Disc Brake Master Cylinder With Drum Brakes?

[RustyRatRod]

I know crazy question. Here's the reason. I have the correct disc/drum master cylinder, proportioning valve and both lines to install on Vixen. What would be the result if I went ahead and did that but left the front drum brakes on for now, until I can afford the 14" tires and put my 14s on it? All this in the quest to keep her runnin and drivin, instead of putting her down until I can afford tires, which may be a while.

 

[69cudaownr]

Interesting question. I would assume you'll get 60% braking to the front drums and 40% to the rear like a disc set up. What I'd be interested in knowing is if the pressure is stronger to front brakes, which could risk the wheel cylinder seal going.

 

[Professor Fate]

Not crazy!
There's always the debate as to whether it's an actual proportioning valve or just a distribution block, so assuming it's basically a distribution block I'd go for it. The biggest difference in the master cylinder (assuming the new and old are the same bore diameter) is that the disc master won't have a residual valve in it for the front discs, and those are leftover necessities from when masters were mounted low in the car; most agree that they're not necessary anymore for drums, anyhow. And you'll have a bigger reservoir for the front brakes.
Line pressure should remain the same if the master's bore is the same, so my bet is that you won't even notice a difference in braking- plus you'll have a safer dual pot master now.

 

[RustyRatRod]

Not crazy!
There's always the debate as to whether it's an actual proportioning valve or just a distribution block, so assuming it's basically a distribution block I'd go for it. The biggest difference in the master cylinder (assuming the new and old are the same bore diameter) is that the disc master won't have a residual valve in it for the front discs, and those are leftover necessities from when masters were mounted low in the car; most agree that they're not necessary anymore for drums, anyhow. And you'll have a bigger reservoir for the front brakes.
Line pressure should remain the same if the master's bore is the same, so my bet is that you won't even notice a difference in braking- plus you'll have a safer dual pot master now.

I'm sorry, I wasn't clear. What I have is the correct MS, lines and the proportioning valve for a disc brake car. I'm sorry I didn't make that more clear. So I would be swapping out the present distribution block for a proportioning valve if I do it.

 

[Professor Fate]

No, I got it- and yes, I'd go for it, unless someone can chime in with a good reason not to.

 

[Tooljunkie]

I did it in the Fargo. Master cyl was for discs all around. 2 residual valves. Rear brakes will lock up if braking hard on gravel. Otherwise im happy with my brakes.

 

[mvh]

A 60s-vintage drum brake MC has a residual pressure valve in the brake line orifices... Your new disc MC will not have one in the front chamber (it is only used for drums, not discs — this is also why you don't use drum MCs with discs). Since the purpose of the RPV is to ensure the brakes apply instantly, you may find yourself in a situation where the rears respond more quickly than the fronts. This is not good. I would not recommend it.

 

[67Dart273]

Rob that is exactly what I did with the 67. I have 4x disk, 73/4 Duster/ Demon up front, and the Versaille disk in rear. I just ruptured the residual valve rubbers in the fittings and away we went. With disk, I'm not even running a prop valve.

 

[Dana67Dart]

Ohyyyy!

People,

1. The man has 4 wheel drum brakes now
2. He is NOT changing to front disc for a while
3. In the mean time he wants to put the disc / drum master cyl and assorted distro block / proportioning valve on the car
4. He would have residual valve on the rear drum brakes
5. He would NOT have a residual valve for his front DRUM brakes.
6. He would have a proportioning valve to the rear drums.

His question is:

Will this work for now? ( And be safe)

 

[72bluNblu]

Ohyyyy!

People,

1. The man has 4 wheel drum brakes now
2. He is NOT changing to front disc for a while
3. In the mean time he wants to put the disc / drum master cyl and assorted distro block / proportioning valve on the car
4. He would have residual valve on the rear drum brakes
5. He would NOT have a residual valve for his front DRUM brakes.
6. He would have a proportioning valve to the rear drums.

His question is:

Will this work for now? ( And be safe)

The disk/drum M/C definitely won’t have a residual valve for the chamber intended for the disk front brakes.

But, it may not have a residual valve for the rear drums either. Like @Professor Fate said, at some point “somebody” decided the residual valves weren’t really necessary even for drums, so, a lot of re-popped and remanufactured M/C’s don’t have them regardless.

The larger chamber size for the front won’t be an issue, and combination valve shouldn’t be an issue either. The residual valve(s) probably won’t be.

If you do find that the rear brakes actuate faster than the fronts because of a residual valve going to the rear, you can add a residual valve into the front line. You’d want a 10psi residual valve for drums.

Wilwood Disc Brakes 260-13784 Wilwood Residual Pressure Valves | Summit Racing

Just remember to remove it when you add the front disks

 

[RustyRatRod]

Ohyyyy!

People,

1. The man has 4 wheel drum brakes now
2. He is NOT changing to front disc for a while
3. In the mean time he wants to put the disc / drum master cyl and assorted distro block / proportioning valve on the car
4. He would have residual valve on the rear drum brakes
5. He would NOT have a residual valve for his front DRUM brakes.
6. He would have a proportioning valve to the rear drums.

His question is:

Will this work for now? ( And be safe)

Correct. You scored high in comprehension. lol

 

[Murray]

Go for it! On your M/C, behind the brass flare fittings in the M/C body is a little rubber "flapper valve" and its purpose is to retain some fluid pressure when at rest. Disc brakes don't require this, so your disc brake M/C which will be doing service on a drum brake will need this to be added.

 

[Inertia]

Without the prop valve to shut off rear pressure at "x" lbs. there would be the chance of rear lock-up under heavy braking, particularly in the rain.
Most modern wheel cyls have the spring in them to replace the residual valve in holding cups against wall .
If you have an adjustable proportioning valve, then it's up to your adjustments.
Tire size and w/c diameter also a consideration .

I'd do it, - with extra caution IF caught in the rain !

 

[72bluNblu]

I don't understand the comments about the rear drums locking up. The rear drums will function as normal- he has a disk/drum master cylinder and a prop valve for a disk/drum set up. So the rears will do exactly what they're supposed to.

The only question is whether or not the front drums will actuate at the same speed as the rears. If they're slow, then a residual pressure valve will need to be added to the front, probably best located between the prop valve and front brakes.

Now, there may not be a residual valve in the rear line either, in which case, everything will probably actuate at the same time like it should. But it should be blindingly obvious if the front drums actuate slower than the rears, since they do most of the braking it will feel dramatically bad on the brakes if the fronts are lagging, and that will be a problem all the time, not just when it rains.

 

[RustyRatRod]

Without the prop valve to shut off rear pressure at "x" lbs. there would be the chance of rear lock-up under heavy braking, particularly in the rain.
Most modern wheel cyls have the spring in them to replace the residual valve in holding cups against wall .
If you have an adjustable proportioning valve, then it's up to your adjustments.
Tire size and w/c diameter also a consideration .

I'd do it, - with extra caution IF caught in the rain !

It's the factory disc/drum proportioning valve.