A common problem and a question we answer more than we can remember is “Why does my pedal feel soft?” in my power brake system. One of the most overlooked things is probably the simplest fix.
Extending out of the booster is a pin. This pin simply extends into the piston in the back of the master cylinder. Have you ever set this distance? If you answered no, not only is this something you must do, it may be the reason for your soft pedal. Have we confused you? Take a look at the picture below and see if this helps.
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”, when you first push the pedal, you will have to overcome this distance and get the pin to the master cylinder. This will cause your soft 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.
"How do you fix this?" is probably the question you are asking and how do you determine what your gap is? The simple thing to start with is a handy little tool that we offer. It is a Push Rod Depth Gauge (p/n: AC2003) and can be seen 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.
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 as there may be different options.
Hopefully setting this gap will take care of some of your problems with a soft pedal. If it didn’t, you have at least taken proper steps to making sure your brake system is performing and set up correctly.