sucking brake fluid from master cylinder

[Guest Randy Berger]

JRBARTLETT posed a question on another thread (Caddy smoking white) and I didn't want to hijack that thread. He asked if the smoking could be caused by the intake vacuum pulling brake fluid into the power booster via a bad seal and then pulling that brake fluid into the intake manifold. I have heard this story several times and it included the bendix treadle-vac. I have the treadle-vac system in my car and I have a spare unit. I have studied it and cannot figure out how the vacuum could possibly puill the brake fluid from the canister that is part of the power booster into the intake. Now before some one says "it just vacuums it up dummy", try this experiment. Fill a glass half full of water. Put a straw through a very tight hole in a cap that fits snugly over the rim of the glass. Now position that straw an inch above the water. Now without hurting yourself try to suck some water up through the straw. Get your teenager to try if it gives you a headache. Were you able to suck up any water? No? If not, then please explain how the intake vacuum does this without that intake actually being immersed in the brake fluid. Years ago (1963-64)I had a bad power booster on my Packard and the brake fluid was disappearing but I had no discernible leaks. After studying the shop manual, I pulled the power unit from the car, removed the vacuum piston and found my missing brake fluid. That canister contained about 1/2 quart of brake fluid. But I never noticed any smoking and do not believe any fluid got sucked into the intake. I will admit the possibility exists if the canister is so full of fluid that it covers the vacuum intake of the power piston. But that is one &^%$ of a lot of fluid. You should have discovered you had a problem long before that happened. Any physics majors want to tell me I'm all wet?

[MarvMarvs]

Randy, you definately have a point. I have never heard of this sort of thing before. If I hadn't had these 2 things going on simultaneously, and somebody hadn't said anything, I would have never even considered it. I am seriously thinking of pulling the booster/master-cylinder unit and replacing it with a rebuilt unit. I have not done that yet because they're not all that readily available locally, and will probably have to have one shipped or pull this one and have it shipped or taken to be re-built. Mark

[heidigirl]

Mark,

inspect the vacuum hose between the check valve and the treadle vac. if its wet, it is a sure sign that all is not well within the booster.

heres what the manual says about testing a treadle vac:

with the engine stopped, and the transmission in neutral, apply the brake several times to exhaust all of the vacuum in the system. depress the brake pedal, hold foot pressure on the pedal, and start the engine. if the vacuum system is operating, the pedal will tend to fall away under foot pressure, and less pressure will be required to hold the pedal in the applied position. if no action is felt, the vacuum system is not functioning.

stop the engine and again exhaust all vacuum in the system. without starting the engine, depress the brake pedal and hold foot pressure on the pedal. if the pedal gradually falls away under foot pressure, the hydraulic system is leaking.

if the brake pedal travels to within 1 inch of the toe board, the brake shoes require adjustment or relining.

a faulty vacuum check valve can be tested by shutting off the engine and after waiting several minutes, applying the brakes. there should be sufficient vacuum reserve for several applications.

with the pedal going to the floor (or almost):

A brakes need adjustment

B air in hydraulic system

C hydraulic leak in lines or at the wheel cylinders

D fluid reservoir needs refilling

E cracked drums

F power brake leakage at:

1 compensating valve or seal

2 hydraulic plunger seals

3 outlet fitting

4 sand hole or crack in the master cylinder

there are any number of places that can rebuild these units. for what it is worth, Karps in Upland, Calif. did mine about 10 years ago, and i haven't had any problems with it. the cost was $200.00.

[Guest Randy Berger]

I think you and Heidigirl missed my point. I would like someone to explain how the engine vacuum can suck up the brake fluid without that vacuum opening being immersed in the fluid. I don't think it can, but am willing to be proven wrong. This query isn't to check to see if you have a bad power booster, but rather to have someone explain rationally if they believe that engine vacuum can suck the brake fluid from the booster if the opening isn't immersed in the fluid. It would take far more than a 1/2 quart to do that.

[Guest Albert]

It could suck up the brake fluid, but the vacuum chamber of the master would have to be almost full of brake fluid, and you step on the brake at the same time.

[Guest Randy Berger]

Albert, that's what I thought after looking at the layout, particularly since the Packard unit is on an angle. I would think most people would have disassembled the power booster long before they accumulated that much fluid.

[heidigirl]

randy,

re your earlier comment, i was answering a question that really wasn't asked, and i inadvertently hijacked your thread. my apology to you.

[critterpainter]

I don't know how it can happen.....but I have seen it happen. Used to run an old Hough loader with the little cummins engine in it. It had a hydro-vac on it. How the *&^&% thing worked I don't know. The brakes were ppor to shot and brake fluid was always being added to it. This went on for years (maintanance where I used to work was LOW priority). Finally the boss decieded to pour some money into the engine. The mechanic found a gummy messy intake and brake fluid in strange places. He would up rebuilding the engine and the brakes. Things worked much better after repairs.

Bill

[Guest Randy Berger]

Heidigirl, no need to apologize - it happens and a lot of interesting data comes to light. But I do appreciate the thought

[Guest Randy Berger]

Bill, I don't know how a hydro-vac works, nor how it was mounted in this particular instance, but I bet the vacuum port was immersed in the fluid.

Thanks for your input.

[Guest]

the crappy booster on a '58 cadillac, and other GM products had the master cylinder on top of the vacuum booster, the booster vacuum- vacuum line was at the base. If you had a leaking o ring, from the piston, it when to the bottom of the booster can, and got sucked into the carb via the 1/2" dia line at the bottom. simple gravity. Below all that is a "t" connector which has a line going to the vacuum tank reserve unit the driver fendor, this also, was the low point of the system.

Most of the time people boil the brake fluid by a rubbing pad, or driving with the parking brake, and are suprised when the fluid is missing with no puddle.

His white smoke is blow-by.

[Guest Randy Berger]

Yeah, I just looked at a diagram of a hydro-vac and the vacuum hose is connected at the bottom of the booster. I can see how that could suck fluid into the intake.

[MarvMarvs]

Thanks for the knowledge guys! This one had me thrown for a loop. I got a chance to one of the members of my car club who really knows these old cars. I described the problem and right away he told me just what is being said here. And that the white smoke was probably brake fluid being drawn into the intake and turning into white exhaust smoke! A much better situation than a blown head gasket.