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Trouble with filling/bleeding my brake system

Dandlac58

Well Known Member
Hi VAF Friends, Mentors, Experts!

Trouble: Unable to fill brake system to remove air bubble mushy/sponginess on three of four brake pedals.

Situation: emptied brake fluid and flushed entire system from top to bottom with mineral spirits. Cleaned brake housings and pistons and added new O rings on the pistons. Using the rebuild kit from MATCO, I rebuilt all 4 MATCO master cylinders, two on the pilot side L & R brakes and two on the co-pilot side L & R brakes. A "T" running from the remote fluid reservoir feeds both sets of brake lines. Checked to ensure all brake lines were connected properly to the master cylinders and to the brakes at the wheels. Proceeded to fill all lines from the bleed fitting at the bottom of each brake on both wheels. Pumped 5606 until it was flowing without bubbles through an overflow line into a bottle we had set up at the remote reservoir in the engine compartment. No leaks detected at any point in the system. Yet the only brake working properly is the co-pilot right brake, the other three are mushy/spongy. Also, if I hold pressure on the co-pilot right brake pedal, the pilot right brake pedal firms up, but when i let go of pressure on the co-pilot brake then the pilot brake goes mushy/spongy. Are we missing a filling process/procedure for the 4 Master cylinder brake setup on the RV7?

What in the world am I doing wrong, spent 7 hours today on a process that should take 30 minutes and at a stand still as to how to move forward. Any help is greatly appreciated.

Thanks!
Dan
 
Do the same sequence again, but during the filling process have somebody cycle the brake pedals as it is done. I have had this help purge air from the lines.

There is a filler system sold that works on the principal of gravity. It look like a medical intravenous injection and is hung higher than the outlet of the master cylinders. Then the fluid is injected via the same caliper bleed fitting with the hose going up to the IV bottle. Since the bottle is higher than the whole system it not only has good head pressure it also forces out the air in the system from the bottom up. With a measured volume of fluid in the bottle you can walk away or leave it overnight. You can also do the above mentioned pumping action on the brake pedals every so often. Then close the caliper bleeder valve and disconnect the IV line. Here is a picture of it:
 

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Careful

I would be careful using an solvent in the system. The solvent might be trapped and then cause problems. If it was me, i would remove the fluid one more time, and try and get everything out, and then fill with fresh fluid, just to make sure no solvent remains. Others have filled systems from the bottom up like you describe so I am not sure why it didnt work this time, except most others dont not flush with solvent, that I know of.
 
Do the same sequence again, but during the filling process have somebody cycle the brake pedals as it is done. I have had this help purge air from the lines.

This ^^^
Also, next time consider the Royco fluid with the higher flashpoint.
 
I had a problem bleeding from the bottom up because the pedals were not fully retracted. Apparently as soon as you move the pedal the opening to the reservoir closes to prevent backflow. If the pedals are not fully retracted the valve will be closed and prevent the fluid from entering the master cyl which stays full of air. I put bungee cords on the top of the pedals to make sure they were fully retracted and that fixed the problem

You can also add external springs to the master cyls if needed

xcvb.jpg
 
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You always bleed from calliper to reservoir, that’s the way it’s done in the aviation industry. Air bubbles want to rise, help them!
 
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Make sure...

Make sure the master cylinders are FULLY extended, especially the last 1/16". If they are not, and you try and fill/bleed, you will have problems.
 
Parking Break?

If you have a parking break, it can cause trouble. Berenger breaks are harder to charge too.
 
With an initial bleed (new system or completely overhauled) I have found that you must exercise the pedal during the bleed process and often times must aggressively shake the pedal assembly during the bleed (as you shake it around or exercise it, you can see the bubble coming out the upper end - Keeping doing it until this stops). Simply pushing fluid through the system is not enough to purge all of the air out of these master cylinder.

Larry
 
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I think this may be the issue...

I had a problem bleeding from the bottom up because the pedals were not fully retracted. Apparently as soon as you move the pedal the opening to the reservoir closes to prevent backflow. If the pedals are not fully retracted the valve will be closed and prevent the fluid from entering the master cyl which stays full of air. I put bungee cords on the top of the pedals to make sure they were fully retracted and that fixed the problem

You can also add external springs to the master cyls if needed

View attachment 18229

I also just talked to MATCO, they said the Master Cylinders must all be fully extended in order to open the bypass valve so fluid can flow all the way through the system and up to the remote reservoir. Will be trying that today and may disconnect the push rod on the master cylinders from the brake pedals just to make doubly sure the bypass valves are open.

Will report the results and thanks to all for the thoughts and recommendations.
 
With an initial bleed (new system or completely overhauled) I have found that you must exercise the pedal during the bleed process and often times must aggressively shake the pedal assembly during the bleed (as you shake it around or exercise it, you can see the bubble coming out the upper end - Keeping doing it until this stops). Simply pushing fluid through the system is not enough to purge all of the air out of these master cylinder.

Larry

Or, you can use a tool like the Mighty Vac vacuum bleeding system and not have to do any of that...:rolleyes:
 
I also just talked to MATCO, they said the Master Cylinders must all be fully extended in order to open the bypass valve so fluid can flow all the way through the system and up to the remote reservoir. Will be trying that today and may disconnect the push rod on the master cylinders from the brake pedals just to make doubly sure the bypass valves are open.

Will report the results and thanks to all for the thoughts and recommendations.

I am sure they told you that the bypass occurs in the last 1/16" of the stroke...and they aren't kidding!
 
Pulled the top of the brake pedals...

Make sure the master cylinders are FULLY extended, especially the last 1/16". If they are not, and you try and fill/bleed, you will have problems.

... all the way to the stop on each pedal.

How do you know that you actually got the cylinders fully extended? I just bought this RV7... is it possible that the bolts which hold the master cylinders on to the brake pedals are not far enough apart and therefore are not actuating the bypass valve on the master cylinders?
 
How do you know that you actually....

got that last 1/16" Bob? I pulled the top of the brake pad as far to the rear as possible but still no joy in filling the system. As I understand it, if the bypass valves are open on both of the master cylinders for the brake you are filling, you shouldn't need to pump the brakes, just fill until you have fluid coming out of the top of the remote reservoir?

I am sure they told you that the bypass occurs in the last 1/16" of the stroke...and they aren't kidding!
 
got that last 1/16" Bob? I pulled the top of the brake pad as far to the rear as possible but still no joy in filling the system. As I understand it, if the bypass valves are open on both of the master cylinders for the brake you are filling, you shouldn't need to pump the brakes, just fill until you have fluid coming out of the top of the remote reservoir?

I think the only way to be sure is to remove one bolt on the cylinder and see if it is fully extended…
 
As I understand it, if the bypass valves are open on both of the master cylinders for the brake you are filling, you shouldn't need to pump the brakes, just fill until you have fluid coming out of the top of the remote reservoir?

NOt in my experience (built two RV's and have rebuild 4 RV full brake systems). Simply pushing fluid up from the bottom is not enough to evacuate all of the air in the master cylinder, like it does on a car (actually an auto MC must be bench bled to get the air out, which is why they don't push fluid from the bottom, they use vacuum to PULL it to the bottom). Something about the design allows a lot of air to remain in the MC doing this. I use pedal exercising and shaking to get it out. Bob's approach with vacuum I am sure does well also.

MC's are notoriously hard to bleed, unlike the rest of the brake system.

Larry
 
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Something about the design allows a lot of air to remain in the MC doing this. I use pedal exercising and shaking to get it out. Bob's approach with vacuum I am sure does well also.

MC's are notoriously hard to bleed, unlike the rest of the brake system.

Larry

The problem is easy to visualize. If the MC piston is partially or totally in the ‘brake on’ position, it blocks the reverse flow of fluid back to the reservoir. But in the ‘brake fully off’ position, the piston rises slightly above the port to the reservoir, allowing there to be a trapped air space. You need to exercise the MC piston to get this air out.
 
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+1

This ^^^
Also, next time consider the Royco fluid with the higher flashpoint.

A big highlight on Bob's suggestion

The Royco 782 is worth every penny for the higher flashpoint. There have been a lot of brake fluid fires over the years with the 5606 fluid.

A lot of people also replace the O rings with Viton O rings for the higher temperature rating to reduce the chance of a melted O ring from some un predicted high temperature braking event.
 
Dan,

On my RV-8, I’ve never had trouble bleeding the brakes from bottom to top. However, we had the same difficulty you are experiencing while bleeding the brakes on a friend’s RV-7 using the same method.

I decided to bleed the brakes using the old school automotive method of applying brake pressure, opening the bleed valve, holding the brake pedals fully depressed, closing the bleed valve, releasing the brake pedals, and repeat. It took a while, but it resulted in solid firm brakes. I believe we used the brake pedals whose master cylinders are closest to the reservoir. That pressurized the fluid through the second set of brakes downstream. Worked like a charm.

Good luck!

Jerry
 
She's taxiing!

Super Tweety made her maiden taxi on 7A gear and rebuilt brake system! Worked beautifully.
Thanks to each and all of you who took the time to make comments and provide advice. Its help like this which grows a strong community of friends!
 
Check and Re-Check as you rebuild Master Cylinders

Thank you Jerry, once we got the Master Cylinders all dialed in, we were able to bleed the brakes fully and completely, removing all air on both sides in about 10 minutes and all is wonderful. The universe is in harmony again!

I will only add this bit of experience based on doing these brakes....bench test the master cylinders before you ever put them on the airplane and if they dont work on the bench they wont work on the airplane... the bench test is very simple, fill them from the bottom port and then cap the bottom port, try to push the extended chrome shaft down, you should not be able to move it more than about 1/16th as the bypass valve closes. No fluid should come out of the top port. If it does, tear them down and check both internal springs and the three o rings and static valve to make sure they are all correct. Without admitting anything.... haha... if even one of them is not correct, the system will not function but if they are all correct, it will function like a dream!


Dan,

On my RV-8, I’ve never had trouble bleeding the brakes from bottom to top. However, we had the same difficulty you are experiencing while bleeding the brakes on a friend’s RV-7 using the same method.

I decided to bleed the brakes using the old school automotive method of applying brake pressure, opening the bleed valve, holding the brake pedals fully depressed, closing the bleed valve, releasing the brake pedals, and repeat. It took a while, but it resulted in solid firm brakes. I believe we used the brake pedals whose master cylinders are closest to the reservoir. That pressurized the fluid through the second set of brakes downstream. Worked like a charm.

Good luck!

Jerry
 
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