The Stovebolt Site Forums The Parking Lot Big Bolts Bad HydroVac Symptom

I'm not near to worrying about the HydroVac unit yet, since I have no 'pedal' at all, and haven't even checked the brake fluid level yet, but if the seals in a HydroVac are gone, what happens when you step on the brakes? I assume that, for safety's sake, the truck will still stop, but not quickly, and only after exerting a lot of force on the pedal.

Since I'm just going to be driving around the yard until I get the rest of the brake system fixed, I'm not planning to get a replacement HydroVac unit until I've got a lot of other things done.

If you get all the air out- - - - -and that can be a big IF, the brakes will work, but you're going to have a low, stiff pedal. Normally, the master cylinder connected to the pedal is simply a trigger device, something to control the Hydrovac. If it has to become the primary source of brake pressure, it must force the fluid through the inoperative booster unit, which takes up most of its capacity. Low pedal- - - -poor brake operation. I don't think I'd take very many test drives that way!
Jerry

What Jerry said plus, the hydro vac is probably where your fluid went when the master ran dry,
Scott

It's also possible to bypass the hydrovac by disconnecting the lines from it and splicing them together. You'll end up with no power boost, but also no leakage that empties the main MC. Just get accustomed to pumping the pedal a few times before you move enough fluid to apply the brakes. Keep the shoes adjusted very close to the drums, not quite dragging, but almost.
Jerry

I'm not planning to put it on the street without a fully working system. Its not really a problem with the 3800 because the hand brake works nicely, and that is just fine for the yard, which is fairly level and about 100 feet on a side. The parking brake on the 4400 is the drum on driveshaft type, though, and it probably doesn't have much stopping power right now.

I just want to reassure my wife that it won't wind up in the kitchen the next time I move it. The last time I wiped out her entire tomato crop, all of her pumpkins and most of her water melons.

One way to tell if it's working or not is to do a direct check. Pump the brake a bit then start the motor, if it's functional you should feel the pedal go down when the engine starts. The other method is having the engine idling and depressing the brake, you'll get a slight change in the idle as the manifold draws air from the HydroVac.

The HydroVac has a bypass valve in it so that if you have no vacum pressure or the engine dies you can still use the brakes. It is best to adjust the shoes out as much as possible, I can stop my truck with a dead pedal no problem (I did rebuild the entire system tho, so your results may vary).

Pull the plug on the rear bottom of the hydrovacs cannister. If brake fluid comes out your hydrovacs shot. These are designed to only hold an ounce or two of fluid.

To see whether your hydrovac holds vaccum, hook a vaccum gauge up to that hole and start the engine. After about 30 seconds turn the engine off and check the gauge. You should read at 17-20 pounds. It should also take awhile for the reading to go down.

If it doesnt, chances are the control valve diaphragm is shot or the check valve is clogged. If the diaphragm is shot, you need to rebuild the hydrovac. The check valve is located in line from the lid of the cannister to the intake manifold. It should have 4 screws holding it together. Pull it apart, after being used for however many years its usually safe to say that some gunk or fluid will stick that valve one way or the other.

It works by being pulled upward by enegine vaccum. When the engine dies, gravity takes effect letting it drop and sealing vaccum pressure inside the cannister so you get one more good power puch of the brakes. Anyways, clean it up real good and make sure theres no scratches on the little brass valve. If there is you may be able to polish it up or you may need a new check valve. Unless the valve is stuck up, it wont effect the hydrovac when the engines running.

Check this out for a rundown of how they work

Another test, push down the pedal slightly with the engine off. Turn on the engine. If the hydrovac is working youll feel the pedal move slightly downward as the vaccum engages the booster.

Jeff

Stark, I can't say how it will work in Cabool but over here in Ava when I drove my 2ton home from Rogersville I got it here on nothing but the front brakes working.
We bled all the brakes and I thought they were working but after tear down the hydro was shot and the rear cyls. were locked up,and I was a little amazed by the trucks ability to stop with just the fronts working.
If the front cyls were locked up like the rears I would have not been able to stop. When you try out the brakes aim the truck away from the house !!
Dan

Nice to know someone is close. As I said, I haven't even started on the brakes. I kind of enjoy working on brakes, actually.

The problem with not aiming at the house is that I only had limited room to unload because of the 4400 and the '73, so 'Dumpy's' nose is up against the edge of the back patio. It's the first thing we see in the morning. That's one of the reasons I need to get it rolling.

Had similar issues with frozen brakes, only the fronts were solid rust and the rears sort of worked. the shoes were WAY out of adustment and the M/C leaked like mad, it was not a fun start to the trip home (I decided it would be cheaper to pay for a tow before the accidednt rather than after).


The common failure part I saw (was even stuck on my NOS part) is the relay valve. It's about a 1/4" bore and doesn't take much to stick it. Without that the M/C pressure wont open the vent and allow the vac press to work the booster, a total disasembly and clean is essential. Luckily you can still get rebuild kits for these things, or there's a company in the NorthEast that my local Napa was gonna send my old one to before I lucked into a NOS part on E-pay. And as a last resort, brand new ones are available from bendix, they are still used on forklifts.

Good luck!

Thanks Freq2002. That is really good information.

i run my 1 ton without the hydrovac and it seems to stop ok, it it necessary to hook it up? i do plan on redoing it though..

My old beast stops OK without it, but there is a huge difference now that it works. I'd never wana try to stop it loaded to 24K without it working tho!