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Master cylinder and brake line questions.

Kern Dog

Life is full of turns. Build your car to handle.
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I bench bled a master cylinder today and found that while pushing the piston in and out, the front port got fluid first. For the first 1/3 of the piston travel, the rear port wasn't getting any pressure or bubbles.
The front port serves the rear brakes, the rear port serves the front brakes.
It seems odd that the rear would get pressure first. I always thought that master cylinders sent fluid equally to both ports at the same time.
Is this normal?

Secondly.....
I have read of people running brake lines in manners different than stock and leaving out a proportioning block or distribution block. I'm looking to do the same.
I plan to run the master cylinder REAR port to a T where it will feed the two front brake hard lines.
For the rear, I plan to use a simple "union" style fitting like shown below:

883 R.JPG


884 R.JPG


Does anyone see a problem by running the brake lines this way?
 
I read somewhere that you have to modify the stock master for disc drum setup or you have to run an adjustable prop valve. Because you need to limit the pressure to the rear brakes or its going to lock them up first. Because apparently it doesn't require the same PSI back there on the wheel cylinders as it does for calipers.
 
I bench bled a master cylinder today and found that while pushing the piston in and out, the front port got fluid first. For the first 1/3 of the piston travel, the rear port wasn't getting any pressure or bubbles.
The front port serves the rear brakes, the rear port serves the front brakes.
It seems odd that the rear would get pressure first. I always thought that master cylinders sent fluid equally to both ports at the same time.
Is this normal?

Secondly.....
I have read of people running brake lines in manners different than stock and leaving out a proportioning block or distribution block. I'm looking to do the same.
I plan to run the master cylinder REAR port to a T where it will feed the two front brake hard lines.
For the rear, I plan to use a simple "union" style fitting like shown below:

View attachment 1377254

View attachment 1377256

Does anyone see a problem by running the brake lines this way?

Make sure your MC does in fact work in the "traditional" manner.
When we converted my son's '65 Valiant to dual reservoir MC I installed it in the car then bled it -- only to find out that the ports were backwards to how I had thought-- so in fact the front port fed the front brakes and the rear port the rear brakes.
After looking over the instructions included with our new MC sure enough the front goes to the front rear to the rear IMG_5276.JPEGIMG_3469.JPEGIMG_3484.JPEGIMG_3488.JPEG
I would also recommend a variable proportioning valve for the rear brake line.
 
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I didn't think that 4 wheel drum, disc-drum or 4 wheel disc would make a difference in this case but to divulge all, this is a 4 wheel disc setup.
Technically, a 4 wheel drum and a 4 wheel disc setup should need no proportioning IF the wheel cylinders or calipers are sized with the right bias.
4 wheel drum cars always had larger front wheel cylinders and often wider drums up front. 4 wheel disc systems use larger front calipers often paired with larger front rotors.

The issue with the master cylinder ports being backwards is not unique. I've seen this before. Dr Diff sends zero instructions with his master cylinders so it is not clear what to do. If one were to simply hook up the lines in the same manner as a stock master cylinder (Like I did) the rear brakes would be getting pressure first and you'd have to press the pedal further to get the fronts to get pressure.

What the hell is this?

1669178932919.png


Thanks, DodgeBoy5150. This makes sense. I'm going to email Dr Diff with this tech sheet and see what he thinks.
 
I was also unsure about my MC from Doctor Diff, but plumbed it in the traditional manner with the front port for rear brakes. (It would have been nice to have an instruction sheet so I could have been 100% sure.) I guess it is working OK. I used the adjustable Prop valve he sent me. I notice the tech sheet above seems to be a different MC. It has a 3 hole mount. My Doctor Diff MC uses a 2 hole mount. This is what I got from the Doctor:
DSC01603.JPG
 
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This is interesting. I have Dr. Diff aluminum master cylinders on all my cars hooked up in the traditional fashion. Disc/drum setups. Sure would think Cass would at least post up on his website if the tube need to be reversed? Also, the 2 ports are different sizes on his.
 
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Well typically the dual reservoir master cylinder is Murphy proof as the rear brake line has a different size fitting than the front. Standard on all cars from the factory. My '67 drum/drum still had a distribution block under the master cylinder. I just changed to a disc/drum distribution block and no proportioning valve n all is good. I do believe the distribution block has different size ports inside to distribute the brake pressure front/rear as necessary.
 
Well typically the dual reservoir master cylinder is Murphy proof as the rear brake line has a different size fitting than the front. Standard on all cars from the factory. My '67 drum/drum still had a distribution block under the master cylinder. I just changed to a disc/drum distribution block and no proportioning valve n all is good. I do believe the distribution block has different size ports inside to distribute the brake pressure front/rear as necessary.
My 64 Fury, as well as all Mopars 1966 and earlier, only have a single line from the original master cylinder. The brake lines must be split by a distribution block for the fronts and rears. Since the plumbing is being re-done, I don't consider it "Murphy proof" without instructions. I hesitated on which way to plumb mine when converting to front discs. And watching YOUTUBE didn't help, as some disc brake kits use cylinders that are plumbed opposite from others. The Doctor Diff MC seemed to have dual chambers the same size, so I basically went with my gut instinct that I was plumbing the right way...
 
The plastic reservoir units seem to have an open channel between them so that fluid is actually shared. I fill one hole and the fluid fills both sides.
I started a thread on FABO about this as well. Lots of A body guys have converted from the 9" drum brakes to front discs and are using these aluminum master cylinders.

This is the unit I have in place. The different port sizes listed match what I have. Since they list "Primary and secondary", I figure that the ports are NOT reversed as I thought was the case.
RAYBESTOS MC39178 Specifications
Number of Ports2
Primary Port Thread A1/2"-20
Secondary Port Thread A9/16"-20

It still seems strange that the port at the front gets fluid first. I just don't understand that.

The Mancini master cylinder is made by Strange Engineering and actually does not use the 3 bolt mount, they include brackets that allow a 3 bolt design but the master cylinder body itself is actually 2 bolt style.
 
My 64 Fury, as well as all Mopars 1966 and earlier, only have a single line from the original master cylinder. The brake lines must be split by a distribution block for the fronts and rears. Since the plumbing is being re-done, I don't consider it "Murphy proof" without instructions. I hesitated on which way to plumb mine when converting to front discs. And watching YOUTUBE didn't help, as some disc brake kits use cylinders that are plumbed opposite from others. The Doctor Diff MC seemed to have dual chambers the same size, so I basically went with my gut instinct that I was plumbing the right way...
The aluminum w plastic caps dual master I got from Dr Diff had 9/16-20 and 1/2-20 on it. But I completely understand your dilemma when you only have 1 main line to the distribution block.
 
Today I lined up all the master cylinders that I have gathered....

IMG_1207.JPG


IMG_1208.JPG


Out of view is an aluminum 15/16" unit and the aluminum 1 1/8" MC on my car.

Three are cast iron 4 bolt units. One at 1" and two of them 15/16".....They all had 9/16" front ports, 1/2" rear ports and when bench bled, pushed fluid equally in time front and rear.

Two of the aluminum units were 1.03" and had 9/16" front, 1/2" rear ports and pushed fluid equally.
One aluminum one was 15/16" and had weird 3/8" ports. The front reservoir was longer but the whole reservoir was sloped. See picture below:

D Diff MC 1.JPG


It pushed fluid to the front first.

The final two MCs are the ones I've been working with lately. Both 15/16" and 1 1/8" units have identical ports and push fluid to the front reservoir first.
 
I read somewhere that you have to modify the stock master for disc drum setup or you have to run an adjustable prop valve. Because you need to limit the pressure to the rear brakes or its going to lock them up first. Because apparently it doesn't require the same PSI back there on the wheel cylinders as it does for calipers.


Smaller bore rear wheel cylinders are supposed to help with the disc/drum conversion proportioning.

I have a factory disc brake 69 road runner that has the small rear drums and a factory proportioning valve in the rear brake line. Early attempts at disc/drum proportioning I guess.
 
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Smaller bore rear wheel cylinders are supposed to help with the disc/drum conversion proportioning.

I have a factory disc brake 69 road runner that has the small rear drums and a factory proportioning valve in the rear brake line. Early attempts at disc/drum proportioning I guess.
I think you have to remove something in the master cylinder. Inside where the line hooks up. This may have been a drum drum conversion to front disc using the same master.
 
Whatever front/rear set-up you run,
wether it be drum/drum, disc/drum,
or disc/disc, you'll want the rear to
react first, if only for a fraction of
a second. This reduces nose dive
under hard braking situations. An
adjustable proportioning valve
allows you to dial this in to a finer
reaction time. When the weight
transfer hits the front first, that
inertia and momentum transfers
the weight of the vehicle forward
causing loss of traction to the rear.
What you strive for is to hold that
inertia/momentum back just long
enough for the rear tires to hold
those forces equally. This reaction
is referred to as brake bias. That
is why some reservoir masters
had residual check valves built in.
I run 2lb residual check valves for
both front and rear on a 4 disc
set-up with an adjustable
proportioning valve in the rear
brake distribution. The two lb
valves ensure contact of the pads
at all 4 corners where brake pedal
reaction is minimal. To much bias
to the front will result the load to shift
forward, causing the rear to lift.
Resault is nosedive and reduced
braking to the rear.
20211227_151541_HDR.jpg
 
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Whatever front/rear set-up you run,
wether it be drum/drum, disc/drum,
or disc/disc, you'll want the rear to
react first, if only for a fraction of
a second. This reduces nose dive
under hard braking situations. An
adjustable proportioning valve
allows you to dial this in to a finer
reaction time. When the weight
transfer hits the front first, that
inertia and momentum transfers
the weight of the vehicle forward
causing loss of traction to the rear.
What you strive for is to hold that
inertia/momentum back just long
enough for the rear tires to hold
those forces equally. This reaction
is referred to as brake bias. That
is why some reservoir masters
had residual check valves built in.
I run 2lb residual check valves for
both front and rear on a 4 disc
set-up with an adjustable
proportioning valve in the rear
brake distribution. The two lb
valves ensure contact of the pads
at all 4 corners where brake pedal
reaction is minimal. To much bias
to the front will result the load to shift
forward, causing the rear to lift.
Resault is nosedive and reduced
braking to the rear.
View attachment 1382323
Great info, thank you for that.
 
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