The Stovebolt Site Forums The Shop Area Driveline Hydrovac bench testing

I have a hydrovac on my '61 that has never really worked since I added it a few years back. It has remained installed, but with the vacuum line disconnected. When it's hooked up and I step on the brake, the engine dies. I figure there is a vacuum leak somewhere.

I have a hand vacuum pump that I use for bleeding and I connected it to the vacuum connector on the booster and I can get it to hold vacuum without issue. It will hold for minutes, until I release the pressure. So I think the diaphragm is good.

I re-set the ignition timing and put a vacuum gauge on the intake manifold and I'm getting between 17" and 18" of steady vacuum pressure. So I should have enough vacuum.

There is NO check valve on the hydrovac, and I know that I need to address that to preserve the diaphragm, but I'm wondering if the lack of a check valve would cause the engine to die and the unit to not work.

Any ideas on how I can figure out why the unit is giving me headaches? What else can I test to determine why it is not working?

Troy,

Can you post a link to a picture of your Hydrovac?

Do you have two vacuum lines connected to your Hydrovac? One on the control valve and the other on the canister? If you do you may have the reversed. The vacuum line from the intake manifold goes to the canister and the fresh air connects to the control valve.

Where did you connect your hand vacuum pump to make your test, at the canister or control valve?

Mike B

Here's a link to flickr: https://www.flickr.com/photos/troymckaskle/25741248476/

The rubber tube running below the booster, blocked off with a bolt is the vacuum line from the intake manifold. I connected the vacuum tester to the nipple that is immediately below the bleeder on the front of the canister. (aluminum colored fitting in the picture).

The visible nut on the back of the hydrovac (left in the picture) is for the rear mount.

The fresh air intake air cleaner is on top of the Control Valve. The vacuum line connects to the silver nipple located on the bottom.

Here's how it works. When you step on the brake pedal it puts hydraulic pressure on the control valve which opens the fresh air intake allowing 14 psi of atmospheric air pressure to push on the back side of the Hydrovac piston. The piston then push's on the Hydrovac's Master Cylinder which increases the hydraulic brake line pressure by about 800-1000 psi. The engine vacuum removes the 14 psi air pressure from the front side of the piston via the vacuum line allowing it to work.

Make sure you have the vacuum line connected to the silver nipple. Is this where you attached your test pump? The vacuum side should be air tight.

Mike B

I connect the blocked-off hose in the picture to the silver nipple when everything is all connected up and tightened down.

The silver nipple is where I attached my test pump. I pumped it up to -20 PSI (according to the gauge on the test pump) and it will hold there for at least 5 minutes.

When I start the truck and connect the blocked off hose to the silver nipple, the motor dies when I step on the brake.

So you're saying with the line plugged the truck runs fine and stops (hard) but doesn't die?

Is this a new problem or a problem you inherited?

Sounds like somethings not right with the Hydrovac, but your test says it's good

Mike B

What happens when you pump up the vacuum with your tester and step on the brakes? I would think that would duplicate the problem conditions would it not?

Interesting... If i pump up the vacuum with the tester and then step on the brakes, the vacuum gauge instantly drops to zero. It loses vacuum.

I am not a Hydrovac guy but I think the problem starts there.