amount of fluid pushed out of the MC for a given force on the pedal doesn't change. The pressure on both sides of piston #2 is the same.

 

timgr - Nice sketch and explanation.