Overhaul of a Bendix Multi-Vac Brake Booster
By Bob Burgoyne
Cool! Bob Burgoyne of Peyton, Colorado, has found a vendor for Hydrovac kits! Bob has taken the time to put together this great step-by-step on overhauling the Bendix Multi-Vac Brake Booster. I know a lot of folks (like me) wonder what to do with their rugged looking hydrovacs. It's cool to know there's hope for them after all. So here's the Tech Tip ... and as Bob says, "it’s all about sharing information and I hope that some others will benefit from it!!!"
We have another Tech Tip on the Bendix Hydrovac system by Jeff Erickson. He rebuilt his from scratch. Check this one, too.
Click on the images for a much larger picture to show all the details. Bob worked hard on getting these right ... so you need to see it ALL!
With winter and shorter days fast approaching and my 1964 Chevy C60 dump truck stripped down to a rolling chassis, I decided to push her into the back yard until spring and work on a few of the smaller parts that I had removed during the disassembly process. The two 10' x 12' sheds that I built the previous year came in handy for my toys. One housed my two Honda 500cc utility 4 x 4 ATV's and the other all the parts that I had removed from my truck. I bought thgree wire Metro racks and after assembly, bolted them to the studs. Then I inventoried all the parts on the shelves. Having already restored the heater box assembly, radiator support panel top cover, cab cowl vents and several other smaller items, I turned my attention to the Bendix Multi-Vac brake booster.
As you can see from the image here (don't forget to click the picture for a larger image), the Multi-Vac unit, although functional, was pretty tired looking and required more than a casual cleaning and repainting. All external surfaces were rusted and partially covered with tar and debris from years of road abuse and, as I later found out, most of the internal rubber components were in pretty sad shape. In reality the unit was in surprisingly good shape given the fact that it hung beneath the cab and endured 40 years of the elements.
After removing the unit from the chassis of the truck I noticed a small identification tag on the locking ring bolt with an alpha-numeric identification code. The code is shown below:
I contacted our own "Spanky" Hardy and he graciously offered his expertise and research services. Spanky indicated that those were Bendix part numbers and that they cross referenced to GM part number 3874183.
An exhaustive search of the web didn't yield any information regarding parts or overhaul kits for this Bendix Hydrovac unit. Just when I was about to abandon the project for the winter and work on something else, I stumbled across a link that lead me to another link that lead me to Reman, Inc (formerly BSC Parts) -- talked to their resident expert, Rick Eskew (404.659.8723 or 800.462.0062). He has over 35 years experience in the booster business.
I checked out the site and it seemed encouraging, so I dropped them a detailed e-mail with all the particulars. The very next morning I received a response from one of their customer service representatives indicating that they not only had an overhaul kit for my Hydrovac but also offered completely remanufactured units (core charges applied). I subsequently called the company to get verbal confirmation and also just to say hello. I had a great conversation with their representative and, as an added dividend, discovered that they can also provide master cylinder and brake cylinder overhaul kits (or remanufactured units) for my 2-ton truck.
Later that evening, "Honey, I need to spend some money…"
With the overhaul kit on order it was time to disassemble the Multi-Vac. Prior to tearing it apart, I studied my 1963 Chevy Truck Shop Manual. One chapter is devoted to various brake boosters and includes a very detailed overview of how to disassemble, inspect and reassemble the various power booster units that came in our trucks. Also included are detailed illustrations or "exploded views" of the various booster units.
I took many digital images from all angles to properly photo-document "what goes where." Although the unit is pretty simple, I like to get into the habit of photo-illustrating all the restoration activities for future reference and inclusion into my web site. I then took it apart and kept all the parts in a small Rubbermaid tray for safe keeping. The picture here illustrates the major components of the Multi-Vac --->
Initial cleaning was done with a stiff bristle brush to remove the heavy dirt and debris. After subsequent cleaning in the parts tank to remove any grease and oil, I washed the parts in soapy water and dried them. This may seem like over-kill, but it helps keep the reclaim unit on the sandblaster cleaner and eliminates the possibility of getting grease and oil products in the system.
Prior to sandblasting the valve body, I used rubber cap plugs and/or electrical tape to cover all the valve ports and cylinder bore. I knew that some media would find its way inside, but this precaution was simply to prevent pressurized media from hitting the honed areas. Sandblasting was pretty straight forward and in a mere couple hours, I had all the pieces of the Multi-Vac cleaned to bare metal.
I used clean cotton cloths pre-moistened with isopropyl alcohol to wipe any media residue for the larger assemblies. For the valve body, I used a variety of nylon bristle bore brushes and warm soapy water to clean the cylinder bore, ports and threaded openings. I didn't want to take my nice clean bare metal assembly and put it in a sink full of warm water, but I couldn't think of a better way to completely remove any and all media residue that undoubtedly found its way into the valve body. After a thorough washing and rinse in warm water, I dried it using a clean towel and then further with a heat gun to ensure there was no residual moisture. This approach may seem odd, but I couldn't justify the cost and time necessary to find a company that has a chemical dipping tank.
I am restoring my truck as close to original as possible. However, I am not going overboard because I plan on using my truck and, other than an occasional weekend rally, have no intentions of showing it. As such, I decided to choose my own colors for the Multi-Vac unit. Given the few pieces I had to paint, I elected to use Rustoleum from the can; a product readily available at most hardware stores and home centers. The primer was a standard light grey and I chose Hunter Green for the valve body and a hammered finish Gold for the diaphragm dome halves and locking ring.
I again inserted cap plugs into the plumbing connections of the valve body, and also masked a circular area on the half of the dome that mates with the valve body.
Given the inherent porosity of spray can paint, I applied 3-4 light coats of primer to each piece, allowing approximately 15 minutes between coats to allow the previous coat to partially dry and also to prevent running. Once the final coat was applied, the pieces were allowed to dry for a minimum of 48 hours. Each piece was then lightly sanded using dry #400 wet-or-dry sandpaper and wiped down with a clean dry cloth. The color coats were applied in the same manner as mentioned above. The two pictures above and three below show the completed pieces.
The overhaul kit from BSC Parts (now Reman, Inc) arrived via UPS which included the main and control valve diaphragms, cups, o-ring, seals and control valve spring. I also purchased three new bolts used for securing the valve body to the dome assembly.
Assembling the Multi-Vac was simple and straightforward and step-by-step instructions were clearly illustrated in my 1964 Chevy Truck Shop Manual. Putting the Multi-Vac back together was merely a reverse of the steps used to disassemble the unit.
As received, several areas of the rubber diaphragms were covered with a white powder-like residue. I suspected this was a release agent used during the molding process, so I cleaned it off using "Bleach-White" (a readily available product used for cleaning white walls and raised white letters on tires) and warm soapy water.
All rubber components (minus the diaphragms) were lubricated using brake fluid prior to installation. The hydraulic cylinder components were assembled first and inserted into the cylinder, then the control valve was assembled and installed and finally the main diaphragm assembly. All moving parts were exercised to ensure free movement prior to moving to the next step.
The following pictures illustrate the completely restored Bendix Multi-Vac Power Booster:
Bob & Gina Burgoyne
11540 Cranston Drive
Peyton, CO 80831
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