The brake booster is the vacuum powered actuator that helps you push the brake master cylinder.  This is power brake option we  buy on our cars.  Power brakes are not optional on the GMC motor home.  Without the booster you have to put both feet on the brake peddle and pull off the steering wheel trying to push hard enough to stop.  This shows it is time to replace the brake booster.  Boosters normally fail by leaking vacuum to the point that with the motor running you might get one pump of the brakes' and/or the peddle becomes hard to push.  Our motor homes are 20 plus years old.  Your booster needs to be replaced if you have not replaced your booster or have a documented replacement.

Replacing the booster does not require special tools.  The hydraulic system does not need to be opened or bled. They require no critical adjustments.   Several hints and tricks  will help you complete this job.  The booster is a critical part of the braking system for the GMC so if you are not comfortable working on these components, take a copy of this article to your favorite repair facility.

Purchase the replacement booster unit before you start the job.  The GMC booster is unique and you must get the correct unit.   Sources are available at the end of this article.  The two-inch push-rod between the peddle and the booster  is unique to the GMC.  Do not  replace this push rod.  They are not available from any source.

Click for DetailsThe booster is found at the driver-side front-access door where you check your oil.  See (A) on the photo. The booster attaches to the firewall with four bolts and the brake master cylinder (B) attaches to the booster with two bolts.

The master cylinder (B) can be unbolted from the booster and gently pulled forward toward the front grill.  Do not remove the brake lines. The cylinder will move forward enough to be out of the way and not damage the attached brake lines.

The four nuts that hold the booster in place are inside the driving compartment.  Later models  of the GMC have a part of the plastic dashboard that covers the area and hinders access to these nuts.  The trick is to cut out the bottom of this plastic enough to reach the nuts from the bottom of the dash. This plastic provides no structural support so no damage is done to the dashboard. The cutout cannot be seen from the front, and you will not have to remove the dash board.  Be very careful that you do not damage the plastic plunger assembly on the booster when you cut out this plastic.  I used a Dremmel tool with a router tip to make the cutout.   Even if you do not want to replace your booster now, it would be a good idea to make this modification so you can replace the booster on the road when it lets you down.

Disconnect the brake peddle and remove the booster out the front of the coach sliding it over the master cylinder.  The new booster goes back in the same but reverse order.

The first time took me four hours.  The second time would take less than two hours. I did not know how much of the plastic to remove, so I had to make several cuts.  I will soon have a picture here to show the cutout.

Here is  Arch's version on an older model GMC


Dremmel tool
Flat ratchet wrench