Technical Pressure bleeding brakes

Hi fellas,
I have a physics question about pressure bleeding brakes (where you affix a plate [with a pressure line] to the top of a master cylinder.
How do the air bubbles react under pressure of ( let's say ) 15 psi?
Will the introduction of pressure force the air down the lines without opening any bleeder valves?
Or will air bubbles just stay in place until a bleeder valve is opened then progress equivalent to the draining of fluid?

 

Don’t really know what to say but I’ve made my own pressure bleeder and only run around 5-7 pounds. Seems to me that the air bubbles come out pretty quick. So much so, I even recheck with a second opening of each bleeder. One difference: my master cylinder is below the floor in the vehicles I have used this with.
Hope this helps.

 

The air is trapped where it is until the bleeders are opened.

 

The added pressure may make the bubbles smaller , but being trapped in non compressible fluid they will not move through it.
Probably.

 

It's probably a good idea but, if you have to get down to the wheel cylinder to open up the bleed screw, why not pressurize from the wheel cylinder and force the bubbles up? Aircraft industry has been doing it that way on airplane brakes for almost 100 years, works great as long as they are on the ground . Know what I mean?

 

^ I bleed motorcycle brakes that way.

 

Bled backwards you just have to be a bit more patience and don't get all gung-ho with the pressure. Depending on the design of the seals you can blow them backwards if done wrong.

I like to pressure bleed my brakes at about 5-7 psi (master to wheels because that's the way my device works). The pipes are narrow enough that the air bubbles don't move in the other tubes and will generally be moving fast enough at that pressure to correctly burp out.
Just make sure to keep an eye on the feed reservoir. It's easy to bleed a whole bunch of air back in...

 

Basically correct. The OP needs to realize that brake lines come in a kazillion different shapes with every kind of bend and high and low sections. If you apply pressure to a line that has air trapped in a high bend, the air will simply stay there but compress......But, if its a small bend and not too high the air may push thru to the next high bend. In years past, when having trouble bleeding brakes, people have been known to jack the rear of a vehicle up to make it easier to push air past a bend, or to park on a hill. Air always wants to rise. Air sitting in a long level line can be pushed to another location....ie: the next high bend.

When I use a pressure bleeder, I fill the bleeder tank and hook it to the master cylinder adapter before I put the adapter on the master cylinder. Then I open the flow valve so that brake fluid is pushed through the rubber hose to the adapter....eliminating any air in the line.

I then fill the master cylinder with fluid to get rid of the air in the reservoir.

Then I put the adapter on the master cylinder and it will immediately begin pushing fluid . A small amount of air will remain in the master cylinder at the top, and it will just stay there while the fluid flows.

Then start opening the bleeder screws one at a time...closing each one before doing the next one.


I think its best to open all four bleeders at least two times apiece.


Note: You can buy kits with prebent brake lines for many reasonably newer vehicles. They can be hard to install because of having to snake them into place. They also sell the copper nickel brake lines with fittings already installed and cut to the proper length. They come in coils (no pre-bend). They can be easier to install and they last much longer than steel lines and are much easier to work with than Stainless Steel lines.

 

That's heavy duty. Have to move fluid thru the lines to get air out. Moving the fluid with some pressure on it helps, doesn't take a lot of pressure.

I made one from a spare master cylinder cap and a 1 gallon garden sprayer.

Can also use gravity for pressure, spare cap, tube and hang a fluid reservoir above the master cylinder

 

This, they move though the line just as they would with any other method. The bubbles are going to flow with the fluid they don't move on their own. They may compress a tad but not enough to write home about.
Ekimneirbo explained it about as well as it can be. The only thing is that the constant pressure from the bleeder will push the air bubbles out though the tube with the fluid a lot easier than pumping the pedal, stopping and then pumping the pedal again. Once you start moving the fluid and bubbles they keep moving. Making sure that you aren't introducing air that will have to be bled out in the end helps speed things up. Same as with bleeding by pumping the brakes it works better if you start with the bleeder with the longest line going to it and work to the shortest.

The Ammco pressure bleeder that I used in High school as a student in the 60's and as the instructor in the 80's was designed for 15 lbs max. Somewhere between 7 and 15 with one of the bug sprayer bleeders is plenty. The sprayer it's self probably won't handle more than that anyhow.

As Usual the ******** artists come out any time you start talking brakes with stuff that they picked up down at the spit and whittle club that doesn't really work. I cringe every time I see a lot of their nonsense. That includes Gravity bleeding that is often suggested when there are lines that are higher than the master cylinder or spots in bends in lines such as over any rear axle that are higher than the bleeders. Common sense says that will not work.

The reverse bleeding deal is one of those things, I would like to see that explained in detail. Most likely a read on Facebook thing. PhilA if you are able, give us a detailed set of instrucions on exactly how to do that as it has never come up in any of the shop manuals or stack of auto mechanics Text books I own.

 

Thanks fellas for the excellent comments.

 

Reverse bleeding is NOT ********... I've used it a lot on aircraft and it will work on automotive systems. It actually works pretty well as air will always travel "up" in a liquid. You just remove the cover from the master cylinder, connect the pressure bleeder to the bleed screw on the brake cyl. or caliper, open the bleed screw and start the fluid flowing. You'll get fluid and any air in the system coming to the surface in the master cylinder. Just have a bunch of rags handy to clean up any fluid that overflows from the master...

 

How would having residual valves in automotive hydraulic brake systems affect reverse bleeding? Would you just have to overcome the valve pressure rating? I think I'll stick to traditional methods.

 

First let me say that the hydraulic fluid does not care which way you push it through a tube. The dynamics that make it more or less viable are the other components in the system that the fluid has to deal with while being pushed.

I'd have to agree with Tomcat11 though I admit I have never tried reverse bleeding in a car. In most small airplanes you only have two wheels with brakes on them and it's pretty much a straight path upwards. That would give some credence to a simple system with brake lines that are virtually perpendicular being easier to push air upwards straight to the master cylinder with no residual valve or other component to interfere. So I can see the benefit of trying to push air up rather than trying to push it virtually straight down when it comes to airplanes.

Moving over to a car/truck, there are long virtually level tubes with bends and high and low spots and other valving to contend with. I don't feel that trying to push fluid thru from the opposite end would provide a benefit. One area that comes to mind is the "T" fitting on a rear end. If someone installed new brake lines, as when building a hot rod, the lines would be full of air. When you push fluid thru from the right rear wheel and it reaches the "T" fitting...........not all of that air is going to go upwards at the T. Some would go up but the fluid would also compress and push some air to the left hand side of the T toward the left rear wheel. When you close the bleeder on the right rear, the compressed air would push back and some would end up back on the right side. It would seem that it would take several times to purge all the air from the rear axle lines. Granted thats supposition on my part. But in my wee mind that seems logical when trying it on a car.

 

I have always used Maureen in the driver's seat and me at the wheels, has worked every time

 

I use speed bleeders, super easy. But, I don’t hear much about them so maybe they’re undesirable for some reason

 

A Mighty Vac tool is a handy tool to have. Brake bleed function makes it a one person operation. It is also a great diagnostic tool with a tee and vacuum gauge plumbed in for testing distributor vacuum advance and in other situations where vacuum leaks need to be ruled out.

 

Speed bleeders work, plain and simple.
I have installed them in my vehicles and Motorcycles, I have a pressure bleeder, and a vacuum bleeder but I prefer the speed bleeders.

 

I don't see the problem with this; that air is going to go where the fluid goes, regardless of which way the fluid is traveling, up, down or sideways. If you are starting from a system that's full of air and you're bleeding from the master cylinder, it's also going to take a couple of go arounds to get all the air out. Always has for me.

And by the way, these were not small airplanes that I was usually working on and they had lots of twists and turns in the brake lines... Sometimes you would be successful bleeding from the master cyl. and sometimes you'd get a stubborn one that would require bleeding from the wheel cyl. or caliper.

 

So how much pressure do the residual valves open at? 2 psi or 10 psi is the standard... a pressure bleeder set at 15 or 20 psi will easily overcome that.

Not saying you have to use "reverse" bleeding or even have to try it. It's just another way of doing the job that sometimes may work as well as the traditional way.

Myself? I've started installing Speed-Bleeders in everything and haven't looked back.

 

Yes, those are the common pressures and of course anything over would overcome the valve(s). Just wanted to see if anyone could come up with a potential issue doing it. Folks can and will do whatever they want or think is going to work. Speed bleeders are ok but can leak air back in. Pushing air back through a master cylinder might trap it there.​

 

Nope, never had that happen on anything. It just bubbles up out of the compensator port...

 

Compensator valves that have rusted stuck tend to fall foul of reverse bleeding- the cup seals aren't designed for a reverse flow of fluid. Granted, usually the failures occur when people get a bit gung-ho and the frustration over the system not bleeding causes action to be taken that would otherwise not be required.

Vehicular systems tend to leak past the threads when you open the bleeder screw, so reverse pressure bleeding is also a fair bit more messy. Coupled with the slave cylinders being the dirtiest part of the system, a lossy method (master to slave) is described in the service manual because it holds the least risk of introducing foreign particles into the hydraulics.

 

I appreciate that sometimes there can be difficulties bleeding brakes but 99% of the times its a trouble free, straightforward process, which makes me wonder why it seems to get so complicated when discussed.

Anyway, if someone invented clear brake lines we'd be able to see the air bubbles, and where they're hiding. Im off to the patent office!

Chris

 

Warbird1 "and neither does the air"

If air always traveled exactly like brake fluid then it would never take multiple trys to bleed a system. If a tube is filled with a supply of fluid, and the fluid is pushed thru the tube, then all the air would be expelled on the first try when using a pressure bleeder. The fluid would fill every nook and cranny if there was no air trapped somewhere in the system.

 

A RESIDUAL PRESSURE VALVE is a one-way valve. Reverse bleeding may not be possible in that instance (drum brakes or low mount MC).

https://www.brakebleeder.com/solutions/bleeding-brakes/how-it-works/

FORGOT - (As Usual)

If you have a FRONT DISC/REAR DRUM METERING VALVE (free-standing or within a combination valve), you would have to have a SPECIAL TOOL to hold the valve open either pressure bleeding or reverse bleeding the circuit.

The METERING VALVE is also one-way.

 

Correct me if I'm wrong but the valve has to allow the fluid to return back to the M/C at higher pressures. When the pressure drops to the rating of the valve it closes. If you reverse bleed, it would seem the pressure would have to remain under the rating of the valve or it will close. That's where, as you say, it acts like a one way valve.

 

Correct, but will all air be expelled with a controlled line minimum pressure of 10# (reverse bleed)?

The ten pound residual pressure in the drum brake line is designed to not allow the wheel cylinder cups to flutter and/or collapse possibly ingesting air or leaking fluid when hydraulic pressure is suddenly released/returned to the MC by the pressure of the shoe return springs. That amount of residual pressure has no effect on correct shoe return. All it does is hold residual pressure to keep the cylinder cups expanded (roundness to seal properly) and to maintain brake pedal height.

The actual shoe adjustment is controlled by initial/self-adjustment and condition of the return springs. The shoe return amount and stop is controlled mechanically.

I hope that made sense ...

 

Makes total sense. You are correct. I miss spoke and will correct my post for the record. I see no need to reverse bleed in any case.

 

Take a length of clear tubing, fill it with fluid allowing some air bubbles to form. Then connect it to a pressure bleeder and open the valve. With enough flow, you'll see that all the air will be expelled pretty quickly. Flow is the key here, it won't work with just a trickle...

And I haven't even touched on gravity bleeding yet... I've done in and it can work in certain situations.