what master cylinder for disk/disk set up?

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69-318

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in my 69 dart iv got larger 11.75" disks for the front and jeep cheroke 11" disks in the rear and im needing a new mater cylinder. my 76 dart parts car has power brakes (booster) i wanted to incorporate into my set up. id like to get a more compact master cylinder from a newer model from napa or rock auto but im not sure which one will work with my set up. could i bolt one right to my booster? or would i have to get a matching master and booster?

on other threads people seem to prefer manual brakes over power but im not sure about how much pedal effort my set up will require? theres a 4 speed going in this car so my hands and feet will be all over the place at any given time. i just dont want to be having to pull on the steering wheel to stop the car if a suprise or emergency comes up.
 
Couldn't you use the one off the jeep you got the rear disks off of? It should be a short aluminum 2 bolt. I would think it would be easy to use the entire booster assembly or get a 4bolt to 2 bolt conversion plate. (Mancini sells them as well as 2 bolt M/C that should work)
 
duh i never thought of that. on saturday ill check to see if the cherokee is still around i guess it would be the right set up as it has disk/disk.
 
I made an adapter plate to convert the cars 4 bolt pattern to the master cylinders 2 bolt design. I used 1/2" aluminum plate.
Doctor Diff/Cass Eslick is a man to contact for this stuff. Also consider The Ram Man. Sorry, I have no links for these people, but a Google search should work.
In my case, Dr Diff sold me a lightweight 2 bolt aluminum master cylinder for my 4 wheel disc setup. I had power but the low engine vacuum made slow speed driving pretty dicey. I decided to return the car to a non power setup. The weight savings will probably be around 12 or more lbs. I installed the master yesterday but i have a leaky line that needs to be replaced. Oh, Dr Diff suggested to use a 4 wheel DRUM combination valve in a 4 wheel disc app.
 
do you have any issues with out the power assist?

comp cams is grinding me a cam that will have some vacuum but not lots to spare. i dont know if i should just ditch the booster or if i should keep it for the added assist
 
69-318 said:
do you have any issues with out the power assist?

comp cams is grinding me a cam that will have some vacuum but not lots to spare. i dont know if i should just ditch the booster or if i should keep it for the added assist
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No power assist? NO problem!
I replaced the leaky line yesterday and drove the car today. The lack of power assist means that the cars braking power is consistant whether the car is running or not. Fat cam with 8 psi of idle vacuum? NO problem! car stall out and you need to stop? No problem.
My 67 Dart GT has a 73 Duster manual disc setup. It stops great. My brother in laws 72 Duster has a manual disc from some other 73-76 A body. It too stops great. last year I sold my blue Duster that had the same setup.
There are a few things to learn though when you deviate from factory stock. The 73-76 disc/drum setup when swapped over to a drum/drum car works like factory stock as long as everything is matched like the factory did it. Since yours is a 4 wheel disc system like mine, you need a dedicated disc/disc master cylinder and a drum/drum combination valve. My master cylinder is a 15/16" bore. The pedal travel is a little longer than with a power booster, but it does NOT feel touchy in any way. It does require more effort than a power setup, but I like the feel. I HAD to do something though. I hated how my car felt at low speeds! The '509 cam produces piss poor idle vacuum and the brakes suffered as a result of it.
Vacuum can? .....Sure, its been done. I thought about that but decided against it for three reasons: One, I was interested in CONSISTANT braking under every condition. No, my car doesn't stall often, so THATs not why I chose to go manual. The second reason I nixed the vacuum can is that I wanted simplicity. The third reason was to shed some weight and make the engine bay less cluttered. The power setup and iron master cylinder weighed 17 lbs. The aluminum M/C weighs 3 1/2 lbs. Every ounce helps. A lighter car accelerates better, stops faster, and handles better than an overweight one.
 
well i think ill skip the booster all together then and just get a MC from a disc/disc vehicle. what is the benefit of using a drum/drum combination valve as opposed to no valve at all? i noticed on a disc/disc MC i was checking out i think one line is bigger than the other would this be enough to have a little more braking up front or do i still need the combination valve?
 
You wrote that you intend to get a master cylinder from a disc/disc vehicle. Thats NOT what I did. My M/C may have an OEM connection, but I'm not sure of what it is. I ordered mine through Dr Diff. The important thing is the BORE size. It is crucial in non power applications. It needs to be either 7/8" or 15/16" to have a decent feel along with the proper pressure at each corner.
The reason for NOT using a disc/drum prop valve? Those are designed to limit the line pressure to the rear drums, while giving full pressure to the front brakes. Using one of these in a 4 wheel disc system will result in low pressure at the rear calipers and poor performance. A 4 wheel drum combination valve works in an approximate 60/40 front to rear ratio.
 
Or you could not use any factory based proportioning. Run a 2-way splitter in the front and run an adustable prop valve for the rear. Even if you use a factory drum prop valve you will likely need to adjust your rear line pressure some because your setup is vastly different than anything that mopar produced way back when. Remember everything affects how the brakes work including ride height, tire selection, and vehicle weight. All of these factors will be different from factory.
 
i was planning on using an adjustable valve for the tuning or the rear brakes so would this take the place of the combination valve? i thought drum brake combination valves had a built in 10lb residual valve to hold the brakes out so theyre ready to grab which would be no good for discs? the MC iv been looking at has a 1.06" bore so would this be too big then? another one was .875". thanks for all the info guys i was hoping to get a MC today but i dont think ill have time.
 
Frankenduster said:
A lighter car accelerates better, stops faster, and handles better than an overweight one.
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Yes. best point about the A bodies is the lighter weight and the more you remove, the better still. Get it down under 3,000 lbs and it feel like a different car.

Just don't let very weak legged people drive the car
 
Larger rotor size has nothing to do with master cylinder size requirements. Using the larger late B body rotors in place of the smaller ones has no effect on other brake system components. Inlarging front caliper bore diameter (73 to 75 A bodies used a 2.60" diameter piston bore, all other A or B body single piston calipers were 2.75" diameter) may or may not require a different bore master cylinder, replacing rear wheel cylinders with calipers when changing out to rear discs most probably will.

As master cylinder bore diameter increases, the amount of travel needed to displace a given amount fluid decreases resulting in shorter pedal travel. As the bore diameter decreases, the piston stroke must increase to displace the same amount of fluid. Use of a master cylinder with too small of a bore results in too much pedal travel, and the pedal bottoms out on the floor before adequate pressure is generated to stop the rotor from turning. With any given amount of wheel cylinder/caliper bore surface area, inlarging the master cylinder bore diameter increases the amount of foot pressure required to move the master cylinder piston. High pedal pressures are why large bore master cylinders are used with power boosters. The short pedal travel resulting from the combination of a low effort power booster and large master cylinder diameter could also explain why the brake pedal action in older power drum brakes systems seemed more like an on off switch.

Drum brake cylinders do not require the volume of fluid that disc brake calipers do. You must use a master cylinder with adequate reservoir capacity for the size calipers you are using. Most disc/drum or drum/drum master cylinders are marginal in the capacity of the reservoirs intended to service the drum brake cylinders.

Residual check valves haven't been used in most or all OEM drum brake master cylinders since the advent of the metal wheel cylinder seal expander (late 70s).

If you are going to run without a power booster, depending on pedal ratio, a 15/16" or 1" bore is usually about right.
 
thanks for all the great info guys. i ended up going with a MC from a 2002 intrepid which has a bore diameter of 15/16". the only issue i have is it is mounted pointed slightly upwards so ill have to mount it at the same angle to keep the resevoir flat. there was one from a 2002 caravan which is mounted parallel to the ground but has a 1.06" bore so i went with the one from the intrepid. the MC has a fromt line thread size of 12mm and the rear line is 10mm.
 
I have tried 3 different NON power master cylinders in the past 2 weeks, all with a different feel. The absolute WORST was a large bore unit 1 5/32". Very short pedal travel, rock hard pedal and the car wouldn't even skid on ice. Next was a 75 Dart Disc/drum unit, one inch bore. It was a little better, but still not quite right. The one in my car now is a 15/16" from Dr Diff. While the car still doesn't stop as quickly as I'd prefer, it is MUCH better than the large bore M/C. I can testify firsthand to the theory of bore size in relation to pedal effort and effectiveness.
 
Just to point out not all master cylinders have the internal groove required for safe use on a manual brake application. You do not want to have your master cylinder pushrod fall out of the back of the master cylinder.
 
it will be a while before i get to try the MC that i got as im still building the car. mine has a cup in the back thats maybe 2" deep that the pushrod goes into so there is no actual seal around the pushrod. and the cup returns to the end of its travel on its own when the brakes are released. like i said before ill have to mount it pointing upwards slightly to have the reservoir sitting flat. there was a smaller MC on an eagle talon with a remote reservoir that might have worked but i didnt have time to research it. i wish the revervoir from the caravan would have worked but the 2 chambers on the MC are about 1/8" closer together. oh well no one will see it anyway i plan on doing most of my driving with the hood closed :burnout:
 
They say some have some kind of valve inside, And that it can be removed. Whats that all about ???? Dose anybody know....
 
So with my 4 wheel disc setup (11.75 Front/11" Rear) I need to be looking at Dr. Diffs 2 bolt unit and a drum/drum prop valve?

I've gotten stuff from Cass before, so that's no biggie. But where does one buy a drum/drum prop valve?
 
superdart said:
So with my 4 wheel disc setup (11.75 Front/11" Rear) I need to be looking at Dr. Diffs 2 bolt unit and a drum/drum prop valve?

I've gotten stuff from Cass before, so that's no biggie. But where does one buy a drum/drum prop valve?
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From a junked 4 wheel drum car!

I may be wrong on this, but I think that in the case of a drum/drum car, the unit is called a combination valve. There is no "proportioning" in a drum brake car. Only disc/drum cars had a built in valve that reduced pressure to the rear.
 
i was thinking of using an adjustable proportioning valve to tune in my rear brakes on my disc/disc set up. the earlier drum distribution block had a built in residual valve to keep a few pounds pressure on the brakes so air wouldnt seep in but with the new style wheel cylinders now days im not sure if its still seccessary. but anyway the junkyard should be full of drum/drum vehicles with decent distribution blocks.
 
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