There is no pressure differential valve in the system.


Lawrenceville, GA

slumloard44 Posted -



Forgot to answer this question.

In short no. It cannot be done with the OEM MC. The MC will not supply either enough pressure or volume to the calipers.

Now, if you wanted to maintain a somewhat OEM appearance for a DISC/DRUM conversion w or w/o a booster, FORD used a single reservoir dual outlet MC in 1965/66 that allowed DISC/DRUM w or w/o power assist and the reservoir was much larger to supply the needed volume for the calipers. These were referred to as 'FRUIT JARS' if you remember that far back.



If planning to use the OEM MIDLAND BOOSTER, only the FRT disc line could be routed through the booster slave cylinder (either w DUAL RSVR MC or the SGL RSVR DUAL OUTLET) as the two systems have to remain isolated from one another.

You only need power assist on the FRT DISC as they require more pressure/volume over drum. The rear drum will work fine with manual pressure.

IMO, a late/early seventies DUAL RSVR MC is not that noticeable if FORD (BENDIX) design. If you need to stay with the OEM MC, you could juggle shoe size/wheel cylinder size to make the drum brakes more aggressive.

Hope all of this made sense.

Now all of the above is IMO, and you need to verify before attempting.

EDIT -1945 HRS 2/11/19

The above text was changed somewhat hoping to make it more understandable.

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charliemccraney

Posted 10 hours ago

There is more to a dual master cylinder than simply bolting one on to get the safety feature.  I know because I had a rear brake line leak that resulted in no brakes at all.  I don't know what else is required but do not let it give you a false sense of security in a non stock application. ----------------------------- Just wondering. With your install, did you include a Pressure Differential Valve? It has a provision for a sending unit to trip a warning lamp in the cabin in case of a failure?

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Valves FAQ

What does a proportioning valve do?
A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions.

What are the symptoms of a bad proportioning valve?
If your prop valve has gone bad your rear wheels will lock up easily, especially on wet surfaces.

What does a metering valve do?
A metering valve or "hold off valve" is used in the disc portion of a disc/drum system to hold off the application of the front discs slightly allowing the slower reacting rears to catch up. This provides rear stability on wet surfaces and reduces excessive pad wear.

What are the symptoms of a bad metering valve?
Your car will nose dive and the front pads will wear too fast.

What does a residual valve do?
There are two different uses for residual valves. The 10 lb valve is used to hold a residual pressure to the drum brakes to keep the shoes out close to the drums giving a higher firmer pedal. The 2 lb valve is used in the disc when the master cylinder is lower than the calipers to prevent back siphoning of the fluid from the master.

What are the symptoms of a bad residual valve?
The brakes will be very spongy and you will need to pump the pedal to get good brakes.

What is a combination valve?
A combination valve incorporates metering and proportioning into one valve providing all necessary valving for disc/drum systems. 
See below

Should I use an adjustable proportioning valve?
Not if you can help it. The adjustable valve will only provide the proportioning function and not the metering that is needed.

SOURCE- https://www.mpbrakes.com/page.asp?p_key=AD8E13B3C7E0409FB4FF6C7C7267C59A

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Great resource.  Answers all questions I had.  Thanks!
https://www.mpbrakes.com/page.asp?p_key=AD8E13B3C7E0409FB4FF6C7C7267C59A

There is more to a dual master cylinder than simply bolting one on to get the safety feature.  I know because I had a rear brake line leak that resulted in no brakes at all.  I don't know what else is required but do not let it give you a false sense of security in a non stock application.
I was fortunate to realize I had the problem in my driveway.


Lawrenceville, GA

You're right, especially on a dedicated restoration.



Yes, but you are giving up the safety feature of the dual reservoir MC.



The dual reservoir MC is a safety feature. Depending on what part of the hydraulic system fails will dictate as to how the system reacts. The system is meant to be able to get off the road with a somewhat safe stop. It is not intended for continued driving

An operational safety brake is paramount as you have no idea what to expect with a hydraulic failure, especially if a PDV (pressure differential valve) is not used with the swap to isolate the failed system (front/rear or possibly both) and give the driver a warning lamp indicating a system failure.

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Everyone uses a dual master cylinder. No way for this to look right under the hood. The '65 Mustang had a single master cylinder with power and non  power brakes. Would the stock master cylinder and boster work with front disk brakes and a proportining valve? I realize the dual master cylinder is a safety thing but I had a rear break like fail with a dual master cylinder once and the emergency brake would have helped as much. I would have hit anything that would have been in front of me at the time anyhow. I have this obsession with keeping things looking as close to orginal as possible.

miker Posted

As K points out, keeping the rear adjusted is really important with the conversion.

That was oldcarmark Mike, credit where credit is due.

On a 55 FORD/BIRD drum brake, there is an eccentric on the FRT primary shoe that requires adjustment before the normal star-wheel adjustment is attempted.

I have also noticed the vendor who supplied a BIRD FRT DISC CONV KIT using no proportioning valve (with MIDLAND BOOSTER) is now supplying one with his kit. I understand the using of larger rear wheel cylinders to achieve somewhat correct brake bias but it still has to have a proportioning valve. The FRT DISC are much more aggressive that the original drum and the hydraulic LINE pressure will cause rear brake lockup more quickly with DISC/DRUM..

The use of a FRT METERING VALVE is also useful as it will somewhat delay the FRT DISC aggressiveness leading to a more balanced braking system.

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When brakes lock and the tire actually grabs the road surface, you lose steering control and the rear end may swing on you, especially on a wet surface. This why ABS came along.

There is one kit manufacturer that defines the propositioning valve while actually describing a metering valve. They are all over the place.

I hate to bring this subject up in a forum as it usually turns into a food fight. But anywho, I just wanted to bring this out.

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