Bendix Hydrovac Overhaul
by Jeff Erickson
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      I thought I'd send in a Tech Tip on rebuilding the old Bendix Hydrovac units. I have the images (that are referenced in the article) also posted in my Photobucket account.

      I did some research but it didn't seem like there was much info anywhere about these units. Stovebolt does have a Tech Tip by Bob Burgoyne on an overhaul of a Bendix Multi-Vac Brake Booster. The difference with this project (that I know of) is that this type of booster was used from 1947-60ish??? The newer booster had the front and rear brakes as seperate systems, proportioning valve, etc. This old booster has front and rears on the same system. That is why the master cylinder was in front of the older system and behind the new one.

      There was one booster used before this model of hydrovac that was an extremely simplified version of this one. The other type was a newer version of vaccum booster. I'm assuming my unit is its predecessor? I think (key term 'think') that Burgoyne's booster is the kind where the brake pedal itself actually links up to the booster. When the booster is activated, it then pushes fluid through the master cylinder and so on. On mine, you push the pedal and it goes through the master cylinder which then pushes fluid through the booster. As pressure builds, all the little doodads (extreme technical term) work to open and close valves to make the booster function and help out. In other words, Bob's is new fancy technology.


It's a good "get 'er done" project

Bendix Hydrovac Unit        I've seen several posts with questions about the old Bendix Hydrovac units and there's a substantial lack of information available on how they work and how to service them. It's a pretty neat unit. You CAN get the kits for these units. I had to email a dozen places before I got anywhere and most places wouldn't even email me back. They pop up on eBay now and again. But for the price of a kit, versus what they want to rebuild them, I'd rather save some money and do it myself. It's not real hard. Actually, if your up for it, it's pretty fun.

        Now there is a lot of confusion on what type of lubricating fluid to use in these specific units. I did a lot of research and it seems inadvisable to use any type of petroleum-based fluid. I'm not certain about how the replacement seals would handle real oils. My understanding is that oil will eat away at the seals in the old units. After searching the internet for several hours, I decided on using mineral oil. Its not petroleum-based specifically, but created as a byproduct of petroleum refining. I used the pharmaceutical grade. It's readily available, better quality and won't hurt those old seals. It's also a great oil for this purpose. If you have any fear of using this, find a corn-based hydraulic fluid instead. Mineral oil can be found at any grocery store or pharmacy between the enemas and laxatives. (Wow, this would be a great place for a comment, but I'll contain myself ~~ Editor) Good dual purpose stuff! I put a light coating on all parts during reassembly.

First a little bit about how it works

        There are a ton of different models and sizes of Hydrovacs so they are not all the same. Open up this image and follow along:

        As pressure is applied to the brakes, fluid is passed into the Hydrovac (yellow). It by-passes the hydraulic brake piston (# 20, thanks to the check ball), and pressure goes through the lines to the wheel cylinders. At the same time it also passes through a pathway in the unit and applies pressure to (# 10) the control valve hydraulic piston. As more pressure is applied to the brakes, the control valve piston pushes up on the metal rod on the rubber diaphragm (# 7). The metal portion of the diaphram has an air pathway through the center which allows air flow circulation throughout the unit (through the tube (# 9) to the rear of the power cylinder piston). The atmospheric / vacuum valve (# 8 and # 13) is a two part valve located above the diaphragm. It blocks air coming into the Hydrovac due to the spring (# 14) under the intake tube. Brake pressure changes cause the vacuum valve to seat on the rubber diaphragm (shown in pink), sealing the air passage through it and effectively blocking any air coming into the power cylinder (# 1) between the ports (# 6) located in the end plate of the unit and the connecting tube (# 9).

        As pressure builds and the diaphragm is pushed even further forward, the atmospheric valve portion opens (shown in purple) allowing air to enter the power cylinder (air path shown in green) and letting the vacuum from the engine pull the power cylinder piston (# 2) forward which pushes the hydraulic brake piston (# 20) into the hydraulic cylinder (# 22). The check ball has pressure from the brake lines pushing back against it now (orange) and seats inside the piston allowing no brake fluid to pass by it back into the master cylinder. Therefore, the piston pushes more brake fluid forward in the lines and braking power increases.

        The whole system uses brake pressure to overpower the springs and engage / disengage the control body valves. The reason this system works the way it does is that the vacuum created by the engine is able to travel throughout the Hydrovac unit (shown in red). Therefore with vacuum in front of and behind the power piston, there is equal pressure on both sides. So until the valves close off that circulation and opens the air intake, the power cylinder piston won't move. Closing one valve stops that circulation throughout the unit. Opening the intake allows the cylinder to pull air from behind so the vacuum can pull the power cylinder piston forward.

        Did you get all that? Its a little confusing.

Time to overhaul

        Now that you know how it works, it's time to overhaul it. (Here's a view from the top) It's a great idea to have a nice big flat box to put the pieces in. As you remove a piece, this will help you remember how they go back together.

        Disconnect the brake lines and mounting bolts and take it off the truck. Now mount that sucker in a vice by the hydraulic cylinders end cap. Loosen the lock nut located at the base of the hydraulic cylinder. Unscrew the unit from the cylinder. Now unscrew the cylinder from the end cap. The copper crush gasket is a special kind and can't be reused.

        Next, remove the snap ring that holds the intake tube on, and remove the intake tube and gasket from the control valve cover plate. Some of the newer units don't have an intake tube but actually have a filter screen under the snap ring instead. Remove the clamps from the hose. Remove the five machine screws holding down the control valve body and set aside the body, spring and diaphragm. (End plate with hydraulic nut removed)

        I removed the tube from the control valve body so I could clean and paint it. Using a 1 1/8" socket, unbolt the hydraulic cylinder nut and flip it over. Using needle nose pliers, pull the hydraulic piston from inside the nut. I pulled the snap ring and washer from that piece and made sure there was no gunk in the grooves. Unbolt the J-hooks from the unit and remove them. Pull the end plate from the power cylinder casing. You'll want to set the piston on a firm surface and push down on the end plate. This pushes the pushrod up through the end plate allowing you access to the hydraulic brake piston. As it's up, you'll have to pry the spring up on the piston and using a toothpick or similar item, push out the tiny rod that connects the piston to the pushrod. Separate these parts and move on to the end plate.

        Looking inside the main bore, you'll see another rubber seal and snap ring. Remove those. Watch out! Mine had a spring under the stop washer and it wasn't shown in my book and you could put out an eye -- or worse -- lose those parts. Mine had a snap ring, a stop washer, a spring, a two piece seal retainer, a rubber cup seal, and a chamfered fiber washer. There is also a seal, located in the bottom below that, that will need to be driven down and out of the end plate. I used a small socket and hammer. The piston plate on mine was in great condition and I didn't feel the need to take it apart.

        A lot of the originals consisted of a leather braided seal that wrapped around the power cylinder's piston. Mine was a later type and had what looked to be a felt ring under the rubber sleeve of the piston.

        Now as one last thing to do, check out the two hydraulic pistons. If you have all new seals available, the fastest and easiest method is to just cut them off being very careful not to score the surface they sit on. You'll also need to remove the c-clip and washer from the small piston. My larger piston was very clean and looked great so I felt no need to remove the snap ring, retainer, spring and check ball. I would advise using compressed air to blow out that area. Pull out the banjo bolts, bleeder screws and any fittings you want to clean up.

Got a mess yet?

        Now it's time to stop and look at what a fine mess you made! CLEAN EVERYTHING! Remember, cleanliness is next to Godliness and this is no exception. Dirt and gunk will dramatically shorten the life span of the seals inside.

        There are some things to specifically look for when tackling the cleaning. When you pull apart and clean out the hydraulic brake cylinder, check it for roughness, pitting or any kind of damage inside. If it's not a good smooth surface, find another unit to use. Also take a look at the check ball inside the hydraulic brake piston. If that ball has any kind of damage, you need to replace that whole piston, as it relies on seating properly to function right. You'll want to make sure the atmospheric / vacuum valve is in good shape. Clean it up with a fine steel wool or a toothbrush -- this to needs to seat properly.

Ready to put it all back?

        Okay are you ready for reassembly? Everything clean enough?

        I laid out all my parts (here's more) so I wouldn't forget anything while reassembling (and I still did ). Flip the end plate over so your looking at the underside. I had a bit of difficulty hammering in the new seal and would recommend letting it sit in the freezer for a few hours to shrink it down to aid in assembly. After that's in, flip over the end plate again. (Push rod seals) Remember a light coating of oil on all these parts.

        Insert the chamfered washer (chamfer side down -- my replacement was steel while the original was fiber), the new rubber cup, the cup retainer, spring and stop washer. It's a pain but with persistence, you'll be able to get the new snap ring down in there to hold everything in. Here's an image of the end plate after the seals were installed.

        Now lube up those little cups and insert them onto the control valve piston (cup, washer, cup, making sure they face the right way). (Here's a picture of the hydraulic piston disassembled.) Put the new C-clip on and crush it being careful not to slip and damage the new seals and making sure that clip is good and secured. You don't want that to come off inside the unit. (Hydraulic nut and piston assembly) Put the washer and snap ring back onto the hydraulic cylinder nut, dip the piston in oil and insert it into the nut. Put a new gasket on the back of the nut and screw it into the end plate securely. Set a new gasket on the end plates control valve portion and put the new diaphragm on also. Set the large spring into the control valve body. Now you can either use some studs in the holes to help guide alignment or be VERY careful setting the control valve body over the endplate and diaphragm. (End plate with hydraulic nut installed)

        Install new screws (8-32 machine screws x 1/2" length -- you can also use internal tooth lock washers) and snug up tight. Make sure the atmospheric / vacuum valve doesn't get cockeyed when putting it back together.

        Diaphram and gasket assembly - Control body / diaphragm spring - Control body intake assembly

        If you took off the tube from the control body. Now is a good time to put it back on. Make sure it's snug and perpendicular to the end plate. Set your new gasket in the top of the body, put the spring on the back of the intake tube, center it over the atmospheric valve, compress it and install the snap ring to hold it all together.

        Again, I lube all these parts with a light coating of oil, which will really help if you need to rotate the intake tube later to line it up.

        Insert the new rubber seal into the bore of the endplate where the hydraulic cylinder goes. It's hard to do but carefully spread the new rubber cup over the hydraulic brake piston making sure it's on the right way. Oil helps it stretch over but makes it harder to hold onto at the same time. Dip the piston in oil and insert it into the hydraulic cylinder tube and push it out to the other end -- but not all the way out. Put another rubber seal over the end of the hydraulic cylinder on the unthreaded portion below the lock nut making sure the lock nut is backed all the way off. Take your power cylinder piston and spring and put the end plate over them. Compress them (I used clamps to hold it) and pull up the spring on the piston end that protrudes from the hydraulic cylinder, being sure not to pull the piston out. Insert the pin and release the spring to hold it in place. Now pull the clamps off gently and while partially compressed, screw the cylinder into the end plate until it bottoms out on the o-ring. Put the new copper washer into the cylinder head and screw that on to the cylinder. Attaching cylinder and piston to push rod - Copper crush gasket - Attachment of hydraulic cylinder to push rod and end plate - Picture of both gaskets - Another view of both gaskets

        My unit had a gasket and a rubber ring which go around the lip of the end plate and seal that and the power cylinder, so I replaced both like they were. I coated the inside of the power cylinder casing and dipped the piston in oil before assembling the two halves together. Make sure the tube connections line up when connecting.

        Install the J-hooks securely and hose clamps securely. Install new bleeder screws, banjo bolts, copper gaskets and brass fittings (or direct fittings to attach the brake lines), and install the vacuum intake fitting. Mount the unit back in the vise (like it was on disassembly) and tighten it until the bleeder screws line up with each other. Tighten down the lock nut and you're done. Casing installed - Upright view of fittings attached

        If you want to protect your paint job, throw some tape on it when you clamp it and that will help quite a bit. For touch up, spray a little paint in the lid of a spray can and use a brush to touch up any nicked spots.

        Reinstall back in the vehicle. Pull the plug out of the bottom of the cylinder and add about an ounce of mineral oil or until it dribbles out -- it doesn't take much. Make sure the fill plug is towards the bottom of the unit when reinstalling!

        Now you'll want to bleed the valves starting with the control body bleeder. When that's done, move onto the bleeder in the hydraulic cylinder cap. Now rebleed all four wheel cylinders and your all set!

        If you have any questions feel free to send me a private message through the forums or email me.

        Good luck!

Jeff Erickson
1947 GMC COE 1.5-ton
1953 Chevy 3100
Bolter # 10264
Kingston, Washington

       Be sure to check out our extensive Forums discussions -- from General Truck talk, Electrical Bay, Big Bolts, Panels and Burbs, Engine and Driveline, Paint and Body, Interiors, Tool Chest -- The Stovebolt Collective can help in your quest and walk you through the mire and magic of working with old iron. ~~ Editor.  

v. September 2007

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