Dual master cylinder f100 brakes

[skidmarks]

Using a dual master cylinder with f100 drum brake front and rear.

Master cylinder is a 1" bore from a AMC product.

Of the 2 ports, which is front and which is rear or does it matter?

With this style master cylinder, are inline residual valves needed?

[51 MERC-CT]

When I installed a drum/drum AMC master cylinder on my Merc. years ago the master cyl. had the residual valves built in and no others were required.

[40 Deluxe]

A word of caution: A dual master cyl. conversion such as this may not work as intended! If pressure is lost front or rear, the pedal will go nearly to the floor before the remaining brakes engage. If total pedal travel is anything less than the OEM application, you will lose all braking and probably crash!
Save your neck and your car by testing your setup. When everything is hooked up and bled, you likely will have good brakes with little pedal travel. But now, open a front bleeder screw and try a panic stop. If the rear wheels don't slide and the pedal goes all the way to the floor, the system failed. Time to re-engineer! Repeat on the rear brakes and the fronts must lock up or again, fail.

[Talkwrench]

Originally Posted by 40 Deluxe

A word of caution: A dual master cyl. conversion such as this may not work as intended! If pressure is lost front or rear, the pedal will go nearly to the floor before the remaining brakes engage. If total pedal travel is anything less than the OEM application, you will lose all braking and probably crash!
Save your neck and your car by testing your setup. When everything is hooked up and bled, you likely will have good brakes with little pedal travel. But now, open a front bleeder screw and try a panic stop. If the rear wheels don't slide and the pedal goes all the way to the floor, the system failed. Time to re-engineer! Repeat on the rear brakes and the fronts must lock up or again, fail.

Correct. I think a lot of people, just expect it to work because "its a dual setup"

[Mtn Wudy]

That’s interesting ....
You think your adding a safety factor in you car and you’re creating a hazard...
A friend had that very thing happen on his old Willy’s pick up...
I’ll show him this post ...
Your learn something new here,,thanks for your input..
Carl...

[V8 Bob]

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Originally Posted by Mtn Wudy

That’s interesting ....
You think your adding a safety factor in you car and you’re creating a hazard...
A friend had that very thing happen on his old Willy’s pick up...
I’ll show him this post ...
Your learn something new here,,thanks for your input..
Carl...

The dual master is a wise upgrade, but it has to be able to full stroke before the pedal bottoms out for it to provide the added safety, something many still don't fully understand.

[Pete F]

I put a dual master on my 34.
The important question is what diameter master are you taking out.
If the diameter is smaller it will take more pedal travel to push the same a mount of fluid to stop the car so diameter is a big fact.
Second I put residual valves is both lines at the master to hold the fluid on the wheel cylinders so the pedal is always firm.
Third the hole closest to the mounting flange is internally valves for the front brakes
Good Luck

[skidmarks]

Originally Posted by Pete F

I
Third the hole closest to the mounting flange is internally valves for the front brakes
Good Luck

Is this on all dual master cylinders or disc./drum (larger resivor for the disc)?

The one I got is for drum/drum

[skidmarks]

Originally Posted by Pete F

Third the hole closest to the mounting flange is internally valves for the front brakes
Good Luck

Is this on all dual master cylinders or for disc/ drum (larger resivor for the disc) ?

The one I got is for drum/drum.

[Pete F]

Originally Posted by skidmarks

Is this on all dual master cylinders or for disc/ drum (larger resivor for the disc) ?

The one I got is for drum/drum.

The larger resivor is for disc brakes because there pistons are bigger and need more fluid

[skidmarks]

Originally Posted by Pete F

The larger resivor is for disc brakes because there pistons are bigger and need more fluid

Yes I understand that,

The question was, the port closest to the mounting flange goes to.the front brakes on all dual master cylinders for both drum/drum or drum/disc

[john in illinois]

Originally Posted by skidmarks

Yes I understand that,

The question was, the port closest to the mounting flange goes to.the front brakes on all dual master cylinders for both drum/drum or drum/disc

Yes the first piston is the primary piston for the front brakes whether disc or drum.

John

[40 Deluxe]

Originally Posted by Pete F

The larger resivor is for disc brakes because there pistons are bigger and need more fluid

Actually, the size of the pistons doesn't matter, as the system is filled with fluid to begin with, and piston travel is very small. The need for a large reservoir is because disc brakes have no adjustment for wear. As the pads wear, the piston extends further out, but does not retract. So fluid level in the master cylinder goes down. With a small reservoir, the master may go empty before the pads are fully worn (if no one tops it off in the meantime).

[V8 Bob]

Originally Posted by 40 Deluxe

Actually, the size of the pistons doesn't matter, as the system is filled with fluid to begin with, and piston travel is very small. The need for a large reservoir is because disc brakes have no adjustment for wear. As the pads wear, the piston extends further out, but does not retract. So fluid level in the master cylinder goes down. With a small reservoir, the master may go empty before the pads are fully worn (if no one tops it off in the meantime).

Piston size does mater, and is why disc/drum and most drum/drum masters are not 50/50 in fluid displacement output. The different primary/secondary port fitting sizes prevent the front and rear OE circuits from being switched.
You are correct that caliper pad wear is one reason the disc reservoir is larger than the drum.

[PetesPonies]

Originally Posted by 40 Deluxe

Actually, the size of the pistons doesn't matter, as the system is filled with fluid to begin with, and piston travel is very small. The need for a large reservoir is because disc brakes have no adjustment for wear. As the pads wear, the piston extends further out, but does not retract. So fluid level in the master cylinder goes down. With a small reservoir, the master may go empty before the pads are fully worn (if no one tops it off in the meantime).

I think I know where you are trying to go with this, but what you are saying is not totally true. A large bore, will require more fluid to move a given distance. And hydraulic pressure is created by the ratio of the different bore sizes; MC to WC or caliper.
To me the most alarming thing happening today, is these drum drum vehicles are being used with MC with different size reservoirs and no residual pressure valves. Typically a large/small reservoir on a MC means it is disc/drum. When you see a large reservoir you can count out a residual valve and even the opening for fluid flow will probably be larger. You loose some engineered brake bias.

[V8 Bob]

Originally Posted by Pete F

Third the hole closest to the mounting flange is internally valves for the front brakes
Good Luck

All drum/drum masters are not 50/50 in operation, so knowing the correct primary/secondary or front/rear plumbing is important. The port (hole) closest to the mounting flange is NOT always intended for the fronts.

[51 MERC-CT]

Originally Posted by V8 Bob

All drum/drum masters are not 50/50 in operation, so knowing the correct primary/secondary or front/rear plumbing is important. The port (hole) closest to the mounting flange is NOT always intended for the fronts.

Yes, seem to recall that there are two springs in the piston assembly and the weak spring side is the one that goes to the front, which activates the fronts first and when the fronts are pressurized the second (strong} spring collapses to activate the rears.

[PetesPonies]

Originally Posted by 51 MERC-CT

Yes, seem to recall that there are two springs in the piston assembly and the weak spring side is the one that goes to the front, which activates the fronts first and when the fronts are pressurized the second (strong} spring collapses to activate the rears.

This is correct.

[Pete F.]

It doesn’t matter what the master was made for it matters what the piston diameter is.
Different cars and trucks used different diameter wheel cylinders .
If your new master was bigger than what you took out that would be ok , but if the diameter is smaller the pedal will have to go down further to stop the car


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[skidmarks]

Well with the last two post,
Anyone know which port on a 1970 AMC drum drum master cylinder goes to the front brakes?