gravity flushing brakes - risk of entraining air?

[NissanMaxima]

Hi all.
I have used the simple gravity method to renew brake fluid in my cars over the years. I have been pleased. I am aware of other methods and prefer to stick with it. However, I was looking at old posts on the topic of flushing brakes and some people have expressed concerns that this method can introduce air through the threads of the bleeder bolts during the process. Others say that because the reservoir is above all the calipers and bleeder bolts, this is impossible. I would tend to agree with the latter. Some folks on youtube videos recommend putting a dab of grease around the threads to avoid possible air entrainment. To those that have done the gravity flush approach, have you had any concerns with air becoming entrained through the loosened bleeder bolts?

 

[KrisZ]

No, never had that happen. But if you unscrew the bleeder too far, you may see air bubbles in the attached tube. But these air bubbles are being pulled from the outside and are immediately expelled through the bleeder, they never get to enter the caliper. The air cannot enter the caliper against the fluid flow. Otherwise brake bleeding wouldn’t work at all.

I think that’s the phenomenon many people experience and mistake it as air being pulled into the calipers.

 

[jetman]

air getting in when gravity bleeding?
not in my world.

 

[D60]

I have theories on such but they are just theories.

Once on an '06 Taco I kept getting a constant stream of bubbles in the clear line I had attached to the bleeder, and I was using my Branick pressure bleeder.

I found I had simply opened the bleeder too far, and closing it some eliminated the problem.

Now, if you look at how (and where) a bleeder is drilled, I think it makes sense that opening too far would create a "gap" between where the fluid exits the caliper or wheel cylinder reservoir and enters the bleeder, thus interrupting a steady stream of fluid.

I also think this could largely be mitigated by some grease (or "sealant" of your choice) on the bleeder threads. I would say this is probably best practice, though i rarely do it.

NOW, is that air actually entering UPSTREAM of the conical bleeder seat and thus getting into "the system"? I HONESTLY DON'T KNOW. Almost certainly not when pressure bleeding. And gravity bleeding is effectively VERY low pressure bleeding.

I guess the TL;DR is don't crack the bleeder further than necessary.

Also, all the above was contrived by my feeble mind in an attempt to make sense of personal observations. If anyone has other thoughts, I'm all ears.

 

[BrianF]

While not a pro, i have done the gravity one man bleed many times. Never ran into any issue of air entrainment.

The caveat is that the master cylinder is never run dry and I only turn the bleeders out enough to get steady flow.

 

[We're Metals]

I typically gravity bleed for a minute then finalize with 5 pumps of the pedal with the 2 man method. Never had any issues. I find its a lot quicker. I think the key is to have a slight bend in the hose you attach to the bleeder screw to prevent Air for somehow going back into the caliper as remote of a chance as this is.