What Master Cylinder

[V8COOPMAN]

Originally Posted by '28 RPU

I am putting '40 Ford Lockheed style brakes on my Model A. I am using a Vacuum type booster. Here's my question: what dual reservoir master cylinder would I want to use? It's my understanding that the original cylinder bore was 1 1/16" bore, Is that correct? The front and rear wheel cylinders are different bore sizes so how would that work with a dual Master? To top it off I am going to need to use a remote reservoir, Would that require a special master or would you just add a remote reservoir to a regular type master. Thanks!

If using a DUAL M/C that has the same bore on BOTH ends of the cylinder, you will effectively be reproducing the results found using the stock, single bore M/C. You noted that the front and rear WHEEL CYLINDERS for the '40 brakes have differing diameters. There are a couple of reasons for this. One reason being that the REAR brakes are responsible for LESS braking than the FRONT brakes are. A couple of factors figure into this, like the front axle of many cars bear more weight than the rear axle, possibly ~60/40 for instance....YOUR RESULTS MAY VARY!

Additionally, when the brakes are applied, especially in taller vehicles like these old Fords tend to be, weight transfer takes place effectively transferring more weight from the rear braking load toward the front brakes' braking load. This amounts to the REAR brakes requiring LESS pressure than the FRONTS, to help refrain from locking-up the REARS with less weight on them. Meanwhile, the FRONTS with a higher percentage of weight on them are working as hard as they possibly can in the attempt to pressurize (slow) the drum BEFORE lock-up.

Remember, a tire that is locked-up and skidding is not producing anything near the stopping power of a tire that is just shy of breaking-loose. For that reason, and taking into account ALL of the above, there is an awful lot of serious engineering that goes into the sizing of wheel cylinder and master cylinder combinations.

Getting back to the rear wheel cylinders being smaller than the fronts. Considering using a M/C with a constant bore for both fronts and rears, an INCREASE in WHEEL cylinder bore to a LARGER DIAMETER will produce MORE pressure at the wheel cylinder/brake shoes. Inversely, DECREASING wheel cylinder bore to a SMALLER DIAMETER will DECREASE pressure at wheel cylinder/brake shoes. Alternatively, changing the M/C bores in relation to wheel cylinder bores will also change effective pressures.

Undoubtedly, most of us mere mortals don't have the credentials nor the facilities to properly engineer a braking system to the fullest. But there are tried & true combinations that guys like us have swapped and used for decades that work efficiently when using quality parts. Lockheed hydraulic brakes on an 'A'-bone is certainly not a new idea. A lot of guys running that combination wanting to employ a dual M/C tend to use a '67-'73-ish Mustang M/C designed for DRUM/DRUM brakes.

One more thing to remember....ALWAYS test that your brake pedal STROKE has enough room for a FULL stroke WHEN ONE SIDE OF YOUR SYSTEM is OPEN. It takes a LONGER STROKE than normal when one side of your system has a leak in it. You can see WHY if you carefully study the system BELOW!

Coop