Weak brakes...

[flyinmonque]

......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.

[MikeyDale]

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.

[F1R]

Originally Posted by ccsmith51

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.

[ccsmith51]

Originally Posted by AltonD

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.

[ccsmith51]

Originally Posted by F1R

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...

[BillL]

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.

[ccsmith51]

Originally Posted by sblack

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.

[SmilingJack]

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???

[ColoRv]

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.

[BillL]

Originally Posted by SmilingJack

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.