Brake Drag Prevention

6/25/12

By Right Stuff

Booster/Master Cylinder Tech Tips - Preventing Brake Drag in power brake systems.

Adjusting the rod between the booster and the master cylinder:

We sell a lot of different boosters and master cylinders, and they all connect together differently. Regardless of what booster and master combo you have, you need to make sure that the rod that pushes into the master cylinder from the brake booster is adjusted properly. Be sure that when the master cylinder is bolted up tight, the rod protruding from the booster is about 1/16” from making contact with the piston of the master cylinder. Obviously, this is something you cannot see once bolted up, so you may need to measure the depth of your master cylinder pocket and the length of the booster rod. 

The booster rod should almost be making contact with the master cylinder, but not actually touching. In a master cylinder with a deep pocket, the rod will have to enter the master cylinder by about 1.5” before making contact.

-         If the rod is too long, you will notice that the master cylinder does not slide freely up to contact the booster because the booster rod and master cylinder piston are pressing against each other. This will “pre-load” your master cylinder causing your brakes to drag at first and eventually heat up and get worse the further you drive.

-         If the rod is too short, you will press the pedal but you will not actually be engaging the piston of the master cylinder because the booster rod is not making contact. This may be your issue if you are having trouble getting fluid to bleed to the wheels.

If the booster rod is too long, here is how to fix it. There are two different booster styles, therefore two sets of instructions:

1. Our “Delco Style” 9” and 11” boosters include a long and short rod as seen below:

                   

            Long Rod in Booster                  Short Pocket         Deep Pocket            Short Rod in booster   

If you have the “Delco Style” booster seen in the illustration, you will first need to install the correct booster rod. You want to use the short booster rod if your master cylinder has a short pocket and the long booster rod if your master has a deep pocket. This usually is all you need to do to have the proper set-up. (see photos)

If you use the correct booster rod and you still have pressure on the master cylinder due to the rod being too long, check to make sure the rod is fully seated down into the booster as far as it will go. When the booster rod is installed, the end that goes into the booster sits in a small hole in a washer down in the booster. If this washer is off center, the rod will not sit down far enough into the booster. You can re-center this washer with a screwdriver if needed. If your booster rod is seated all the way into the booster and you still have pre-load, you may need to grind down the end of the rod that presses into the master cylinder 1/8” at a time until the pressure is released.  (this is due to variations in aftermarket master cylinder pocket depths and is usually not necessary)

2. If your booster has a rod that is part of the booster like below, and you have a pre-load issue, follow these instructions:

The screw seen at the center of the booster is adjustable. It can be screwed in or out to give you the proper length to mate up to your master cylinder piston. Sometimes this screw will be too long, even when it is screwed all the way in. This causes a pre-load on your master cylinder. In this case, you should completely remove the screw from the booster, take off the small brass neural that is near the tip of the screw and reinstall the screw. Removal of this little “neural” will give you the extra 1/8” that is sometimes needed to get the correct length. Still too long? Remove the screw completely and bolt master cylinder up (this is usually not necessary).

If the booster rod is too short, you need to install the longer booster rod. If your booster does not have a removable booster rod, you may need to order one of the plugs seen in the photo below to shorten the depth of your master cylinder pocket.

Note: Most of our master cylinders have a deep pocket with a plug to make it short if needed. If your booster’s rod is short, you may need to order a plug (MCBUL01) to fill space in your master. See pic