Tech Talk with Master Power Brakes

Explaining a Bellcrank Assembly

Posted by Master Power Brakes Team on Mar 30, 2018 10:15:51 AM

What is a Bellcrank Assembly?

How a power brake conversion mounts to the firewall can be more than what you might think.

We recently had a customer that wanted to mount his booster directly to the firewall and didn’t understand why he couldn’t do that and why the bracket assembly we use is necessary. We take it for granted but in thinking further about it, we thought we might need to explain.

On a lot of our applications, we use a bracket assembly between the booster and the firewall that we commonly refer to as a bellcrank assembly. At first glance, it looks like a simple method of locating the booster on the firewall however there is much more to it than that.


Bellcrank Ratio


So, what is so important about this? Obtaining proper pedal ratio is extremely important towards the overall operation of the brake system. Everything starts with the brake pedal when applying the brakes. If the pedal ratio isn’t correct, the brake system cannot function correctly.

When using a vacuum brake booster, a pedal ratio of 4:1 is necessary. In comparison, a typical manual brake system uses a pedal ratio of 6:1.


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Bellcrank Assembly

How we go about changing the pedal ratio is done on a lot of applications through the bellcrank assembly. Inside the left and right mounting brackets is a pivot arm. This is the important part of the bellcrank assembly.

As the push rod comes from the brake pedal, it pushes on the pivot. The pivot arm has an attachment at the bottom that the pivot arm rotates from and an attachment at the top that then pushes upon the booster. The distance between all three of these points creates a ratio through a simple formula. This ratio when combined with the ratio on the brake pedal creates the proper overall ratio which is approximately 4:1.

If the booster is mounted direct to the firewall, the brake booster is going to need to move down significantly on the firewall for the push rod to intersect the brake pedal at the proper point to make the necessary change in the ratio.

When a bellcrank assembly is used, it allows for a true bolt-in situation where all of the guess work has been taken care of by us.


Got Questions?  Speak With A Specialist Now! 


Now that you know how a bellcrank assembly works and what it does to the pedal ratio, why does that matter?

If the pedal ratio is not changed, a couple of scenarios can exist. When pressing on the brake pedal you will experience either a really hard pedal or the opposite which is a brake pedal that is very touchy. Neither scenario is very fun when driving your car. With the proper ratio on the brake pedal, every push of the pedal will result in a fairly effortless application that can be easily modulated through less or more effort on the brake pedal.


Hopefully this will give a little more understanding of what is going on within the brackets that mount the booster. Every piece of the system has a purpose and understanding that purpose is important toward the proper function of your brake system.

If you have further questions, don’t hesitate to ask one of our technical representatives via email or a phone call.



Topics: How To Series, Video Guide, bellcrank assembly

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