Tech Talk with Master Power Brakes

Selecting the Right Booster-Master Cylinder Combo for Your Classic Car

Posted by Master Power Brakes Team on Jun 28, 2019 2:31:11 PM

If you're looking to convert your current manual brakes to a power brake system, you might be wondering which booster-master cylinder combination to choose. 

Some applications are pretty straightforward with only one option, but we see plenty of situations where two, three or even four different options can work. It then becomes a choice of determining which conversion kit combo is the right one for you. 




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As you can see, owners have some choices to make here.

First, determine what you want to achieve.

To better illustrate these choices, we will be using an example from a recent phone call that we had with a customer about their 69 Camaro.   

Questions to Ask Regarding Booster and Master Cylinder Combinations  

Are you converting to disc brakes? If so, what kind?

Are you looking to leave the OE drum brakes, but looking for a better pedal feel? 

If you are converting to disc brakes, especially a four-wheel disc brake conversion, you cannot have too much assist in the system. At the same time, there are limitations.

In the case of our customer with the '69 Camaro, a 9” dual booster is the preferred scenario. Unfortunately, this particular car has a big block engine with tall valve covers so space is more than likely going to be an issue.

In this case, we are going to be forced to sacrifice a little bit of power assist and choose a system using an 8” dual diaphragm booster. This trade-off will still provide an assist that makes the pedal feel better than manual brakes but not quite as good as a system with the 9" dual diaphragm booster. This is one of the drawbacks to classic cars. Some of them were built without the thought process of using a brake booster and therefore are very limited for space.


Tech Tip: One of the drawbacks of classic cars is they were not originally designed with a brake booster in mind, so space may be limited. 


What's the difference between the 11" single diaphragm booster vs the 9" dual diaphragm booster?

While we are discussing the different boosters, we also get questions about the differences between the 11” single diaphragm booster versus the 9” dual diaphragm booster or some other variation and what makes one different from the other. Generally speaking, the larger the diameter the more assist it will provide. That was true until dual diaphragms became available. This allowed a smaller diameter to provide as much and sometimes more assist due to the fact there are two diaphragms inside making things work. To determine the assist provided by the brake booster, a very simplified equation is to multiply atmospheric conditions by the diameter of the booster and multiple that by the number of diaphragms in the booster. The resulting number is the amount of assist the booster provides. This is a simplistic way of looking at the situation, but it gets you headed in the right direction.

Now, let’s get back to our vehicle from above and determine which system to pick. From the above list, the 11” single won’t provide as much assist as the 8” dual, which in turn won’t provide as much as the 9” dual. At the same time, the 11” might fit somewhere that the 9” dual won’t and vice versa.


Tech Tip: When converting to disc brakes, always go with as much booster as will fit your vehicle. 


Some vehicles will allow for a larger booster, but others will only allow for a smaller option due to space constraints.

As much as we sing the praises of disc brake technology over drums, not everyone sees things our way. With this in mind, what are the best options for owners looking to keep their drum brakes?

In this scenario, you don’t want as much assist as possible because too much booster will create a touchy brake pedal that will throw you through the windshield. In this case, we would recommend a booster that is on the small side and go with a 9” single diaphragm.

MC rear ImageOh but wait, there's more. If you look carefully at the list above for the 1969 Camaro, you’ll see different bore sizes on the master cylinder. The different bore sizes are directly related to the rest of your brake system. The larger piston (like the one used in the Legend Series), calipers typically require a master cylinder with a larger bore size while the smaller but multi-piston calipers require a smaller bore size master cylinder with slightly less volume but more overall pressure. 

Even though we used the 69 Camaro and its related Booster/Master combos for illustration purposes, the guidance above will hold true regardless of make, model, and year. 

If all of this gets too confusing, give one of our sales representatives a call and we will gladly walk you through the steps and put you in the correct system based on you, your vehicle and your needs.


UP NEXT: Pro-Driver Complete Brake Conversion System Video


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