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Weak brakes...

ccsmith51

Well Known Member
I, and my friends, are stumped on this one.

The brakes on my RV-6 have never held very well since I purchased it last August.

About 20 hours ago I replaced the pads and discs. It still didn't hold very well and I thought that perhaps I had not broken the pads in well. I would have to stand on the brake pedals to the point I was forcing fluid past the master cylinder O-ring and still the brakes would not hold past about 1,600 RPM.

Yesterday I pulled everything apart and sanded the pads per recommendations to remove any glaze, roughed up the discs, and put it all back together. I flew and upon landing got on the brakes good. I taxied back and took off and landed and did the same.

Today I went flying that the brakes are the same, won't hold past about 1,600 RPM.

The pedals feel solid, not mushy. There are no leaks in the system except when I press so hard that I think I might break the pedals.

So, any thoughts on what I can try next? The same brakes on my RV-4 we so strong that a couple of times I almost put it on the nose...

Thanks,
 
There is a published procedure for breaking in the pads and this sounds like it was not done correctly. Some say that just stand on the brakes when doing the landing roll out but in my experience this does not work. I have the proper procedure written down at the hanger as a hand out but am at home now. Suggest going to the brake pad manufacture webpage and downloading the info. The scenario that you are experiencing is not new to the RV world but seems to go away as soon as people are coaxed into doing the break in correctly.. Larry
 
Do both brake sets feel equal when applying pressure? If so, my uneducated guess would be the shoes and or rotor.
 
Procedure

I have Rapco pads that I purchased from ACS. On their web page it has a PDF with the proper procedure. Below is what it says, and which I performed:

RAPCO, Inc. Brake Disc Wear-in Procedure
The normal break-in procedure for brake discs using the Rapco, Inc. RA164- discs, is to get the friction material hot (Approx. 850 Deg. F.) by performing the following:
? Use proper maintenance techniques by insuring that the friction material is completely clean and dry of all solvents and hydraulic fluid
? Perform a moderate-speed taxi (10-15Kts ground speed), insuring that you have proper braking (There will be very little brake action at first).
? Allow the brake to cool by taxiing across the airport or down field, without using brakes. If you are not going to fly the aircraft during the break-in procedure, then allow the brakes to cool for 45 min. until the next high-speed taxi stop.
? Once you've established positive brake action, you can proceed to a full-stop landing using significant braking. (Short field landing procedure). At this time the pilot should notice the brakes starting to grab more. If this is sufficient braking action, there is no need for another stop cycle.
? Perform another short field landing, leaving the landing gear extended throughout the landing pattern. This allows the brakes to cool properly before the next heat cycle. After this landing you should have significant brake action.
 
I find if you put your toes to the top of the pedal you will get a bit more power.

Otherwise you may have a strong engine. Results may vary. :cool:
 
Are your brake disks standard issue or stainless? I seem to recall that stainless discs don't offer the same gripping power (friction) as standard steel discs.
 
Do both brake sets feel equal when applying pressure? If so, my uneducated guess would be the shoes and or rotor.

Both brakes feel the same. What I have not tried is flying from the right side and using the passenger side pedals. When we bled the brakes last time we tested. It's sides and he felt the same. I will do a run-up from the passenger side next time and see if it is thes same, which is what I expect.
 
I find if you put your toes to the top of the pedal you will get a bit more power.

Otherwise you may have a strong engine. Results may vary. :cool:

Tried that but the pressure required is still too much and can't hold past about 1,600 RPM.
 
Had a similar issue. Air in the lines. Try one of these. I hooked up a line from the top of the reservoir and just let the bleeder go till I had good overflow from each side. Also, to remove excess fluid from the reservoir, I use a turkey injector syringe from the aviation department at Winn Dixie.

As a added benefit, it sure makes bleeding brakes easy.

https://www.aircraftspruce.com/catalog/lgpages/hydraulicBleedertank.php
 
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Have you checked the brake and cylinder part numbers? Maybe somehow you have the wrong brake or cylinder.

Dave

I am not sure what that means. Can you elaborate? The wheel brake assemblies are Cleveland. I don't know what the cylinders are in the pedals, but they are gold.
 
On my 4 project I noticed that the mounting bracket for the brake assembly, which is a steel disk with 4 holes, was not welded on to its mounting tube properly. It is crooked. So the pads will not be parallel to the disc. I am going to fix that before I assemble the landing gear. I think that this will substantially reduce the contact area and therefore the stopping power.

Could that be your problem? Just a hare brained theory, but you posted looking for ideas and one never knows.
 
On my 4 project I noticed that the mounting bracket for the brake assembly, which is a steel disk with 4 holes, was not welded on to its mounting tube properly. It is crooked. So the pads will not be parallel to the disc. I am going to fix that before I assemble the landing gear. I think that this will substantially reduce the contact area and therefore the stopping power.

Could that be your problem? Just a hare brained theory, but you posted looking for ideas and one never knows.

I believe that is OK. When I put everything back together yesterday I made sure everything moved in and out freely and that the pads appeared to be parallel to the discs.
 
Had a similar issue. Air in the lines. Try one of these. I hooked up a line from the top of the reservoir and just let the bleeder go till I had good overflow from each side. Also, to remove excess fluid from the reservoir, I use a turkey injector syringe from the aviation department at Winn Dixie.

As a added benefit, it sure makes bleeding brakes easy.

https://www.aircraftspruce.com/catalog/lgpages/hydraulicBleedertank.php

My A&P friend and I spent a lot of time bleeding the brakes and think that we got all of the air out. The pedals feel firm and we saw no bubbles.

I have never heard that the reservoir could be too full. Can you elaborate on that?
 
Too full is not an issue. Other than when it heats up and overflows and makes a mess.
 
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How about the pedal geometry relative to the cylinder? Does the pedal provide the proper leverage? Brakes are pretty simple. If the pad is good and the disc is good then they aren't being pushed together hard enough. Either you aren't putting enough pressure into the system because the cylinder isn't being pressed hard enough (geometry?, weak feet?) or there are leaks somewhere. Or there is not enough travel to squeeze the pads tight! Can't think of much else.
 
Brakes

Something sounds wrong. If you're pressing hard on the pedals, that hydraulic pressure is being blocked somewhere. I've seen debris get in lines but usually it shows up as pulling to one side. Also seen pistons get stuck. If it's the same both pilot and co-pilot then the problem has to be downstream. Can't be air. Usually air feels like a soft pedal. If they are new pads and not bed properly, that would be a symptom. Same of they were covered in oil. Keep looking.
 
Larry brings up a good point. Any chance there is a parking brake installed that is not fully releasing? Since you are not the builder, if installed, it is between the calipers and each master cylinder as a single unit.
 
Might check.....

......the calliapers(sp?) could be not getting full travel. You say the system feels solid so I would not suspect air in the system. If you can come up with some worn out pads put them in and pump up brakes and clean the pistons with an old tooth brush. Then retract the pistons and extend them a few times before reinstalling the new pads. If the pistons do not move freely in and out dissasemble and clean and replace seals. Corrosion on pistons can cause this too.
 
I had some trouble bleeding my brakes. The pressure it took to inject fluid into the system was very high at first. I worked the brake pedals back and forth several times before it would flow. I rigged an overflow on the reservoir and once it started taking fluid, I just kept on pumping and circulating till all the bubbles were gone. I stopped several times and worked the brakes to dislodge any bubbles and then pumped more. It certainly wasn't as easy as I thought it would be.....Then I found a leak on each master cylinder and learned that fuel lube is not the right thread sealer to use on brake fittings! So, I got to do go thru the bleeding process the second time after reinstalling all my fittings with Teflon Tape.
 
I am not sure what that means. Can you elaborate? The wheel brake assemblies are Cleveland. I don't know what the cylinders are in the pedals, but they are gold.

What is the piston diameter of your foot cylinders?

What is the diameter of your Brake caliper pistons?

If the RV wheel brake pistons are as normal then I suspect your master cylinders(At your feet) could be larger than normal. This would explain why you exert higher than normal force with your feet and get less than expected braking action.

I think it might be worth checking as it sounds like you have done everything else right.
 
Larry brings up a good point. Any chance there is a parking brake installed that is not fully releasing? Since you are not the builder, if installed, it is between the calipers and each master cylinder as a single unit.

Nope, no parking brake.
 
What is the piston diameter of your foot cylinders?

What is the diameter of your Brake caliper pistons?

If the RV wheel brake pistons are as normal then I suspect your master cylinders(At your feet) could be larger than normal. This would explain why you exert higher than normal force with your feet and get less than expected braking action.

I think it might be worth checking as it sounds like you have done everything else right.

I will measure both next week when I am there. Where can I find the proper specs?

Grrrr, that means I have to pull the pants again... :(
 
These brakes are not that complicated. I have no direct experience with these brakes, but here is some theory to stimulate your thinking. You may have already done all of this nut maybe something will help.


Issue: Poor Braking - low performance - poor braking force

Issue . . . . . . . . . . . . . Possible cause - check this
Low friction factor .. . . . . . . . Break -in or pad/disc mis match
Low Pad force . . . . . . . . . . . . . . a) low fluid pressure . . . . . . . - check pressure
. . . . . . . . . . . . . . . . . . . . . . . b) Undersize piston . . . . . . . . . .- check part numbers
. . . . . . . . . . . . . . . . . . . . . . . c)large master plunger . . . . . . . . - check part numbers
. . . . . . . . . . . . . . . . . . . . . . . d)poor leverage or hitting stop . . . . - check plans

If you post a picture of the linkage, and part numbers, so a 6 guy can compare numbers if you did not get the full monty of plans and documentation.

Since the pedal is firm, it sounds like no air, although that alone would not cause this. Be sure that the pedal force is all going to hydraulic pressure, i.e. not hitting bottom, or some stop. Check lines for any magic device that could limit pressure. Like a pressure regulator. Silly but possible.

You never know what people might do to solve something. Like brakes too good, so installed a pressure limiter and then neglected to adjust it. You never know.

Good luck in your investigation, work the basics, and ensure you know all the components installed.
 
How about the pedal geometry relative to the cylinder? Does the pedal provide the proper leverage? Brakes are pretty simple. If the pad is good and the disc is good then they aren't being pushed together hard enough. Either you aren't putting enough pressure into the system because the cylinder isn't being pressed hard enough (geometry?, weak feet?) or there are leaks somewhere. Or there is not enough travel to squeeze the pads tight! Can't think of much else.

I will look at the geometry next week. I know that when we bleed the brakes there is a lot of pedal travel when the valves are opened at the calipers.

I don't think that I have weak feet!! :) I press so hard that I get a few drops of fluid forced out around the piston on one master.
 
Do you brake lines have any sharp bends in them?

Do you have all metal brake lines or combo of hoses and lines?

If hoses, how old are they?

Maybe it is a flow issue and the lines are restricted???
 
Brakes are the simplistic system in your plane. You push a fluid here, it presses a pad against a rotor there. Nothing to it. People get all twitterpated over things that frankly are not that important. In race cars, we swap brake pads in a pit stop, no break in at all and the next turn your life is on the line when you put your foot on the brakes. It works fine and is done all the time.

I changed my brake pads on my -8 three weeks ago. No break in and they held on my run up no problem (constant speed, angle valve, 10:1, Pmags, very strong engine). Sure I had to push a bit harder than normal but they held fine. First landing I hit the brakes hard, they stopped me well.

If you haven't replaced the brake fluid with a proper bleeding technique, stop measuring calipers and sanding discs and bleed your brakes....you have air in your lines. I would drain your system completely and use the system from aircraft tool supply that pushes fluid in from the bottom....works very well. Bet you an Oshkosh beer that it fixes your problem.

With brake issues bleeding is the very first step. Then you start looking for system issues if you are CERTAIN there is no air in the lines.
 
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Do you brake lines have any sharp bends in them?

Do you have all metal brake lines or combo of hoses and lines?

If hoses, how old are they?

Maybe it is a flow issue and the lines are restricted???

If this were the case then they would be slow, likely uneven, to apply and hard to modulate. Also they would drag when released. Hoses?? Ok, that would be the same. Hoses are known to swell and close the interior but it usually does not work at all and then drags, won't release if it works.

Good thought though for others to add to the list of failure modes.
 
. Bet you an Oshkosh beer that it fixes your problem.

You are on - make it TWO. :D

On 4 wheel vehicle with split master cylinder, yes, but not with a single master slave for each wheel.
 
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Any possibility that you have the wrong kind of brake fluid (ie automotive rather than aviation)? Could that cause the o-rings to swell and bind?

A shot in the dark here...
 
I am not sure what that means. Can you elaborate? The wheel brake assemblies are Cleveland. I don't know what the cylinders are in the pedals, but they are gold.

You can email Cleveland with the part numbers of your wheels, your master cylinders and your brakes, and see if they are the correct combination. If it's not, that could be an issue.

Their email address is: [email protected]

Dave
 
Any possibility that you have the wrong kind of brake fluid (ie automotive rather than aviation)? Could that cause the o-rings to swell and bind?

A shot in the dark here...

Nope, not a possibility. I have the aviation fluid for sure....
 
Thanks for all the wonderful replies and suggestions....

Some afternoon next week my A&P and I will:

check passenger side to see if weak brakes there, too
drain the complete system
recheck all hoses and connections
recheck all pedal movements
remove, clean, and replace O-rings on caliper pistons
refill and bleed system

I'll also pull any numbers I can find off the parts and send to Cleveland for evaluation if the above does not fix problem.

Still open to other suggestions, but will certainly report on my findings.

Thanks again, everyone!
 
Old hoses can bulge and you lose your power to hose flex instead of it being transferred to pistons....if flexible hoses are old, replace.
 
I will look at the geometry next week. I know that when we bleed the brakes there is a lot of pedal travel when the valves are opened at the calipers.

I don't think that I have weak feet!! :) I press so hard that I get a few drops of fluid forced out around the piston on one master.

This should never happen, no matter how hard you press.

The pilots brakes should never pass through the passenger side.

Make sure the pistons are installed correctly in the caliper.

Keep the caliper pins clean and free to move so they won't bind.

Rebuilding the master cyl's is cheap and the slaves are even cheaper and easier. Also, do check the age of the hoses from the pedals to the firewall.
 
This should never happen, no matter how hard you press.

The pilots brakes should never pass through the passenger side.

Make sure the pistons are installed correctly in the caliper.

Keep the caliper pins clean and free to move so they won't bind.

Rebuilding the master cyl's is cheap and the slaves are even cheaper and easier. Also, do check the age of the hoses from the pedals to the firewall.

I assume the hoses are the same age as the plane, 15 years. I don't recall seeing anything in the airframe log about changing them.

For some reason it occurs to me that the pilot and passenger brakes do pass through each other. That is, from the reservoir to the pilot right pedal to the passenger right pedal, to the right brake. And the opposite for the left brake.

If that is incorrect, how should they be routed? I can change the routing next week when I pull everything apart again.
 
I agree, there should be no leaks, ever.

Are you sure the pads are conditioned properly? The Cleveland web site says failure to hold at runup power is a symptom of poor conditioning.
Do your brakes drag? The same web site says excessive brake useage during taxi can wear off the glaze layer, so you have to re-condition them.
 
I assume the hoses are the same age as the plane, 15 years. I don't recall seeing anything in the airframe log about changing them.

For some reason it occurs to me that the pilot and passenger brakes do pass through each other. That is, from the reservoir to the pilot right pedal to the passenger right pedal, to the right brake. And the opposite for the left brake.

If that is incorrect, how should they be routed? I can change the routing next week when I pull everything apart again.

If you must have passenger brakes (that will ALMOST never be used) the fluid line runs to passenger side first, then to pilot side. The pilot side now works directly to the slave without pushing through the passenger master. But, even this set-up will require the pilot master to pull brake fluid through the passenger cyl.
 
For some reason it occurs to me that the pilot and passenger brakes do pass through each other. That is, from the reservoir to the pilot right pedal to the passenger right pedal, to the right brake. And the opposite for the left brake.

Earlier plans for the RV-6/6A did show the lines routed this way if the dual brake option was installed.

Later plans changed to the pilot cyl. being the primary and connecting directly to the wheel calipers, and the passenger side pedals piggy-backed off the pilot side.

Either way will work if everything is functioning properly (it is likely that hundreds of RV-6A's are flying with the system plumbed this way) so I highly doubtful that is the cause of your problem, but the current way is more optimal (which is why the RV-9, 7, 10 and 12 are all configured that way).
 
Earlier plans for the RV-6/6A did show the lines routed this way if the dual brake option was installed.

Later plans changed to the pilot cyl. being the primary and connecting directly to the wheel calipers, and the passenger side pedals piggy-backed off the pilot side.

Either way will work if everything is functioning properly (it is likely that hundreds of RV-6A's are flying with the system plumbed this way) so I highly doubtful that is the cause of your problem, but the current way is more optimal (which is why the RV-9, 7, 10 and 12 are all configured that way).

Thanks, Scott.

I will most likely replumb since I will have the system drained anyway. So, how do I connect the passenger pedals to the pilot pedals?

Do I feed the input of both passenger pedals from the reservoir, then from the output of the passenger pedals to the input of the pilot pedals, and then from the output of the pilot pedals to the calipers?

Or is there some other scheme?
 
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