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New brakes, very inefficient...

High_Flyer

High_Flyer

Well Known Member
Hi,
I have just done the first taxi trials after completion of my RV-7.
Everything is great, except that I find the brakes to be very innefficient.
There is no air in the lines, I've bled the lines very carefully and the pedals are quite hard to press, the course at the top of the pedals is about 1/2 inch or or slightly more, both feel the same.
Also I have installed a Parking brake valve.
Trying to stop from a speed of less than 10 Knots takes probably 30 to 40 feet when braking as hard as I can.
With brakes applied I the plane doesn't move until I run the engine past 2000 RPM (195HP with constant speed prop), so I think that's good.
I'm a bit confused. The brakes are holding well for engine run up but can't stop the plane quickly from taxi speed...
Any idea why?
Thanks,
Alain.
 
Have you conditioned or "broken in" the brake pads according to manufactures instructions yet?
 
The supplied Clevelands are the absolute minimum size for the expected weight and speed. What you described sounds normal. They will bed in and improve.

I've quit doing the conditioning procedure, as I get the same result after a few firm applications during rollout.
 
Although it probably is the normal break-in you are experiencing. it might be worth while to check the clocking of the lever on your parking break valve. I had a similar problem when my plane was new. I thought my weak brakes were do to being new too, however it never improved. I tracked the problem down to the parking break valve. I had the wrong arm on it and it was only partially opening. it was 45 deg off. it would fully close, but just barely open. After I corrected this, my brakes became very effective.
 
Even after following the "conditioning" recommendations, I find that after about 10 landings with firm brake pressure, they get better.

Chris
 
Maybe...

Assuming you actually DO have all the air out of the system:

If you used hose all the way down the legs, the brakes will not function correctly. I've seen this problem on more than a few Rockets, and a change to standard alum line with short hoses on each end (mimic what Cessna did with the C-150) fixes the problem. I would not have thought that the braided line would expand enough to feel the difference, but that seems to be the case.

If your install mimics the C-150, then make a few more landings - use the brakes to stop - and see if that helps.

Conditioning process: not a good idea - especially with the wheel pants installed. One brake fire is more than enough to prove this process is not the best idea out there...

YMMV
Mark
 
Hose

F1Boss said:
Assuming you actually DO have all the air out of the system:

If you used hose all the way down the legs, the brakes will not function correctly. I've seen this problem on more than a few Rockets, and a change to standard alum line with short hoses on each end (mimic what Cessna did with the C-150) fixes the problem. I would not have thought that the braided line would expand enough to feel the difference, but that seems to be the case.

If your install mimics the C-150, then make a few more landings - use the brakes to stop - and see if that helps.

Conditioning process: not a good idea - especially with the wheel pants installed. One brake fire is more than enough to prove this process is not the best idea out there...

YMMV
Mark
Click to expand...

Disagree about the hose. Hose from TS working perfectly on non RV taildragger with slightly higher landing speed then RV. I have never used the conditioning process and agree that it is unnecessary and perhaps even dangerous.
 
Not all hose is created equal

jrs14855 said:
Disagree about the hose. Hose from TS working perfectly on non RV taildragger with slightly higher landing speed then RV. I have never used the conditioning process and agree that it is unnecessary and perhaps even dangerous.
Click to expand...

Is the TS hose a -3 size? Or maybe made better quality than the -4 Aeroquip? If it's working for you, don't fix it!

Best,
Mark
 
Conditioning

I'm just about to start my taxi trials and have brake conditioning in the schedule, is it really not necessary or a good idea?
 
Are you guys cleaning the silver paint off from the new rotors? A little lacquer thinner takes it right off. Otherwise, it is a gummy goo until it cooks off.
 
Thanks a lot for the replies...

9GT said:
Have you conditioned or "broken in" the brake pads according to manufactures instructions yet?
Click to expand...

The supplied Clevelands are the absolute minimum size for the expected weight and speed. What you described sounds normal. They will bed in and improve.
I've quit doing the conditioning procedure, as I get the same result after a few firm applications during rollout.
Click to expand...

No, I have not.
I was expecting some ineficiency due to the fact that the brakes are new and also I have seen on this forum that they are a bit "weak", but what I'm experiencing is worse than what I expected...

Although it probably is the normal break-in you are experiencing. it might be worth while to check the clocking of the lever on your parking break valve.
Click to expand...
I thought about that too! When I assembled the parking brake valve I was very careful to make sure the valve would return to the fully open position when released. The parking brake valve works as expected when "ON", and the planes moves immediately when released, which makes me think that this is not the issue.
I'll double check again ,for that I'l have to crawl under the front deck to verify that...

If you used hose all the way down the legs, the brakes will not function correctly.
Click to expand...
The original setup is using cheap plastic hoses from the brake fluid reservoir to the the brakes master cylinders, and some braided flexible lines from the left master cylindres to the firewall. I've changed the original plastic hoses with braided hoses from Bonaco.
I have also used hoses from Bonaco going from the firewall down the gear legs to the brakes in place of the aluminum tubing.
I never thought it could be an issue as there are dozens of RV flying like that and never saw a post about this...

Anyone else have an opinion about this??

Thanks a lot for the help...
Alain.
 
brake fluid

What kind of brake fluid are you using? When pressing on the brakes, can you see the hose flexing or tightening or moving?
 
try getting your toes up on top of the brake pedal. this will get you more leverage to see what your braking potential really is.
IMG_1361.png
 
I would suggest

A)Bleeding the brakes again, even if you're sure there is no air. I have had a similar problem in the past and even though I was sure there was no air, I re-bled them at they worked much better

B)Give it some time. Assuming you have enough runway to not need heavy braking, re-evaluate after 10-15 landings. As others have suggested, they will bed in and get better.

Chris
 
Maybe pads are glazed

Glazed brake pads will likely require replacement (you can sand the brake pads, but in my opinion it is better to replace). I had the same issue as the original poster (poor braking). I replaced the pads and cleaned/scuffed disks with maroon scotchbrite. Then I proceeded with a break-in to transfer some brake pad material to the disk to create adherent friction (see details in link below).

I found that the recommended RPM for break-in creates too much heat (I actually damaged disks and o-ring at one point). I modified the Semi-Metallic Break-in Procedure to 1500 ft. / 1400 RPM. Let cool for 15 minutes. That seems to work for me (79" fixed pitch 0-320).

http://www.aeroinstock.com/wp-conte...bsite_APSBrakes_Break-InInformation_Final.pdf

Here are some excerpts from the above link:

Nj3VwsN.jpg


iiMMrFe.jpg
 
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brakes

High_Flyer said:
I can't see the brake line while braking... I'd have to check that with someone pressing the brake pedals while I watch.
Why are you asking that??
Click to expand...
If the brake hoses (on the high pressure side) are moving a lot when you press on them, then some of the pressure you want to go to the discs is being "wasted" moving the hose. Based on what I've read here, I doubt that is your problem.
 
rv8ch said:
If the brake hoses (on the high pressure side) are moving a lot when you press on them, then some of the pressure you want to go to the discs is being "wasted" moving the hose. Based on what I've read here, I doubt that is your problem.
Click to expand...

The brakes are essentially a hydrostatic system. No pressure is ever wasted. If you have hoses expanding/moving, that can change the volume, which will drop the pressure everywhere - including what you feel with your toes. But if you maintain, say, 20 lbs of force on the pedals, then you have to get the same brake force, regardless of whether or not the lines are solid or can flex. Unless, of course, you actually run out of travel on the master cylinder.
 
Here's my experience after replacing the linings for the first time:

1) Did the break-in procedure...about four hard successive applications, from about 35 mph to 5 mph. Keep it rolling; no stopping in between.

2) Let brakes cool for 30 minutes. Brakes had somewhat more bite at this point.

3) Went for a 20-minute flight and landed. Pedal pressure was a little higher than normal, but I could easily stop in the normal distance.

4) Next landing with two aboard...still needed higher pedal pressure; could smell the brakes while taxiing after landing.

5) Next landing, everything felt normal again.
 
Tanks a lot

Thanks a lot for all your replies.
What I can get out of all the replies is that my brakesmost likely need "breaking in". I'll do that as soon as I get back home (I'm away for work for a week) and will report further once it's done.
Alain
 
Be sure to re-read the post about "glazing". If that's the case, the best solution is to replace the linings.
 
PCHunt said:
Be sure to re-read the post about "glazing". If that's the case, the best solution is to replace the linings.
Click to expand...

Thanks, the brakes are new, only used during the two initial taxi tests a few days ago and at speed of around 10Kts, so I'm pretty sure they are not "glazed", my braking attempts didn't generate enough heat for that (the discs were cold enough to keep my fingers on them after shutdown and post taxi checks)
 
Brakes Ineffective

It does sound to me that the Parking brake is partly closed.
I found this a bit confusing when I installed mine. It needs checking and if the cause , adjust the position of arm to the brake cable.
 
Based on what you said I would also doubt the brakes are glazed, that would require a lot of heat. I would also say that the beding proceedure (breaking them in) should be done. There are compounds in brake lining material that require some heat which is why you should bed in new brake linings. Since you have SS lines through your system you may always experience a slightly firmer pedal than if you had the plastic lines. This is because the plastic lines flex more when pressure in applied (steping on the brakes) and give a little through their length. This is one of the reasons why in a race car the rubber brake lines are replaced with braided SS lines. It doesn't sound like you have air in the lines since you have firm pedals. If there was air in them the pedal would feel mushy. Before doing any more taxiing and the bedding proceedure clean the rotors with some mineral spirits, gas, or acetone to remove any oils. There may be a coating on them from the factory to keep them from corroding.
 
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Not "may be". These is a coating. As Alex noted, plenty of folks neglect to remove the protective paint before first use.

AlexPeterson said:
Are you guys cleaning the silver paint off from the new rotors? A little lacquer thinner takes it right off. Otherwise, it is a gummy goo until it cooks off.
Click to expand...
 
DanH said:
I hear ya'. As noted, some of us have quit using the bedding process.
Click to expand...
I notes that, but I would recommend following the manufacturers instructions unless you have your experience and knowledge. We have a lot of newbies using the forums. Rarely does following the instructions provided by the factory lead to trouble.
 
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