Shylor,

Understand your concerns but I truly believe if done correctly all is fine.

The risk is slightly higher with vacuum because it relies on a seal between the bleeder screw threads and the caliper. If you think about it you are placing a vacuum on the end of the bleeder, so you could conceivably suck fluid out or air in around the threads. So this seal is important.

I can't see how a pressure bleeder introduces air. Pressure is applied above the fluid in the master. The fluid is essentially an inch or so deep to the openings to the master. Air would have to be forced down through ~1inch of fluid. The weight of the fluid, and the fact you pressure bleed at between 10-20psi makes this unlikely.

When you release pressure, do it slowly at the motive bleeder canister so you don't cause any turbulence in the fluid in the master resevoir but if you do that I just can't see it happening.

Regardless, I've bled with the motive and beat the snot out of my brakes on the track the following day and had no issues with air.

 

I found this:

Pressure bleeding do’s and don’ts

Pressure bleeding on its own is not necessarily a bad thing, but there are several steps one must take to ensure that the bleed event will result in an air-free brake system.

When we talk about pressure bleeding, we are referring to the process in which we pour our brake fluid into a pressure vessel, hook up a pressure source, and run the now pressurized fluid directly into the master cylinder reservoir. One by one the caliper bleeder screws are opened to allow the pressurized fluid to flow through the system until all of the old fluid has been purged. Simple, right?

Well yes, but beware of imitations – not all pressure bleeders are created equal. The professional units (the type you can consider using) separate the pressurized brake fluid from the pressure source (air) using a flexible rubber diaphragm. In this fashion, the pressurized air is kept from forcing its way into the fluid. As we all know, air and fluid should be kept as far apart as possible.

This brings us to the imitations. There seem to be a rash of products available lately that claim to be pressure brake bleeders at a fraction of the cost of the professional units. Like most things that sound too good to be true, well, it’s exactly that.

Like the professional units, these imitations contain a pressure vessel into which new brake fluid is poured. However, in order to pressurize the fluid, an integral pump handle is cycled to build the pressure inside the vessel without any measures taken to separate the pressurized air from the fluid. For those of you who have ever bought a $19.95 do-it-yourself potted plant and bug sprayer from Home Depot you get the idea.

Of course, having pressurized air in contact with the brake fluid will certainly force the fluid through the system just as effectively as the high-zoot professional unit, but as an added bonus we are stuffing air into the brake fluid at the same time. Talk about an unwanted surprise!

While it may not be visible to the naked eye (air can actually entrain itself in the fluid as to be visually undetectable) it’s there right along with all of the nasty moisture trapped inside of it. This of course begs the question: if you are stuffing air and water contaminated fluid into your brake system, why even bother bleeding it in the first place?

Naturally there will be those who argue that the amount of air in question is not important enough to worry about, but think about this for a moment: nearly every automotive manufacturer stores their bulk brake fluid in large containers which are subjected to a constant VACUUM. Talk about an expensive process! If just storing your fluid under regular atmospheric conditions isn’t good enough to keep air and water out, just imagine what shoving 30psi worth of compressed air on top of it is doing.

The professional units can cost hundreds of dollars, and for good reason; unfortunately the cost keeps them beyond the reach of most of us normal folks. Your best bet is probably to get back in the driver’s seat and begin stroking the pedal with your foot again, but ultimately the choice is yours.

 

Someone on the other forum wrote this:

I do not recommend either pressure or vacume bleeders for competition.

The reason is as stated in the article that pressure bleeders dissolve gas INTO the brake fluid which is released when heated. Vacume bleeders will "pull" the gas out of solution and accomplish the same result....air in the system.

Why give your brake system the "Bends". IMHO using a pressure bleeder is like shaking a soda pop bottle and opening it

The only system I would consider is AP's F1 unit as used by F1 teams. It is a non gas interface simultaneous pressure/vacume bleeder.

My recommendations are a set of Speedbleeders, an assitant, if necessary, to pump the pedal and a Mallett or Pegasus Racing catch bottle.

I also recommend bleeding hot and tapping the calipers with a rubber mallet just before and during the bleed process.

 

Shylor,

This is nothing personal, so don't take it that way.

I see stuff like this on the web every day and I have to laugh. There's no data. This is all someone's opinion.

What is the rate of diffusion of air into brake fluid?
What is the vapor pressure of brake fluid? This would have to be overcome to dissolve air into the brake fluid by applying pressure.

So professionals store their brake fluid under VACUUM? Ok. Is that to avoid air dissolving into the fluid or WATER? Air invariably contains water. Even a closed container can be permeable to water. A constant vacuum would be the only way to avoid water diffusing into the container.

Who's opinion's correct? Who knows. There's no DATA either way. We each base this on our experiences. I've bled with the motive, and tracked the living heck out of my brakes two days later for two days straight.

It takes me 15 minutes to bleed with the motive. Being a chemical engineer, my gut is the amount of air forced into the fluid at that pressure for that period of time is not a concern. So, in the absence of data, I'll follow my gut.

 

Thanks for your input. Would you suggest the Motive GM version or the universal? I would like to use it with my other non GM cars. What about the Motive bleeder bottles? And who has the best price including shipping? Now what about Speed Bleeders?

 

I have both the Motive Bleeder and a vacuum bleeder that uses compressed air to pull the brake fluid through the system (it is called Vacula and is available through the Griot's Garage catalog). They are both pressure differential bleeders. In other words the pressure at the Master Cylinder end of the line is higher than the pressure at the caliper end of the line. Both are easy to use and will not get air into the system if properly used.

The problem with the vacuum bleeder is that it pulls fluid out of the master cylinder so rapidly that you have to be careful not to empty it. This wouldn't be much of a problem on an older car without ABS but on the C5 you will get air in the ABS unit and have to go to the dealer to get the unit bled. Yes, you do get some bleeding of air between the bleed screw and the caliper but it all goes into the hose. It just makes it hard to determine if you have all of the air bubbles out of the caliper. That is overcome by flushing for a while so you know you have pulled fresh fluid all the way from the master cylinder.

The Motive Bleeder forces fluid out of its reservoir into the Master Cylinder and when you open the bleeder valve on the caliper you get a nice stream of fluid that is easy to read for air bubbles or lack of air bubbles. Thus you can bleed quicker with less fluid with this unit. The generic GM cap will work with the C5 but only if you keep the pressure down to 15 psi or a little lower. The C5 Service Manual calls for 20 psi but the cap will leak fluid all over if you try that high of a pressure.

Don't ask me about how I found out about the usage problems with these two units.

By the way gravity bleeding works well also and if you have the time can be the best way to bleed the system.

Bill

 

How long does gravity bleeding take?

 

Gravity bleeding takes some time, but if you are doing other things it is OK. If you have an empty caliper though, I prefer the other methods to insure that little bubbles don't adhere to interior nooks and crannys not coming out.

As for the Motive, I think I bought mine on a Cobra web site or somewhere like that. Do a google on it, I got it with all except foreign covers for about the same prices as others wanted for one cover.

 

Gravity bleeding can even be sped up by putting an extra 3 psi on the lid

Understand that in reality the air pushes against the top of the fluid at 14.7 psi (sea level) anyway. Any pressure you put on the motive bleeder is really in addition to that, so if you look at percentage increase it isn't as much as you think.

Even just putting the lid on, pumping up to 3 psi and opening a bleeder will accelerate the fluid a bit more than gravity bleeding.

I pressure bleed mine at 10psi or below. Frankly I'm lazy and it takes a while for the little pump to build up pressure.

 

Okay, okay I'll get a Motive Power Bleeder. Who sells them the cheapest, including shipping?

 

do it the old fashion way, you will get better with time and its safer than the MOTIVE

 

I know how to bleed brakes, I have been doing the two man method for years. I'm just trying to find a easier way.

 

someone is filling people full of