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Hi can anyone tell me exactly when this system came out. I heard it was in 57 for 3 speed cars and treadle vac for power glides. A friend of mine has a pretty much untouched 55 convertable that he's had since the early 70's and looks to have a hydrovac system in it. Any help is much appreciated as always. Thanks
 

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I would like to see pictures of any stick shift power brake cars' booster etc.
 

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Don't know exactly when these were first offered, but the bendix hydrovac units were available for all three years of 55-57 Chevrolets. They were also being installed in other cars during the same time but those had slightly different mounting configurations and housings, so they won't be a direct fit for the Chevy's (but some parts may be interchangeable if you are rebuilding). There are rebuild kits for these, but since there are many little and delicate parts inside, it is not recommeneded that you do it yourself. Fortunately, new brake lines are avaiable repo.

The main difference in the 57 unit is the bent air tube that goes from the back to the air intake valve on the side. On the 55-56 unit, the tube is straight over the top, with the air valve being in a different placement. There are illustrations of this in the 57 shop manual, however, they used the 56 unit in several illustrations (straight tube over top). The 57 assembly manual shows the correct unit and bracket. I presume that they could be interchangeable up to a point, but if you are wanting to go original, then the 57 unit has to have the bent tube.

The mounting brackets are different. The 55-56 units have 6 large and 1 small holes drilled in the rear to mount to the firewall. The 57 bracket has only 3. The fender side of the 56 unit has 2 holes (one not used) and has BENDIX stamped on it while the 57 bracket comes to a opening with one hole for a screw to mount on. It is thinner and lighter.

From what I have found, these were only offered on manual transmission cars that were sold in the U.S. Export cars may have different standards on Hydrovac and Treadlevac applications regarding the transmission that was installed.

I have one of each of these units, complete (not for sale) and wanted to use them on my vehicles (one being a 270 hp 57 Bel Air 2 dr hardtop). The one going on my 57 was rebuilt by Karps Brake Service in California. They did a wonderful job getting the housing (silver) and mounting bracket (gold annodized) restored to original condition and then bench testing the entire unit.

Perhaps the rarest part to find on these is the brass tag that was installed on one of the front mounting screws. Most of the time, they are completely crunched and mangled or missing altogether. The complete tag was a rectangular item with three holes drilled (one for the mounting screw), and had a part number stamped on the top. Mine reads 377372.
 

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Come in Jalapeno!
Bendix hydravacs were fitted to many cars and trucks and go back into the 40's.
My 57 Hardtop has a HydraVac and it's a PowerGlide. It would have been fitted by the dealer, because Treadle Vacs were not dealer installed. It is correct that the manual cars had HydraVacs factory fitted, this was because the under dash details foul the clutch linkages. I have not driven mine yet, and as the unit has been rebuilt, it will be interesting to see how it stops.
 

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Come in Jalapeno!
Bendix hydravacs were fitted to many cars and trucks and go back into the 40's.
My 57 Hardtop has a HydraVac and it's a PowerGlide. It would have been fitted by the dealer, because Treadle Vacs were not dealer installed. It is correct that the manual cars had HydraVacs factory fitted, this was because the under dash details foul the clutch linkages. I have not driven mine yet, and as the unit has been rebuilt, it will be interesting to see how it stops.
Howdy Mr. Allen!

57 BA knows his H-Vacs, doesn't he?

Just to add to his excellent post, there are several other parts reproduced for the 55-57 H-Vac system: The brass tee and the check valve on the carb (also used in the Treadle-Vac systems); the 57 H-Vac mounting bracket (which may actually be discontinued as of this writing); the vacuum hose with original type markings and spring clamps; the vac hose support (V8 only).
 
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