The Stovebolt Site Forums The Shop Area Driveline Hydro Vac Rebuild Necessary?

Hey guys! I haven't been active in a while, been lurking some when I get time, but don't contribute much, because I don't know that much. (Haha)
Anyway, I'm back to working on my '51 2-ton, and I'm rebuilding leaky wheel cylinders. I'm keeping the brake system stock, and was wondering if while I'm at it, I should rebuild the master cylinder and Hydro-Vac? Neither are leaking as far as I can see, but I thought it may be good preventive maintenance. What do you guys think?
I don't know much about the Hydro-Vac, other than I guess it's just a brake booster? When that goes bad, what will the symptoms be?

When driving the truck (before the wheel cylinders started leaking) the first pump on the brake pedal would go to the floor, and the second pump would have pressure and good braking action. What does that sound like a symptom of?
Thanks!

Having to pump brakes to get a good pedal is a symptom of incorrect brake adjustment. You're having to move more fluid than one pedal push can accomplish to get the brake shoes into contact with the drums. A hydrovac works just like any other booster- - - -you're using vacuum against a big piston to make your leg stronger. If the engine speeds up when you push the brake pedal, you've got a vacuum leak. If the engine runs rough and a cloud of choking white smoke comes from the tailpipe when you step on the brakes, you've got a brake fluid leak into the vacuum chamber of the hydrovac.

Fix the leaky wheel cylinders, replace any fluid-soaked brake linings, and adjust the brake shoes properly for minimum clearance from the drums- - - -you'll probably have good brakes without having to mess with the master cylinder or the hydrovac. Be sure to inspect the steel and rubber brake lines for leaks, rust, or damage. There's nothing that increases your pucker factor worse than blowing out a brake line unexpectedly!
Jerry

Thanks, Jerry! I'll get the cylinders fixed and brakes adjusted. I've never noticed any increase in engine RPM while braking, nor any white smoke so I guess the Hydro-Vac is good. If something should happen to it or the master cylinder, would it likely show symptoms to provide warning, or could it be an immediate loss of braking?

Also, I know to adjust the brakes you turn the star wheels on the cylinders, but how do you know when the minimum clearance is reached? Is there a guide on the forum here somewhere?

Do you have access to a service manual for your truck? The big rigs have somewhat different brake systems than the pickups, and I'm not sure what a 51 2-ton would have as an adjustment procedure, or even what design brake system you've got. Bendix-type brakes have a single star wheel adjuster at the bottom of the backing plate, Lockheed brakes have two hex head cams resembling bolt heads at the midpoint of the backing plate, and Huck brakes have a star wheel surrounding the ends of the wheel cylinders at the top. Each system has different adjusting procedures, but they all involve getting the brake shoes snug against the brake drum, and then backing off just far enough to let the drum turn freely without creating an excessive amount of pedal stroke. One of the big rig guys will be along shortly with more specific information and/or a service manual reference.

It might be a good idea to start a thread in the "Big Bolts" forum asking specific questions about adjusting your brakes.
Jerry

Jerry, you beat me to it. I was going to suggest a thread in the Big Bolts Forum. Some very knowledgeable folks there. I’ll ask the Driveline Moderator to move this there.

I looked online at the "Old Car Manual Project" website, and found a very helpful booklet on the adjusting procedure for the 1 1/2 and 2-ton trucks, and I can definitely do it, but geez it's gonna take a bit. I don't have a pressure bleeder tank, but I can just have a helper in the cab, and keep the master cylinder full, right?

Bleeding a hydrovac without a pressure bleeder is going to be difficult. I'd recommend doing a flush with denatured alcohol first, to clean any dirt and debris out of the brake system, blow air pressure through all the lines and wheel cylinders to remove the alcohol, and once the system is clean and purged, catch the fluid you bleed out and recycle it. I've managed to go through a couple of quarts or more of brake fluid trying to get all the air out of a system with a remote booster downstream from the master cylinder. Since DOT 3 or 4 brake fluid is basically alcohol and glycerine if, a few drops of alcohol remain in the system they won't be a contaminant.
Jerry

Great advice 👍 Jerry, after many failed attempts at conventional bleeding my l60 I was able to successfully bleed by myself using an affordable pressure bleeder. I needed to modify top plate for my master clutch cylinder combo and secure with a couple Stanley ratchet clamps. https://www.summitracing.com/parts/mvp-0105/?rrec=true .

After rebuilding all cylinders and all new steel and rubber lines I have good brakes without the hydro-vac, adjust the shoes so they scuff the drums just a little with the rig off the ground. The hydro-vac on mine is leaking internally and was drinking brake fluid, I haven't taken the time to remove it but will probably replace it with a re-man from NAPA. I don't have to pump the peddle at all, it's firm, high and without surprises.

How well does it stop with a max load onboard?
Jerry

Hi 235 Kidd,

My truck is a '44 CCKW (2-1/2 ton). It too, utilizes the Hydrovac. With that said, not sure how much of the following applies to civilian models, so take it for what its worth.

If you already have the system torn apart to rebuild the wheel cylinders, I personally, would not even consider putting it back together without removing, inspecting, and rebuilding the master cylinder. If it's pitted, (which it likely is) you can either replace it or have it sleeved. Same goes for the wheel cylinders.

As mentioned previously by HotrodLincoln, now is also the time to inspect hoses and lines. Replace them if they look even remotely suspect. They are not that expensive.

The Hydrovac on the other hand, is a somewhat different matter. If it worked previously, does not show any signs of internal or external leakage, and the brake fluid is not heavily contaminated, I would consider leaving it alone. Otherwise, I would replace it. As 78buckshot mentioned, NAPA has them. They were rebuilds from Midland the last time I checked. They were also not cheap.

I just went through all this on my truck several years ago (all new hoses, lines, 6 rebuilt wheel cylinders, master, hydrovac, etc.) so I know it is a lot of work. Brake systems, however, (especially on big trucks) are something that one should never scrimp on IMHO. About a year after I got my truck back on the road, a guy in a teeny little compact car made a sharp left turn from the right shoulder directly in front of me. He was so close that I could barley see the roof of his car over the top of my hood. All ten wheels locked, tires screeched, and my passenger nearly banged her head on the windshield, but I did not hit him. I was glad that the system was working correctly. If it had not, I would have creamed him.

I did not, BTW, have access to a pressure bleeder at the time. I followed the TM, used a hose on each bleeder connected to a small reservoir of fluid, and went at it one bleeder at a time. It bled satisfactorily. However, if you have do access to one, it will obviously make your life much easier.

With regards to running without a hydrovac, my truck uses the Huck design which is pretty anemic in the way of self-energizing. I once disconnected the vacuum supply just to see what would happen. Pedal effort was really high, so I could not imagine driving it without assist. Try carrying a load and it would be a nightmare to stop. BTW, if your truck also has the Huck system, let me know and I can give you a few extra tips that may make your life easier.


Good luck!!

Best Regards...

p.s. Hey 78buckshot, where in SE MI do you live? I think we might be neighbors.


p.p.s. If anyone is interested in learning more about Hydrovacs, TM9-1827B covers them in detail. It does not show the Type 3, which is the replacement for most older units, but it does show theory and operation.

A freebie download can be had here: https://archive.org/details/TM9-1827B

Thank you for the information, Bob, I appreciate it! I imagine you are right about inspecting the master cylinder. I traced all the brake lines, and they look good with no rust pitting or wear from rubbing on anything. We live in a dry climate, so that helps keep external rust away for the most part. I looked it up, and a rebuilt Hydro-Vac from Napa is $288, but since mine seems good so far, I think I'll leave it, although I do agree with you a braking system is nothing to skimp on.
So far the brake fluid I've removed from the system has been somewhat dirty...I guess a system flush is in order?
How do I tell if I have the Huck design?
Thanks!

Jerry, my 2 ton is rated at 16,000 lb. GVW. I have not had the max load on it but with 105 bales of hay on it's last trip I had no problem with the brakes, granted that was probably only 4500 - 5000 lbs and the truck weighs about 7400 lbs empty. With the 7.20:1 rear end it doesn't coast much and 45 mph top end keeps me on mostly gravel roads. Hey CrowBarBob, I live north of Chelsea just east of M52, about ten miles west of Hell.

Brake fluid acts like a sponge and soaks up water, then as the trucks sits in storage the microscopic droplets settle to the low spots and start rusting the metal tubing from the inside out. It you haven't replaced the steel and rubber lines, you should do it now, its an easy DIY weekend project AND it might save someone's life!

Mike B

FYI.
https://www.flickr.com/photos/advance-design_parts_co/albums/72157635961538965

Hi 235Kidd,

$288 sounds very reasonable. As I recall, mine was well over $400 several years ago - maybe because it was a higher capacity unit to accommodate six wheel cylinders - not sure...(Or maybe my NAPA guy did not give me as good of deal as he said he was )

Yes. If the fluid is dirty, definitely time to flush. I have never lived in a dry climate, but up here in MI, humidity is fairly high most of the year. As Mike B mentioned, brake fluid absorbs a lot of water. All of my wheel cylinders along with the master were pitted, and had to be sleeved. I also replaced all of the lines, mainly because someone had replaced them previously, and used the "closest distance between point A and point B routing strategy" when running the lines, regardless of proper routing and mounting practice. There were nearly as many kinks as there were bends. Since I never wanted to do it again, I used all stainless steel tubing. I routed it in the factory configuration following the witness marks on the frame.


The Huck design, such as on my truck, features a set of metal caps that fit over the ends of each wheel cylinder. There are two caps per cylinder - one for each shoe. During braking, the wheel cylinder pistons push on these caps.

In each cap, there is a threaded hole that runs down the center axis of the cap, in line with the centerline of the piston. In each hole is a corresponding threaded rod that engages the brake shoe. For adjustment, the cap is turned, changing the relative position of the rod to the cap. The cap has a series of grooves or notches around it's outer circumference. These grooves or notches engage a disc on the backing plate which is attached to a shaft that goes through the backing plate to outside world, and has a hex shape on it that can be turned by a wrench. There are two of these shafts, one for each cap/shoe combination that can be adjusted to compensate for wear.

The bottoms of the shoes are connected to a center pivot point via articulated links.

I don't have any pics handy that I can share, but if you look at page 338 and 339 of the manual on this site, it shows a pretty good image of them: https://radionerds.com/images/8/8a/TM-9-801_GMC-CCKW-352-353.pdf.

Is this the design on your truck ?

Hi Brad Allen,

Those slides on Hydrovac repair are awesome!! Some of the best information that I have seen.

Hi 78buckshot,

You must be near the Chrysler Proving Ground, then. I'm a ways east of you. North of Mount Clemens.

Best Regards.....

Bob, my brake system must not be Huck on the rears, but the front brakes sound like what you're describing. Each rear brake has 2 adjusting star wheels, but the wheels seem to move the shoes themselves. The wheel cylinders have rubber cabs on each end and the rod that engages on the shoe, the rods are not threaded in.

Here is a link to what I have:
Brake Manual