KH disk front 83/4 rear, prop valve and master cylinder brake line question

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nick455440

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Long story shortish, have a 65 valiant, awhile ago had done the duel reservoir master where I ran the back res to the from drum distribution block, and the front res hardline straight to the rear rubber line. Was planning on doing the same but had seen a post where some recommended using a larger reservoir master for the disks, attached is a pic of the original set up,

IMG_5121.jpeg



Thought about moving to this (pic below) master cylinder as it looks like it would work with the current rod set up ( I think the bore may be different) and using the summit proportioning valve (only the small block on the below pic) to run the front disks, and hardline again the other reservoir to ur rear drum rubber line,

Questions - would the original master cylinder truly be too small of a reservoir for disks and would it hurt anything to bypass the distribution block (rears only) and run the hard line straight from the master to the rear rubber line? I put frame connectors in the car and was going to run the brake line on the inside of them, instead of drilling through them, this would make the new SS front to back line to long but if I ran it to the master cylinder it would be a ideal length.. any info is appreciated
IMG_5122.jpeg


IMG_5123.png
 
Yes A larger resevoir is recommended because as the pads wear, the piston starting positions move outward.
A couple routes you can follow:
Pre-67

Or '67 up tandem Master,

For the pre-'67 set up, you must the proportioning valve in the feed to the rear line.
With the tandem master you can use the prop valve in the rear line, or a combination valve as was used by the factory 70-72.
However the '65 AFAIK has no provision for the brake failure indicator to connect to a 'safety switch'.

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If you are going run manual brakes a 1 1/32 master cylinder will require excessive leg effort to stop the car. Something like a 15/16 bore master cylinder would be a lot less leg effort.
 
And you won't need a proportioning valve if you run 7/8 wheel cylinders in the rear.
 
It will be a drum 83/4 rear and KH disks in the front, will check the wheel cylinder size in the read
 
You can't go wrong using stock (pick a year) parts. Like choose 67 or 69 depending on the rear end backing plates if A body. And when you go to the store order all the parts based on the year dart, for example.

Years down the road you will be glad you did.

As for the way you have it plumbed now you don't need the 70 something distro/proportioning/brake warning block.

You have the rear most cylinder going to the front brakes, the front most cyl going to the rear brakes with a proportioning valve in that line.

I would get a 67 to 70 disk drum master cyl.

The 1- 1/32 bore is 0.031 over the stock 1" bore. As mentioned slightly more leg force.

15/16 is 0.0625" smaller requiring less force but more travel.

Personally I wonder how many 1" bore MCs have been rebuilt so many times that they are 0.030 over from the rebuilder.

I did some quick calculations and with the assumption that with all other things being the same....

Compared to stock 1" dia mc...

1-1/32 would increase leg force by 6%

15/16 would decrease leg force by 14 %
If with 1" mc you press with 500 lbs...

1-1/32 it increases to 530 lbs
15/16 it decreases to 431 lbs


As for smaller rear wheel cylinder. To me that just complicated things and we don't know if the op is using 10x 1.75 or 10x 2 or 10x2.5 or 11x3 rear brakes.

So an adjustable valve (which he already has) would be IMHO the right way to go
 
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I basically did what Dana describes, except I ran both front and rear lines through the metering block as per factory installations, then added the proportioning valve to the line going to the rear axle. This is very easy to do without cutting and flaring if you just buy a short (15" or so) pre-made brake line and bend it into a loop, then install the proportioning valve on an angle bracket parallel to the metering block, offset by an inch or two. The rear line just moves over an inch to go into the proportioning valve, and the loop connects the PV to the metering block.

This was on a factory drum brake 1967 Barracuda convertible — I duplicated the setup on a factory disc brake Barracuda (K-H discs and 10 x 1.75 drums), with the exception of using the 72-up metering block and Summit proportioning valve. I used a 15/16 disc brake MC with no booster. Once I had the PV dialed in the brakes were excellent. But I had to dial the PV out to something like 80/20 front/rear to keep the rear brakes from locking up. For this reason I would recommend an adjustable proportioning valve.

Rear lockup was never an issue with the OEM disc brakes on my 67 fastback which included a (purportedly) adjustable proportioning valve from the factory (which I never attempted to adjust).

BTW the pedal pressure is probably more on the order of 200 lbs, not 500 lbs.... I know you were just using a number for comparison, but I doubt many owners could muscle up 500 lbs. even with both feet. I typically do 330lbs on the leg press at the gym, and even the 1-1/32 manual brakes aren't at that level.
 
mvh said:
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BTW the pedal pressure is probably more on the order of 200 lbs, not 500 lbs.... I know you were just using a number for comparison, but I doubt many owners could muscle up 500 lbs. even with both feet. I typically do 330lbs on the leg press at the gym, and even the 1-1/32 manual brakes aren't at that level.
Click to expand...
the pounds of pressure in the system are amplified by the pedal linkage/ratio. It’s not 1 to 1..
 
The proportioning valve doesn't begin to play a role until line pressure is up around 300 psi. or something like that.
Its in the pamplets I linked to above.
They also explain the concept and why its used so I'm not going to repeat it here. Whom am I anyway? Some guy on a keyboard. Take it direct from Chrysler Tech.
 
thanks for all the info, I don’t think the metering block/prop combo I have from summit is adjustable,
one question …looking at the later drum disk master cylinders, mine has a bolt on spring and boot, the newer models do not, should my rod work ok just removing everything but the rubber that holds it in the bore? Thanks all
 
mvh said:
BTW the pedal pressure is probably more on the order of 200 lbs, not 500 lbs
Click to expand...
In hind sight only Arnold could stop the car.


:rofl:

Even 100 lbs is probably the max most people can push with!
 
4mulas said:
the pounds of pressure in the system are amplified by the pedal linkage/ratio. It’s not 1 to 1
Click to expand...
Never said it was.


As I stated:

"with the assumption that with all other things being the same"

IE. pedal ratio, front disks calipers, pads, drums etc.
 
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