So they sell these little bleeding kits that lets you pump a vacuum hand pump to generate inches of pressure and then you open the bleeder and it starts sucking the brake fluid out.


My question is, how does this work all the way up to the master cylinder? I was always under the impression that you had to push the master cylinder in order to push the uncompressible hydraulic fluid all the way to the other side (the bleeder). Then when you let go the plunger goes back and brake fluid is drawn into it from the reservoir making the level lower in the reservoir. Then you repeat.


Rather than doing this from the master cylinder end at the brake pedal if you now apply vacuum on the other end at the bleeder, what happens at the master cylinder? Does it pull on the master cylinder plunger for you? How does it continually draw fluid from the reservoir?

 

Brake lines are open to the reservoir, so no affect of vacuum on the master cylinder piston.

Fluid flows from higher pressure to lower pressure. When you lower pressure w/ vacuum pump and open the bleeder, atmospheric pressure (14.7 psi) vs. vacuum level (perhaps 10 psi) at lower brake line end creates 4 psi differential on the fluid and fluid moves.

Principal advantage of vacuum bleeding is ease of performing single person bleeding.

Some folks worry about bubbles in the brake fluid that comes into the catch reservoir and go to great lengths (grease, sealant on bleeder threads) to prevent air bubbles. The bubbles are not an issue as the bubbles enter at the loosened bleeder and do not enter the fluid in the brake lines. When you close bleeder plug you seal the system w/ only brake fluid trapped in the lines.

good luck

 

I saw a diagram and understand what you mean now about it going directly to the reservoir when the valve is at rest. Great.


So then that leaves only one other problem. What about the air being sucked in through the threads of the bleeder? Is it better to hold vacuum on it and then close the bleeder? What if holding vacuum is actively sucking air in through the threads and I close the bleeder with air at the very base of the bleeder? It's still air in the system right?


Would it be better to close the bleeder without holding vacuum? Or wait a bit for the air to work itself out and then close the bleeder?


Or any way to avoid sucking air in through the threads at all? Opening the bleeder up just a bit? I'd rather not do all of the pulling the bleeder off and teflon tape it or whatever.

 

Read the post. Air entering bleeder and then exiting w/ draing fluid is not a problem.

good luck

 

The secret is to always have vacuum applied, while the bleeder screw is open. As long as all your connections are air-tight, this shouldn't be difficult. Any air that enters through the threads, which should be small bubbles, is immediately drawn into the catch can. You can even help the fluid along, by pressing the brake pedal a little, as long as you keep that constant vacuum on the line. I try not to go farther with the brake pedal, than normal braking.

 

I have let vacuum go to zero w/ bleeder cracked w/o problems. Fluid pressure should prevent any bubbles forming as vacuum goes to zero.

good luck

 

Some people bleed by gravity-drain also.

I'm one who has "complained" about bubbles sucking in at the bleed screw. If your brake system is originally free of air bubbles, that's one thing.

But if you have a soft pedal and you are watching the hose to see whether or not you get any air bubbles from the line, then you can't tell with all the other bubbles. And it was an excuse for me to buy a pressure-bleeder to add to all my junk. I mean TOOLS.

 

I like pressure bleeding in principle, however it requires special pressure cap adapters to pressurize different brake fluid reservoirs.

good luck

 

I have a Motive pressure bleeder, but they don't make a cap to fit the Hondas. The general purpose cap works O.K. Motive has a standing cash prize available for anyone who can design a cap for the Honda.
The pressure bleeders are real handy for the 2-year fluid change.

 

Couldn't you buy a Honda cap and make it fit the tool? I need to look into this.