Stubborn Brake Fluid Air Pocket?

[BUDFIVE]

Build Friends,
These pics show what looks like a stubborn brake fluid air bubble in the remote reservoir adapter at the master cylinder tops. The brakes have been bled manually one corner at a time and the pedal is firm. This “bubble” expands/moves when brakes are applied. The reservoir fluid level is above the MCs but the feed hose from reservoir to MC adapter is level with or just below the MC at times as it travels through the footbox front.
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Parts:
Tilton 75-750 with remote adapter
FFR fluid reservoirs with 1/8NPT to 5/16 barb fitting
5/16” hose from reservoir to MC adapter
3AN male to 3AN male
3AN Brake fittings/lines

The previous Wilwood MC had black remote adapters so it may have had the same pocket and I wouldn’t know it.

The air pockets have been there a week and I re-bled the corners yesterday and they remain. While the brakes feel good now, it seems during handling forces the bubble could move and get “sucked” into the cylinder.

Is it time for a pressure bleed?

[rich grsc]

It isn't a problem

[CraigS]

Rich please explain that.

[rich grsc]

The pocket is above the fluid, no different that a reservoir attached directly to the MC. Fluid is drawn in from the bottom of the fluid reservoir when the MC is retracted, that port closes when you apply the brakes.

[Alec]

Agree, it won't get 'sucked' into the MC. The air will want to float to/ stay at the top. Mine are black, I wouldn't ever know if I had a bubble in there.

[Ray from Long Island]

Rich is right but perhaps also leave the reservoir fluid cap very loose overnight to let the bubble rise naturally?
Ray

[BUDFIVE]

Guys thanks. Ray, I too thought about leaving the reservoir unsealed. It looks like it might help if I raised the front vs rear of the car.

[michael everson]

You might need to pull a vacuum to get rid of the air pocket.
Mike

[rich grsc]

Leaving the top off the reservoir might not be a good idea as brake fluid easily absorbs moisture?
I would just leave it alone

[Bill Elliott]

Leaving the top off the reservoir might not be a good idea as brake fluid easily absorbs moisture?
I would just leave it alone

Rich is correct, exposure to air degrades the quality of the fluid . The absorbion of moisture reduces the boiling point of the fluid. You would have more problems later bigger than that bubble sitting on top of the fluid.

[BUDFIVE]

Great discussion with lots of knowledge bits.
Rich, good point about moisture.
Michael, vacuum at the calipers? If so that should be the same as pressure at MC?
Bill, aside from trying vacuum or pressure, doing nothing might be the right thing.

[i.e.427]

One tip to avoid this from happening is to elevate the reservoirs during the initial bleeding. I struggled with a Mk4 over the weekend that just wouldn't bleed. I finally loosened the reservoir from it's mount and held it up while bleeding and it started moving fluid.

Frank

[BUDFIVE]

Thanks Frank,
I moved the reservoirs to the highest position in the mounts but didn’t try pulling them down, out of the mounts and holding them above the frame rail. Good to know.

[MB750]

I must have burned thru an entire quart of brake fluid doing my system. I got it, but made quite a mess in the process killing air bubbles and moving reservoirs and whatnot.

[CraigS]

If you need to move the reservoirs from their standard mount position isn't that telling you that you need to move them permanently?

[BUDFIVE]

CraigS-interesting thought. I hope once there is no air between the reservoir and MC the fluid has inertia to stay there.

My read on moving (raising) the reservoirs during bleeding is to provide enough gravity force to overcome the fluid’s inertia, getting it to flow from the reservoir to the master cylinder replacing the air already there. That inertia could be surface tension, short rises in the hose, hose ID vs fluid viscosity, etc. I believe my problem is only there because the air is between the reservoir and top of MC, above the force created pumping the brakes. In fact, as I think about this more, bleeding with the reservoir cap on loosely vs off may not be helping me. I think the force can also be pressure at the reservoir. So if I need to I’ll try pressure bleeding because the air pockets bother me:)

“……an object at rest, tends to stay at rest; an object in motion tends to remain in motion…”

[cv2065]

Use an air bleeder. This thing is amazing. 5 minutes a corner.

https://www.harborfreight.com/pneumatic-air-operated-brake-bleeder-with-auto-refill-kit-57057.html

[Wizbangdoodle]

I concur with Rich, not a problem. The only air that will be a problem is when it enters the pressurized section of the brakes, i.e. the piston section of the MC. Think of a typical master cylinder on an old car or truck. You fill it up and then place the cap on and pull the retaining bar over the top to keep it in place. There's a bunch of air between that cap and the brake fluid. Same thing here. As long as you don't drive upside down, you'll be fine. This truly is a non-problem.

[i.e.427]

If you need to move the reservoirs from their standard mount position isn't that telling you that you need to move them permanently?

The short answer is yes. Ideally the entire supply hose and reservoir would be higher than the masters. However, with the platforms we are dealing with, this isn't possible. With the donor master cylinder and remote reservoir, this wasn't an issue. The reservoir was higher than the master. With the Wilwood's/Tilton's it's not. My recommendation is to raise the reservoirs during initial bleeding, so that any air pre-master can migrate up and out. The question was about an air pocket pre-master but post-reservoir and is it a "problem"? For now.......... no problem. I am not a fluid dynamics engineer. I have however been in the electrical field for some time. And I can tell you, they call it electrical theory for a reason. It doesn't always do what you think it's going to do, or what it's "supposed" to. I always raise the reservoirs during the initial bleeding in a Mk4 or Coupe. It's just become habit.

You all do you.

Frank

[BUDFIVE]

Hey y’all—I wanted to share this follow-up:
I tried re-bleeding the brakes this week with the reservoirs removed from the mounts and raised as high as the hoses would allow—the air pockets remained. I then tried pressure bleeding-the air pockets remained. With the general consensus it wasn’t an issue, I put this on the back burner. In my build thread update tonight I mentioned that during my 2nd golfcart the pockets disappeared—apparently the drive vibration and repeated, firm brake application bubbled the air up into the reservoirs. The brakes felt and performed great today. Thanks guys.

[CraigS]

Glad it worked out for you. Nice not to have your brain saying, 'You sure it will be OK?'