Aerated Fluids, Bleeding & Filling

[Autotecheng]

Hello,

I am not an engineer, I am a service technician in the automotive field with some formal education.

Before asking a question here I typically spend hours on a day to day basis researching it and trying to find the answers myself.

Today's question concerns air that has become trapped in hydraulic steering, braking, or engine cooling systems.

Trapped air pockets delay the application of brakes for example, and on some applications can be extremely difficult to completely remove. Aerated fluid seems like it has to be flushed completely.

I don't know a whole lot but what I have read is that brake fluids are federally standardized by their boiling point wet and dry. I know that boiling is a function of pressure and temperature, as well as moisture content. I know that if a brake caliper overheats to an extreme that the brake fluid boils and pressure to the caliper piston is lost (even under the applied pressure from the master cylinder in the range of about 1500-2500 psi). I don't know for sure but I think when the fluid overheats that dramatically it displaces air out past the reservoir. Then as it cools it draws a vacuum which pulls air in past seals, resulting in an aerated fluid.

I've also read that brake fluids are made to be hygroscopic so that they will disperse moisture content throughout the system, making it difficult for a brake line to become restricted by ice in cold climates. But that quality also lowers its boiling point.

Because of the expansion / boiling / absorption of water brake fluid eventually needs to be serviced.

I have been looking into the different ways of flushing the system and bleeding it but I haven't decided on the best approach.

One thing I am curious about is whether drawing a vacuum on the system from the reservoir could raise the boiling point and help to remove trapped air in the same way that vacuuming an AC system does, or that vacuuming a soda bottle would.

Several authors have commented that pumping the brake pedal rapidly and holding it and then releasing it while someone else watches the fluid reservoir will indicate if there is trapped air. They say it will bubble up in the reservoir.

I don't know, I plan to experiment with some fluid samples under controlled conditions.

Another area where aeration is a problem is with power steering systems. Power steering fluid frequently overheats, expands, overflows, and then creates a vacuum. Then even with a correct level the power steering pump cavitates. Cavitation accelerates wear and decreases performance according to my research. Cavitation is directly related to the vapor pressure and inlet pressure of the fluid at the pump.

I'm curious if placing the power steering system under a vacuum could help reduce aeration after refilling it. I have read about people using this technique and I can see why it would work.

Another application where trapped air is a problem is in engine cooling systems. Some systems provide bleeder valves at certain points, and some sources say disconnecting a hose at a high location to bleed air will work. Some manufacturers specify that a vacuum be drawn upon the cooling system and the new coolant "sucked" in. While I have never seen this procedure I can see why it would work. Cooling systems have a lot of crevices and passages that are hard to reach, and the presence of air (as far as I know) increases corrosion which in turn impedes heat transfer and coolant flow. It's also common for cooling systems to develop leaks that draw air into the system.

Actually, come to think of it we use pressure pumps to check cooling systems for leaks and one time I released the pressure too quickly - the coolant spewed out and erupted. It seemed that the air had become infused with it. Come to think of it, by releasing the pressure suddenly I may have made the system boil.

Another thing that I am interested in is that many cooling systems have an expansion tank which is labeled a "degas bottle" in the documentation. I am wondering if the system is designed to collect air at that tank and to open and vent it out when a set pressure is reached.

In fact I am not sure but I suspect that fluid levels in all of the previously mentioned reservoirs are set on the basis of thermal expansion and pressure change.

I think, perhaps, that aerated/contaminated fluids should be flushed out completely, and then a vacuum drawn upon the new fluid for awhile to bleed the remaining air. I am not sure if applying a vacuum could damage the fluid or system.

I'm also interested in simple ways to measure the vapor pressure of a fluid to aid in diagnosis. I intend to experiment with applying vacuum to fluids of different type and age.

I feel that the subject is worth the time I am putting into investigating it. Getting the most and best performance out of my hydraulic systems and knowing I've done my work right is important.

I imagine that all of the problems I face have already been solved professionally in other fields of engineering where they are of a more critical and more professional nature.

Thank you for your advice and if anyone is willing to answer future inquiries via email please let me know,

-Andrew

 

[MikeHalloran]

Random thoughts.
Brake systems are designed so that the bleeds are at high points, where air should accumulate.

I have bled troublesome cooling systems by pulling a vacuum and then filling through the same pipe. It requires a big water trap, or a pump that you don't care much about.

I'd be reluctant to pull much of a vacuum on a brake system. If it's a drum system, a hard vacuum could pull air _and_ wear debris in past the cup seals in the wheel cylinders and maybe the master cylinder. I don't think most disc brake seals would be bothered directly.


Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

I have never had problems purging air from brake lines by normal bleeding process.

I have never had a problem purging air from the P/S system. It does take some steady turns from lock to lock at times, holding the lock for a second or two.

I have had trouble with cooling systems.

Solutions

1) If possible put a bleeder valve at the high point.

2) Use trolley jack or topography to get the radiator fill point as high as possible. Deflect hoses by hand if required.

Basically, air rises.

If the brake fluid boiling point is depressed or inadequate for your application, change it for fresh fliud with a suitable rating.

These systems generally run with a positive internal pressure at most points except the pump/master cylinder inlet.

Cavitation or low pressure over the fluid does not introduce air. It introduces vapour from the fluid as the fluid boils at that reduced pressure. The vapour can quickly condense as soon as the vapour cools.

If you boil the brake fluid, it does not pressurise to anywhere near what you say while it is still vapour, so the pressure cannot condense it. It needs to cool enough so that it is almost all liquid before the stroke of the pedal is enough to build pressure. Remember liquids are to all practical purposes NON COMPRESSIBLE. This means that when you press the pedal the brakes apply and pressure builds to match the resistance.

With a gas, as in boiled fluid still above boiling point, the gas in the system compresses and if there is any volume, the pressure build up as the gas compresses is slight. See the combined gas laws to see how to exactly work it out.

Regards
Pat
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[swall]

I was the materials engineer at a major brake system supplier in the U.S. I never heard mention of air being sucked past caliper or w/c seals. I think some drivers encounter a knock back phenomenon during severe cornering and erroneously conclude that air got sucked in and caused a loss of pedal.Water in the brake fluid can indeed cause fluid to boil at lower temperatures compared to virgin brake fluid. A 3% water content in DOT 3 fluid can lower the boiling point by 50 degrees F.