Tom Piper

While using some clear tubing with brake fluid in it, I found it interesting to try to get the air (bubbles) to flow downward . It isn't an easy thing to do; it takes a sufficient flow to accomplish it.
So, it would seem that gravity bleeding, or vacuum bleeding at the bleed-screw, would not do a good job to get air out of the system. Plus, because it takes sufficient flow to dislodge crud attached to the walls of the tubing and parts, gravity and vacuum usage doesn't seem like the way to get the crud out either.

After reading some on it, I like to use "pedal" flushing (even better than pressure flushing) and Reverse Fluid Injection.
If using pressure flushing from the master cylinder, it is necessary to prepare properly to get all the fluid out. With pedal flushing, it happens automatically.
However, with pedal flushing, it is best to use a limited stroke on the master cylinder so not to push the seals into the normally unused section of the bore.
With Reverse Fluid Injection bleeding (which should only be done with a system with clean brake fluid after a good flushing) is accomplished by forcing the brake fluid INTO the bleed screw and taking advantage of the normal flow of bubbles upward and out of the master cylinder.

For some good FREE information and some excellent tips on how to prepare the master cylinder for **complete** pressure flushing, download the manual from this site -- it is a 40 some page PDF file, so dial-ups beware:
http://www.phxsyss.com/

Plus, if you are interested in buying one of these injectors at a reduced price, go to this site:
http://www.tooldiscounter.com/leftme...category=Brake

I bought the PRO Injector model for $196 and, after doing my '92 yesterday, I am quite pleased with it.

As a side note, my '92 has C5 brakes and there were some questions on pedal travel when using the original C4 master cylinder with the bigger bore C5 caliper pistons.
Calculations using the difference in piston diameters suggest that there should be 10% more pedal travel for the C5 calipers, but the Corvette has a quick take-up master cylinder (kind of like the new floor jacks that raise fast until the load is applied) and I can't even notice it.


Tom Piper

John A. Marker

I have been using the clear plastic tube trick for years and placing the tube in a glass jar of brake fluid so if there is the back flow it pulls fluid and not air. I have always used the brake peddle and slowly depressed it. You also have to constantly check the master to be sure you don't run out of fluid. This method has worked for 30+ years of bleeding them myself.....although it does help to have the wife help when she can.

jackdaroofer

rGravity bleeding works fine...in fact, from what I read, it is the choice of Nascar and other racers.

While you are gravity bleeding you can be doing other things at the same time. Just don't let MC run out of fluid. Crude is generally suspended in the fluid so it flows out just fine

seya

Tom Piper

I certainly respect your opinion.

Tom Piper

C4DC

When I bled my system last week after installing the bias spring, I flushed the whole system and was surprised at how quickly the new ate blue showed with the speed bleeders. I figured I'd have to fill the mc several times to see the color change at the rt rear, but after the 2nd fill it was there. It would seem with the fluid moving with each pedal stroke, air bubbles would get pumped out quickly. This wouldn't be so with conventional gravity or the pump and hold techniques (or at least decreased effectiveness).
Obviously this has nothing to do with bleeding the mc though...
I bet the nascar guys are just flushing fluid and the components are nearly new so no crud in their systems. Just me thinking aloud (and rambling).

Tom Piper

I think of this forum as a place to share information. That was my purpose when I started this thread.
The method someone else uses is not important to me.
What follows is information I found while reading and experimenting.

Flushing and bleeding a brake hydraulic system are two distinct processes.
Flushing is the process of attempting to remove contamination from the system.
Bleeding is the process of attempting to remove air from the system.

Flushing:
Some contaminants are in suspension in the brake fluid, some adhere to the inner surfaces of the system. The amount of neglect has a bearing on how much adheres to the inner surfaces. Even gentle flushing will remove the contaminants in suspension in the brake fluid. It is the contaminants that adhere to the inner surfaces that need to be “coaxed” loose. This is where enough force is needed to create a flow that “agitates” the liquid.
I personally prefer pedal flushing because it creates “pulses” and turbulence. However, pressure flushing by applying pressure to the master cylinder is a good way if done correctly. But, few people do pressure flushing correctly – in order to have an effect on all the fluid in the system (ignoring the fluid captured in some ABS modulators) it is necessary to have the primary cup in the master cylinder down stream from the vent port by using a method to keep the brake pedal depressed – the drawings in the manual I mentioned in the original post above do a much better job of explaining this.
And, it is always best to flush a system well first before attempting to use reverse bleeding so contaminants from the wheel cylinder or calipers are not moved backward into the master cylinder and ABS system.

Bleeding:
For bleeding, the brake system can be thought of as divided into two parts.
Part one is where the fluid flows down from the master cylinder into the inlet of the wheel cylinder or caliper. Part two is in the wheel cylinder or caliper itself where the brake fluid enters at the lowest point and exits the bleeder valve at the highest point – in other words is uphill.
It is important to realize you will probably never get every minute bubble out of the system – your goal should be to get as much air out as possible.
The first part (downhill from the master cylinder to the caliper) of the system is best bled with Reverse Flow of the brake fluid while using pressure to inject clean fluid at the bleeder valve for each wheel. This takes advantage of the fact that air bubbles in the system have a tendency to flow uphill and you are simply aiding the process. (As an added note, while experimenting with bubbles in clear tubing, I found that the smaller bulbbles are the hardest to get moving, they seem to adhere to the inner surface of the tubing and don’t want to move without some encouragement.)
This first part of the system is made up of horizontal and vertical sections – the bubbles move well in either direction in the horizontal sections, but respond much better to upward movement in vertical sections.
Looking at the second section of the brake system (wheel cylinder or caliper), the best way to bleed it is to allow clean fluid under the influence of gravity to fill the caliper and move the air to the top and out the bleeder screw – called gravity bleeding or burping.
So, bleeding is a two step process – first reverse pressure to get air out of the first section and then gravity bleeding to get air out of the second section.

The other nice thing about Reverse Injection is you can create pulses with it (similar to the pulses with the brake pedal) that tend to loosen the small bubbles from the inner surface of the tubing and get them moving.

If you have a system that does not have air in it to start with and you don’t let the fluid level run low in the master cylinder, gravity bleeding is sufficient because there is no air to get out. But, for flushing a neglected brake system, gravity or vacuum may not do the job.


Tom Piper