"Diesel Dan" (1951 COE) has a Hydrovac power brake unit to assist the hyraulic brakes. Twice a year the master cylinder goes completely dry with the truck just sitting. I do start up the truck occasionally to keep things circulating and I work the brakes at that time also. I have checked all of the fittings, the brake lines, the backing plates, the inside of the rims, and the ground under where the truck is sitting for signs of brake fluid, and everything is bone dry! No signs of brake fluid whatsoever....anywhere! I have rebuilt and checked the master cylinder, and it is always dry as well. I pulled off the vacuum hose at the Hydrovac and checked for brake fluid in the hose to make sure that the Hydravac isn't sucking up the brake fluid and it too is dry. Finally, several years ago I replaced the Hydravac with a new rebuilt unit.........same thing, the master cylinder continues to go dry with the truck just sitting. The previous owner had the same problem. The Hydrovac does not appear to have a drain plug to remove so that I can check the canister for brake fluid. The brake fluid has to be going somewhere! Any ideas?

:rolleyes:

The brake fluid is filling the piston cavity of the power unit.

Eventually, the leather seal will fail, and the unit will start to drip brake fluid.

Do not procrastinate with this repair. Brake fluid being hydroscopic will cause major corrosion issues inside the cavity. This will make a simple repair a fairly more complex restoration project. The corrosion will need to be blasted out, then the pits filled with epoxy to prevent the piston from leaking.

Thanks for the information! I kind of assumed that was where the brake fluid was going, and you pretty much confirmed it. However, the Hydrovac sits quite a bit higher than the master cylinder and without the truck running, the brake fluid is still disappearing out of the master cylinder. So, how does the fluid get into the piston cavity when the truck is not running? And, when I put on the new Hydrovac the brake fluid started to disappear out of the master cylinder again within a few months. I have always suspected that the fluid was going into the piston cavity but I can't figure out how....especially when the truck isn't running.

"bowtieollie" is correct. After you clean out the system the only acceptible lubricant for the Hydro-Vac system is " Airusun " oil. Any other lubricant will kill the leather baffle.

Hey MT,

Never heard of "Airusun" oil...is that the type or brand? Where can I get it, NAPA?

Thanks,

I scored a gallon of Airusun oil from a guy who works at an industrial repair/commercial truck maintenence company. This oil is designed to preserve the leather internal parts on hydro-vac brake systems.

dog,

did you ever resolve how the hydraulic fluid was able to move uphill?..did you use airuson oil as mt suggested, and if so, where'd you buy it?..


epi

JunkYardDog I had a truck like that once upon a time, a 1952 Chevy with a tanker body, used it to water down the road. That is when I finally learned to drive a truck without any brakes on it. Wasn't too bad, I needed the job......

Chevrolet: Nope, never figured out how the fluid could run uphill to the Hydrovac unit when the truck is not running. I still have the same problem.

Macky Wacky: As usual.....

strange, huh?..how could it just disappear?..


epi

JunkYardDog
What brand of hydravac unit is it and is the master cylinder attached directly to the hydravac or remote?
Most hydravac units have vacuum and brake fluid seperated by a single seal which doesnt take much to damage allowing the brake fluid to leak into the main vacuum cavity. I the engine is running the vacuum will hurry the leakage.
Bowtieollie is right about brake fluid absorbing moisture from the air (high humidity more it absorbs).
Tony

Hey Tony. I don't know the brand of Hydrovac that I have since it is a new rebuilt unit and the master cylinder is remote. The Hydrovac sits higher than the master cylinder, and it is located up under the left side of the cab. The master cylinder is located down under the floor of the cab next to the left frame rail.

if the diaphragm were punctured, would there be enough vacuum stored in the hydravac to suck the fluid up even after the engine has been shut down?..


epi

The vacuum is applied to both sides of diaphram when brakes released, when brakes applied vacuum is blocked and non hydraulic side is opened to air pressure which pushes diaphram and all connected to it to assist in brake application.
There is some residual vacuum after engine shut down but would suspect not for long.
Find the vacuum supply hose (generally 1/2" inside diameter), disconnect it and poke a blunt flexible plastic rod in to touch the deepest part of the chamber, if it comes back out wet guess where your brake fluid is going.
Use a blunt flexible dipstick with little force as there maybe easily damaged parts in there. Most hydrovac units I have seen the vacuum is connected into the main chamber first then goes through the control valve to the rear chamber. You may also see brake fluid in the hose from the engine if there is a lot of fluid in the chamber.
Tony

JunkYardDog... I am looking for Hydrovac parts on the net and came across your post here on VCCA.
I found an interesting link below, relating to your empty master cylinder question.

http://www.cckw.org/phpbb2/viewtopic.php?t=17&sid=f83676f7db804d1656c80abbe041e0ab

Thanks for the link Radiodoc! That site contains some great information. Never thought of the vacuum in the Hydrovac unit drawing the brake fluid out of the master cylinder after shutdown. I will check into that. However, it takes several months for my master cylinder to run dry, and there are no leaks. The check valve on the manifold is worth checking out as well.

Thanks!!!

:p

Don't know if this will help or not but this Chevy Service News from late 40's talks about the units leaking (pg. 2).

This was posted on the web by Gene.

Service news

I had the same problem with my Vol. Fire Dept. antique truck. After having the hydro-boost rebuilt, the shop owner told me that at least once a year you should put 2-3 ounces of neetsfort oil into the booster to keep the leather diaphram from drying out and cracking. If you look on the booster, there should be a small threaded plug or possibly a small rubber plug that you can remove in order to do this.

John

Well I'm having a problem finding a hydrovac for my truck. It started as a semi with trailer brakes too, but now it is a dump and I've been having the same disappearing brake fluid. I rebuilt the master cyl. but same thing. now I've been trying to find a rebuild kit or just a rebuilt hydrovac and no luck. the serial tag says fc 302 7617 but I can't find where this model even exsisted. Any ideals?

BSC Parts Warehouse may have the parts you need for the Hydrovac unit.Bsc Parts Warehouse
824 Memorial Dr SE
Atlanta, GA 30316
404-659-8722
The Stovebolt web site has a very good writeup on rebuilding the unit in their Tech Tips.

Hello, I have a complete 51 Chevy cab over rolling chassis that has a complete brake system in place and working. I bought the chassis because it had a 302 engine ( not original to the truck) in it. I removed the engine but now have the complete chassis. If any one is interested in the brake system or the whole chassis let me know. I am in Mass. My phone is 508-869-2003

I also have two steering columns for this chassis.

Regards,
Ken

I found another hydrovac at NAPA. It was an exact match to the one off my truck. I've got it back on but now I have a hard brake pedel and no brakes. Any Ideas what is going on? Thanks