Another brake issue . . .

[BDPetersen]

The owner of an F model on our field experienced a left brake failure while exiting the runway after landing. Pedal went to the floor, so to speak. A few days passed before the AI got around to check it out. He found everything normal, firm pedal and no leaks. So we are left trying to wrap our heads around a self correcting bleeding issue? Transient air bubble? 

Any thoughts? He uses the Don Maxwell pump oil can method for bleeding. I’ll pass along the tail low suggestion.

[RLCarter]

brake was dragging, heating up the fluid causing brake fade?

[smwash02]

You'll need to drop the flaps to have similar conditions as on landing.

I've had some challenges recently and while bleeding found my flap pump was introducing air into the system when operated quickly and needed to rebuild the pump.

[Prior owner]

I bought two different new oil cans and filled them with 5606. Then attached a CLEAR hose to each and began to actuate them. They both pump fluid and lots and lots of very very small air bubbles embedded in the fluid. Perhaps there is a better oil can out there that works better, but I haven’t seen it yet. If you are going to use the oil can method, I would recommend using a clear hose to watch for this.

Edited May 17, 2019 by PilotCoyote

[Yetti]

how old is the brake fluid?

[BDPetersen]

Brake was bled on annual just before the flight. Was an issue then, but thought to be resolved.  Presumably air remained. The mystery was how it resolved itself while sitting for a few days. It has now been bled again (in spite of now having firm pedal) this time with nose elevated to possibly tilt master cylinder so as to purge any air bubble.

[Bolter]

On 5/16/2019 at 5:18 PM, BDPetersen said:

The owner of an F model on our field experienced a left brake failure while exiting the runway after landing. Pedal went to the floor, so to speak. A few days passed before the AI got around to check it out. He found everything normal, firm pedal and no leaks. So we are left trying to wrap our heads around a self correcting bleeding issue? Transient air bubble? 

Any thoughts? He uses the Don Maxwell pump oil can method for bleeding. I’ll pass along the tail low suggestion.

(based on my J experience)  What about o-rings in the master cylinders?  I have had nicked o-rings where the chrome plating of the piston rod wore.  Easy to replace once you have the the panels removed and can reach the cylinders.  Standard o-rings inside.   It could be that it let air in, at the cylinder, and that bubble went upstream after given time to sit.  

Also, based on my J experience, when there was a brake system problem with leakage before the cylinders, it affected both brakes equally.  This is not an absolute certainty, but I would be biased to find a problem between caliper and master cylinder as the failure was the left brake only.  

[Yetti]

air gets trapped in the high portion of the wheel well.   It might be actually higher than the brake cylinders.  A tape measure would say so.   There were two sets of bubbles that came out the last side.   Per the A&P usually it take 20 pumps with the oil can.   Second set came out after 40 pumps.

[BDPetersen]

10 hours ago, Yetti said:

   Per the A&P usually it take 20 pumps with the oil can.   Second set came out after 40 pumps.

That sounds like a good explanation. Will pass that on.

[Prior owner]

13 hours ago, Yetti said:

air gets trapped in the high portion of the wheel well.   It might be actually higher than the brake cylinders.  A tape measure would say so.   There were two sets of bubbles that came out the last side.   Per the A&P usually it take 20 pumps with the oil can.   Second set came out after 40 pumps.

I wonder if this is a non-issue with a standard pressurized brake bleeding setup, as the fluid delivery has higher pressure and volume/flow is higher and constant enough to drive the air out more efficiently?

Edited May 20, 2019 by PilotCoyote

[deanders]

This happened to me several times in my E model. I removed the master cylinder and overhauled it using new orings from Spruce Aircraft. There is a check valve of sorts in the master cylinder that is the issue. I haven't  had any  issues since the overhaul.