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Tech Talk with Master Power Brakes

Is a Master Cylinder Change Required in Drum to Disk Conversion?

Posted by mpbrakes1 on Mar 21, 2016 5:01:35 PM

master-cylinderWe have been asked a number of times if there is such a thing as a manual disk brake conversion kit? The simple answer is yes. The power booster component almost exclusively affects how your brake pedal "feels" to you under foot.

In a situation where your classic car has multiple drivers, some of whom are used to the "foot feel" of power brakes - maybe they spend most of their time driving a newer car or light truck - a power-assisted brake pedal can makes it easier to go between vehicles.  The power booster allows you to maintain relative consistency with how much pressure to apply when stopping either of the two vehicles.

You DO NOT NEED to upgrade or convert to a power assisted apply system, however, when making the upgrade to disc from drum brakes. Recently, we got an email from a customer planning on making that drum to disc brake conversion but interested in sticking with a manual set-up.

"Dave" has a '66 GTO he uses as a daily driver and wanted the enhanced performance and stopping ability of disc brakes. But as the sole driver of the car, Dave was comfortable with the pedal effort the manual brakes currently required and wanted to keep that same level of brake feel.

His question centered around the fact that he had bought a master cylinder approximately five years ago in an effort to make the car safer by getting rid of the single bowl master cylinder. At that point, he was only concerned with that safety factor of the master cylinder. Things change and now he is interested in converting over the last step of safer brakes and going with a disc brake conversion kit. We were able to go back and look at his previous purchases and did find out that we had in fact sold him the proper master cylinder that will allow him to keep the same master cylinder while doing the disc brake conversion.

Let's assume for a minute though that we didn't have access to Dave's previous purchase. How can you tell if your master is appropriately sized to operate a new (disc brake) system? The bore size is critical when selecting a master cylinder for use in either a power or manual apply brake system. The wrong size master cylinder added to a power assist set-up could actually provide for a pedal feel that is even harder than what a well working manual system would ever provide. Measuring a master cylinder bore size is actually quite easy for most folks. A quality ruler is generally all that is needed.  After pulling the master cylinder forward on the studs, measure the inside of the bore on the back of the master cylinder. In general - the smaller the bore size, the more line pressure you will have in the system at a particular pedal effort. Conversely, larger bore size gives less movement of calipers for the same foot pressure and therefore almost always translates into what most would consider a hard pedal.

Looking at the chart below will give some further insight into how different bore sizes can greatly affect the overall brake pressure along with how much pedal effort is needed to get to the same point:

[table id=1 /]

Here comes a pitch for calling in an expert to ask the question. With your measurement in hand, call us at
1-800-382-9772 and ask the question: “will this be appropriately sized for my vehicle?” We're here to help you appropriately size brake components.

Topics: drum-to-disk conversions, Master Cylinder

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