The Stovebolt Site Forums The Parking Lot Big Bolts Midland Hydrovac Rebuild

Not finding a whole lot of information on hydrovacs? Perhaps yours doesn't look remotely like Bendix models that are in the Service Manual? Or perhaps there's no serial number on your mystery hydrovac? Is that immense bulb at the end held together by a band? Then you've got a Midland, so read on!

While I was shortening the frame of '51 6500 down to the more manageable length of a 6100, I had to relocate a Watson auxiliary transmission. In order to keep either driveshaft from being too steeply angled or too short I had to run the mounting brackets right where the hydrovac was. Now I had heard of this mysterious unit before and at first I was content to simply move it. ...But it was mysterious, and since I am curious moving turned into rebuild, restore, and then relocate.

Dropping the hydrovac out of the truck frame is simple enough: disconnect the air, vacuum and two brake lines. Take out the bolts holding the hydrovac to the truck and voila! And then, having read the tech tip on rebuilding a hydrovac, I called up Rick Eskew from Reman, Inc. for a kit with new software. Of coarse, I expected my 60 year old truck have all the old original equipment on it. After all, I've never spent hours cursing the previous owners for all the modifications that they tried to do. So I ordered a rebuild kit for a 374000 model Bendix Hydrovac because, not only would that have been original, there was a small aluminum tag that actually read 374000. .....

Then I noticed that my hydrovac didn't look like the one in the Truck Service Manual. After disassembling a few components and sandblasting them I found 'Midland' cast into both the Power Control Valve and the Hydraulic Brake Cylinder. Google led me to this website where, with a little patience, I found that I had a Midland type single diaphragm hydrovac casting #C3388.

A quick call back to Rick Eskew and he verified that a Midland with 10" diameter band and 1/2" fittings was model #C3388. We finally got the correct kit ordered and Rick assured me that he would refund the cost of the original kit once I mailed it back and he received it.

Enough story time; this thread is worthless without pictures!

Parts where dissambled and sandblasted. When sandblasting the Hydraulic Brake Cylinder, be sure to plug the holes for the fittings with screws and jam a shop rag in to the big open end. The interior of that cylinder is honed out and blast media will ruin it...

Sandblasted Hydraulic Brake Cylinder
Poor Man's exploded diagram of the cylinder

I took the advice of the existing tech tip and washed all the components with warm water/soap and then alcohol to get rid of the brake fluid. I use POR-15 for all of my truck parts, which won't stick if there's any oil residue on the part. I also use Q-tips for the hard to reach places.

If you're using POR-15, don't worry about that fine layer of rust that instantly appears on all that metal you just ran under warm water. The Metal Ready prep is a zinc phosphate/phosphoric acid compound that puts works even better on rust than clean metal. I did make sure that honed surface inside the cylinder was extra dry and clean, however.

Components prepped and waiting on POR-15
Components drying after first coat

All the exterior surfaces and the interior of the power control valve (big bulb) get a least 2 coats of POR-15. Since POR-15 is pretty much impervious to most chemicals, I'm not too worried about it breaking down inside the power control cylinder. The exterior surfaces also get a top-coating of POR-15 Chassis Black for durability.

The rebuild kit should arrive in several days and I'll post pictures of the reassembly.

Shom,
Cool rebuild! Thanks for posting such complete details. That POR-15 looks like a pretty cool product. Do you have any experience with their Floor Armor product? I was looking to really straighten up my garage and when I was checking out the POR-15 it caught my attention as being a lot easier than some of the other products I have been looking at.