Tech Talk with Master Power Brakes

Got A Soft Brake Pedal? How to check the Booster Pin to Master Cylinder Gap

Posted by Master Power Brakes Team on Feb 26, 2018 10:24:00 AM

A common problem and a question we frequently answer regarding power brakes systems is:

“Why does my brake pedal feel soft?”

One of the most overlooked things is probably the simplest fix.

Let's take a quick look at the placement and role of the booster push rod (pin) in the master cylinder. 

Extending out of the booster is a push rod, or pin. This pin simply extends into the piston in the back of the master cylinder.

Have you ever manually set the distance of the pin?

If you answered no, not only is this something you must do, but it may also be the reason for your soft pedal.

Have we confused you? Take a look at the picture below and see if this helps. 

Main Booster Image-2

Ideal Gap Distance from Pin to Master Cylinder

Master Power Brakes recommends that the distance between the pin from the booster and the hole in your master cylinder be approximately 0.020”.

If this distance is greater than 0.020”, you will have to overcome this distance to get the pin to the master cylinder when you first push the pedal. This will cause your soft brake pedal.

If the distance is less than 0.020”, the master cylinder may not be returning the entire amount and therefore causing a different problem of brake drag.

The fix?

The simple thing to start with is a handy little tool that we offer called a Push Rod Depth Gauge (see below).

Simply set against the back flange of the master cylinder, push in on the center slide and then flip it over and lay it against the front edge of the booster. The center slide should be 0.020” from the pin on the booster. If not, determine the distance needed and adjust the pin in the booster to the proper length. Some boosters have a threaded pin in the end and others can only be adjusted by grinding on the pin. If you don’t have one of our tools, then break out the ruler and straight edge and you will have to do a little math, but the end result is the same.

depth gauge master power brakes


 Step 1Step 2


Check the Depth 

Step 1 - Check the depth of the hole in the master.     

Step 2 - Flip the tool and check it against the booster.


Adjust the Distance

Now that the depth has been checked, you can adjust the pushrod to the proper length. Some boosters have an adjustable center pin that can be simply threaded in and out. Other boosters do not have a threaded pin and will require that the pushrod have some length ground away.

If you are in the rare instance that you need to lengthen the pushrod, STOP what you are doing and give us a call (there will be different options here, and we'd like to help walk you through it).



Hopefully setting this gap will take care of some of your problems with a soft pedal. If it didn’t, check out our blog on the top 8 causes of a soft brake pedal


READ: Why is my Brake Pedal Soft? How to Diagnose and Fix



Click Here To Go Straight To Our Depth Gauge




Topics: How To Series, Video Guide

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