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Right Brake Won't Bleed, RV-6A

petehowell

Well Known Member
Here's the situation. Buddy has an RV-6A. We found a leaking, cracked flare at the right caliper and fixed it. Having a whale of time getting the air out of the right side brake system when we bleed. Pilot side brakes only. Has a soft pedal that can be pumped up to solid, then goes soft in a short amount of time. Left side has solid pedal, no problems.

Here's what we have done. Garden sprayer pressure bleeder from the bottom up. We've run several qts thru the system and out thru the reservoir. No bubbles coming out at the reservoir. Still a soft pedal.

We took the pressure side line off the master that goes over the top of the pedals and tried to get any air out of that - then rebled from the bottom up - soft pedal.

Replace master cylinder on the right side, re-bled, still soft pedal.

There is section of pressure side line that goes from the flex hose to to solid tube then up and over where the battery box used to be then on to the gear leg bulkhead fitting. That area looks ripe to store a bubble, any ideas how to flush it out better?

There are no fluid leaks we can see anywhere. Any tricks we are missing or areas to inspect further?

Thanks for any Ideas.......
 
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My building buddy has a Cozy MKIV that has a parking brake valve. Any time we do brake work on it where the system is opened, the parking brake valve will trap and hold some air when we bleed the brakes, even using the pressure pot method from the bleeder valve, he would always have a soft pedal. Took forever to trouble shoot but he has clear flex lines and we finally removed the valve from the mounting bracket and manipulated it around as the lines were being bled and out popped several air bubbles. Their may be a spot somewhere in your friends brake system holding air that get trapped and the pressure pot bleeding just can't push it out.
 
no parking brake

Hi David,

No parking brake, but the line hoop over the battery box seems suspect to us. The line has bee successfully bled previously......
 
Bleeding Brakes

Some planes are notorious for being hard to bleed. For difficult cases I like a pressure pot from the bottom up with the highest flow the system will allow, then while flowing tap on the sides of the brake cylinders with something solid or rapidly pulse the brake pedal a small amount. Sometimes this will dislodge the air bubble from it?s hiding place. To keep the mess to a minimum I have a fitting with a hose on it that replaces the reservoir fill plug during bleeding. Good luck.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
What size is the line the runs up to the battery box? Larger lines can be very difficult to purge, but since you said it has been done successfully before...
The higher the velocity the better. Instead of using a bleeder valve I would break the line at the closest fitting and pressure bleed it. Keep the pressure up high (10-15psi) the whole time.
Another, more difficult, trick is to insert a very small diameter line (miniature EVA tubing) into the brake line - measure the length so that it reaches just to the peak of the loop. You can then use a vacuum pump or suck on the line to remove air. you put the open port where you are inserting the tube in a cup of fluid so you do not introduce more air.
 
1/4 in tubing

The tube is the Van's plans std small 1/4", same that goes down the gear leg.


What size is the line the runs up to the battery box? Larger lines can be very difficult to purge, but since you said it has been done successfully before...
The higher the velocity the better. Instead of using a bleeder valve I would break the line at the closest fitting and pressure bleed it. Keep the pressure up high (10-15psi) the whole time.
Another, more difficult, trick is to insert a very small diameter line (miniature EVA tubing) into the brake line - measure the length so that it reaches just to the peak of the loop. You can then use a vacuum pump or suck on the line to remove air. you put the open port where you are inserting the tube in a cup of fluid so you do not introduce more air.
 
I have one which is exhibiting similar behavior, have flowed quarts of brake fluid, bottom up under pressure, with no bubbles coming out the reservoir, and still a slightly soft pedal, but enough to hold at run up, but wish it was better.

I solved the other side by 1) bleeding bottom up, 2) bleeding top down, 3) bleeding bottom up. And it finally resulted in a rock solid pedal. Tried this same sequence on the trouble side to no avail.

Had to disconnect the master cylinder from the rudder pedal to allow me to shake the cylinder and adjacent lines while bleeding under pressure. Gave me just enough movement in the lines to shake a bubble free.

Makes sure the garden sprayer bleeder isn't introducing air into the stream of fluid. I ended up not using the pump handle to pressurize the sprayer. Instead I installed a fitting into the top of the sprayer tank and pressurized the headspace above the brake fluid in the tank using my air compressor and a quality regulator. Ended up using about 15 psi.

Put a little dab of grease on the tubing where you connect the fluid feed to the bleeder valve to keep air from being introduced there.

Once you get this solved, please post what you think did the trick.
 
Get it yet Pete? How much pressure on that sprayer? I tested mine with a gage to count x number of pumps to 30 psi. Then you might have to crack the fitting loose at one of the master to get the flow higher until that bubble comes through.

Hope you got it already, though.

Great to see you at OSH and meet Andi.
 
Impossible to say for certain but perhaps your master cylinder is leaking internally? Do you have firm peddle pressure and brake application but the peddle creeps while holding brakes? If so, replace the master cylinder.

I hope this helps.
 
Thanks Guys

The owner's dad had a med procedure so we are taking a few days off. Some responses below.

Fred - Master cylinder is has been replaced, so unless old and new had same defect, I think we can rule that out.

Bill - I dont know the pressure, but I have a way to measure and will check it. We the hose off at the master and bled there, but did not try just cracking 0 maybe we let some air back in - good idea. (Andi loved meeting you, too!)

Dave - Shaking the master, and dab of grease on the nipple - we will try, Top down bleed - will try this, too. Several other systems bled easily (RV7/9) with this system have been easy and solid pedal, and clear line shows no bubble out of the sprayer, but we will try an RC fuel pump next to see if that could be it.

Thanks guys! will report back!
 
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Try by unhooking bottom bolt on the master cylinder and swinging it up past horizontal, then pump. Any air should move out.
 
Mounted upside down

If its mounted upside down as per early design you will never get all the air out until you turn the master cylinder right side up to bleed the system.

Cm
 
Fixed! - caliper problem..........

The owner figured it out - the brake line was coming in contact with the wheel pant and forcing the caliper to retract the puck when the brakes were released - the soft pedal was not air, it was caused by needing to force the puck out a long distance to contact the disk again.

Additional info - this plane has custom pants that allow the brakes to be bled with only an access panel removed.

the brake line was repositioned, and the pedal is firm all the time now.
 
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Pete,

If a soft pedal can be pumped up and becomes hard, that points to seals in the master cylinder or some anamoly with the calipar (warped rotors have similar symptoms). Air in the lines will have the pedal soft until enough pedal travel has occurred to compress all of the air and then it goes hard, often progressively. This should be repeatable any number of times with the same amount of soft travel before going hard. If it is air in the lines, you could pump a hundred times and you'll still have the same amount of travel before the pedal goes hard.

Larry
 
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