The Stovebolt Site Forums The Shop Area Engine Shop Mystery part/brake question

Okay, I'm hoping you guys can help me figure out what this thing is under my truck, and if it's part of the reason I have no or very limited brakes.

I'm still waiting on my service manuals to arrive, but wanted to get an idea of what I've got because I have never seen a setup like this before.

The mystery part is a round canister... maybe 6" across and 6-7" long, give or take. It bolts to the passenger side frame, directly across from the master cylinder. It appears that the brake line from the master cylinder goes to it, and lines to each wheel come out of it. There is also (presumably) a vacuum line that attaches to it from the base of the carb. Is this some form of vacuum canister or brake booster? How can I tell if it's working properly?

Like I said, I have essentially no brakes. If I pump the brakes a lot, the pedal will become firm about 4-5" off the floor, and the brakes seem to work. As soon as I let off the pedal, I'm back to not having brakes again. I'm planning on pulling the wheels this week/weekend weather permitting to see what condition the rest of the system is in (It's monsoon season in Kentucky). Sorry for the long post, but hopefully it's got enough detail for someone to point me in the right direction.

Thanks,
Chris

Yes, it is the brake booster.There should be a bleeder valve on it too.
The vacuum lines do develop holes and or cracks in them with time and age.Are the lines hooked up?

The brake lines are hooked up, but as far as condition, I haven't checked that closely yet. I do not see any fluid leaking out of them, which I consider to be a good sign. I'll check the vacuum line for holes/cracks. I kinda figured it was a brake booster.. but didn't know these old trucks actually had power brakes back then. Learn something new everyday.

You need to pick up a shop manual. You gotta brake booster buddy. It's hard to stop without it. Actually, my diagnosis is pits in your master cylinder, or faulty brake tube or hoses. If these are not sound, the booster can't help like it should.... Don't know if you have done this, but an inspection of the tubing and hoses on an old truck is well worth the time. My '48 2-ton was parked when I bought due to "brake problems." I looked at tube full of pinhole rust, then carefully removed, labelled each piece, and then bent new clones from new tube. If the hoses have any checks or cracks, it's time to replace with fresh stock. Just my two cents, or would you rather eat the Lexus?

I woulda guessed booster, too, but that sounds different than the one on my '44 ton and a half. First off, mine's on the driver's side frame, just behind the master cylinder. Second, mine is strictly a booster to the master - no brake lines come or go from the booster cannister - just the one vacuum hose. The Master Cylinder still does the work. My understanding of the booster was that it just physically made the brake cylinder easier to push.

Am I wrong, or did something change?

-Michael

You don't say what size truck you have or year. I assume it's a 48 by your "name"? Brake booster's are called Hydro-vac's on these. They can be on opposite frame side or on same side outside frame as on my GMC COE's. Yes, there is another brake cylinder on it, and Yes, bleeders, two on it. And they have to be bled in order to do right. Use ONLY a GOOD fitting flare wrench or six-pt socket or wrench on them. Use penatrant oil sparingly on them, let set a day or two and start to loosen, if it moves, STOP, and re-tighten some, let set awhile, go back and move some more. Work them back and forth till they come all the way out. Clean them up good or replace, make sure holes go all the way thru with wire poked in them. Replace and bleed. Most times these will break off, and the cylinder is rusted up and pitted just like the Master Cyl is/was. The chamber has a vac line to it and a leather Piston seal that dries up and needs replacement. It's a very complicated unit, and parts are hard to get. Most brake shops just send whole unit out to rebuild, around $250 I'm hearing. My 54 has one, I got it to bleed, don't think it works at all and have been driving with "regular" brakes for 5yrs. now. An old 59 F-truck I once had with same system would stop on a dime like power brakes and make vac sound when I let off the pedal. Mine does none of this. They do work, yet with a load are NOT very good. I plan on working on or sending it out this year to make them right. Biggest problem on bleeding is the two rear wheel cylinders are hard to bleed because of position they are in in the drums. Have to do them in certain order too. I once had a 76 F-truck with about same unit on it, so pretty sure same systems were used long time. I was thinking of maybe using a later model unit on some of my old trucks as I rebuild. Lots of later models went to a Hydra-vac system that runs off the power-steering pump and uses no vacumm and are on firewall with MC. These are used because of Diesel's not having Vacumm to run brakes on. We are now putting those on all 3/4-ton HD's and one-tons in truck plant I work at (Chevy's). You will find that sitting around will cause you to replace most of the brake systems on these old trucks and be very expensive and hard to find parts to do it. Good Luck!

P.S. Yes, you'll need a Manual, it's all in there, including Hydro-vac rebuilds/set-up. It's not so easy to understand tho, maybe that's why shop's just send them out. Carter's used to show a rebuild kit in catalog, but I think it was based on availability and not always in stock.

Thanks guys! The shop manuals have been ordered, just haven't come in yet. The truck is a 48 1.5 ton Chevy. It sounds like I've got the Hydro-Vac system. I've got a new master cylinder for the truck that I plan on installing this weekend. Also going to replace the wheel cylinders too, because this truck was parked years ago due to brake problems. I did a quick visual inspection of the lines, and they *appear* to be solid, but I do need to give it a more thorough look when I tear things apart this weekend. Hopefully I'll get lucky with the master cylinder/wheel cylinder replacement and bleeding the lines and this will fix the problem. But if not, no big deal.. just keep replacing the bad parts until it's fixed. I got it running, now just have to stop it.

If I were going to the trouble to replace the master cylinder and the wheel cylinders, I would think about bending up some spanky new brake lines. There is no point in pushing the rust in those old lines into your new brake parts..

Just a suggestion: if you're replacing the master cylinder, etc., pull a few pieces of tube for a little looksee. My gut feeling is all of it should be replaced.

Easy enough.. I'll bend some new brake lines while I'm at it. Might as well do it right, since I'm gonna have everything apart anyway.

Thanks!
Chris

Blue, don't forget the basics, make sure the brakes are properly adjusted. I remember years ago I used to to help a guy by hauling a load of gravel now and then with his dump truck. It didn't have much for brakes, he blamed the hydravac and had run it that way for a couple years. It's a wonder what a little adjustment will some times do. Mike

Blue 48,

Suggestion: go ahead and replace all your brake tubing now; flex-hoses too. It's worth the peace of mind! This is a lotta truck; "no brakes is no good!"
I've had enough "loss of brake" experience that I won't even think about taking my '55.1 1-1/2ton out on the road until those brakes are completely re-plumbed!

Mikestem,

Sounds like your truck might have an earlier type of booster which is a straight vacuum booster on the pedal linkage; I've seen brake linkages on the pre-'47 trucks with an "extra eye" on the clevis that looks like where a brake booster can could be connected.

The Hydro-Vac was a Vacuum booster and Hydraulic cylinder that was connected "in series" between the regular master cylinder and the wheels; it "senses" hydraulic pressure in the system and then triggers the booster, and there's a series of check valves which make it all work correctly.
The nice thing about a Hydrovac is that they were a "remote unit";the only connections needed were Vacuum from the engine and place to "splice it into" the brake lines.
Chrysler & **** used similar systems on their cars in the '50s, mf'd by Kelsey-Hayes, and Midland-Ross, repsectively.

The vacuum section on the Hydro-Vac on my 55.1 still works; when I was running the truck around the yard last summer, I'd hear the thing grunt & groan as I operated the brake pedal(The air cleaner for the unit is inside the cab on the right side cab corner!).

I will probably by-pass mine while I'm re-doing the brakes until I can afford to have it professionally re-done; I don't plan on any heavy hauling for a while anyway.

I've looked at the shop manual illustrations and I'm in no hurry to pull this puppy apart!