Brake question re: heel brakes that don’t “return”

Hey all, I’ve found some stuff on here about this, but we haven’t fixed the issue. My -12 is the only cub type plane I’ve flown, and the brake setup is the only type of this sort that my a&p has worked on. We’re looking for some input.

our setup: Ak bushwheels double puck calipers. Scott master cylinders with northlands brake boosters.

We’ve noticed that when the brakes are bled well, there is very very little travel in the brake pedals between no brakes, and applied brakes. We also have had squeaky/lightly dragging brakes after replacing disks and pads. We thought maybe the spring in the booster was worn, and therefor not returning the pedal to “zero” brakes. So we bought the overhaul kit from Ak bushwheels and replaced the plunger, piston, o rings, spring, etc. we noticed before re-installing that the piston in the cylinder once pushed down, does not readily return back to “zero” from the spring force pushing on it. We lubed the cylinder walls with hydro fluid, and still the piston tends to just stay compressed at the bottom of the cylinder once pressed down.
Anyways, re-installed and bled and filled, and the problem is still there. If the brakes are bled well and full of fluid, there is extremely little travel in the brake pedal, and it doesn’t seem to want to return the pedal to “zero” and release the brakes.

any input on how this system SHOULD work, and tips on how to fix this problem will be greatly appreciated.

Your question has a couple of parts to work through. The first part where you say there is very little travel when "well bled" makes me think maybe you have bled the brakes too well which would result in having very little gap to close between the brake pads and disc which would cause dragging if anything changed such as temperature causing the brake fluid to expand slightly. You should ideally have a little brake travel to close the gap then some additional travel in the pedal to modulate brake force. Sealed brakes like the North River system or stock Scott diaphragms don't self-regulate very well which is why regular attention to brake fluid levels is needed. Too much fluid forced into the system will cause them to drag, not enough and you get air in the system. Delicate balance.

The second part is the brake piston dragging. Since the system relies on the piston returning fully to draw the brake caliper pistons back, you really need free movement of the brake pistons. If there's resistance to the pistons sliding in the master cylinder bores that needs to be addressed first which is what it looks like you've done. Were you able to identify if it was just the seals dragging or is there some galling in the cylinder or piston that might be the culprit? It may be that the o-rings are new and very tight in the bores which can cause issues. Again it seems like you've done your due diligence on that end with lubing everything but the piston dragging seems to be the main problem.

I had this issue earlier this winter before going on skis and it was related to the cold temps just causing things to get sticky and the pedal wouldn't return. Once it warmed up to above 15 degrees my plane had no more issues although I went straight to skis at that point. Not saying that's your issue but its something to consider if you're parking the plane outside.

Properly bled brakes filled to the top will have almost no travel in the pedal. It is a closed system so once full the fluid has no where to go as the rotor turns and tries to push the pucks back into the caliper/master (they all have a bit of runout). You will always get a bit of squeak until the pads are moved back enough to not make contact. Make sure you caliper glide pins are clean and move freely, also the pucks in the caliper move freely. Airframes has the O rings for them and they are easy to pop out/clean/replace. I top off my masters every 25 hours or so and have a bit of caliper drag first few landings not a big issue. Not uncommon problem to have cold fluid warm up and lock brakes if you are adding fluid in cold hanger and go out on a very hot day. Sounds like normal operation if the caliper is working smoothly.
DENNY

Great minds think alike.:smile:
DENNY

Kodiakmack said:

Hey all, I’ve found some stuff on here about this, but we haven’t fixed the issue. My -12 is the only cub type plane I’ve flown, and the brake setup is the only type of this sort that my a&p has worked on. We’re looking for some input.

our setup: Ak bushwheels double puck calipers. Scott master cylinders with northlands brake boosters.

We’ve noticed that when the brakes are bled well, there is very very little travel in the brake pedals between no brakes, and applied brakes. We also have had squeaky/lightly dragging brakes after replacing disks and pads. We thought maybe the spring in the booster was worn, and therefor not returning the pedal to “zero” brakes. So we bought the overhaul kit from Ak bushwheels and replaced the plunger, piston, o rings, spring, etc. we noticed before re-installing that the piston in the cylinder once pushed down, does not readily return back to “zero” from the spring force pushing on it. We lubed the cylinder walls with hydro fluid, and still the piston tends to just stay compressed at the bottom of the cylinder once pressed down.
Anyways, re-installed and bled and filled, and the problem is still there. If the brakes are bled well and full of fluid, there is extremely little travel in the brake pedal, and it doesn’t seem to want to return the pedal to “zero” and release the brakes.

any input on how this system SHOULD work, and tips on how to fix this problem will be greatly appreciated.

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Being a STC holder of brake cylinders I have run into similar situations replacing the older booster type cylinders.. Those cylinders are machined and left bare aluminum.. I would bet that the pistons are scored from use and have ridges built up causing them to stick.. I hard anodize my cylinders to keep that from happening..

Brian.

What about the little U-shaped brackets that hold the front pedals to the floorboards? They are supposed to allow the pedals to rotate freely. They often require shims.

The pistons do not return 100% of the distance. If your brakes are dragging you have too much fluid in the system. Take out a couple of drops. Fill the cylinder only to the bottom of the brass adapter to leave a small air bubble for expansion purposes. It takes very little pedal travel to apply and release the brake.

Also be certain your brakes are free to move on their pins.

Thanks everybody. We will keep tinkering. Is there any recommendation to hone the cylinder, similar to honing the caliper cylinders? As for the pins at the caliper they are all new within a few hours and are lubed with the appropriate lube and move freely. I plan to do what we can to clean/lube the forward pedals as mentioned without having to remove those suckers. As is, the brakes work and are acceptable, we’re just trying to make sure we’re not up in the night with our understanding of this system.

Kodiakmack said:

Is there any recommendation to hone the cylinder, similar to honing the caliper cylinders?

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Did you find corrosion in them? If NO leave them alone. They sound as though you have them correct. Just a little too much fluid. They are not supposed to move very much. Since the brake cylinders don't move much, very little fluid moves.

I think he's talking about the master cylinders not the brake calipers.

Just check the brake booster cylinders for scoring and if some is found then try and smooth it out with a light honing should be fine.

Crash said:

I think he's talking about the master cylinders not the brake calipers.

Just check the brake booster cylinders for scoring and if some is found then try and smooth it out with a light honing should be fine.

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That's what I'm talking about too. The caliper pistons don't move much, so they require very little fluid from the master to apply the brakes.

The original Scott cast pedals can wear out and bind against the piston rod end. Not a problem with the Scott plunger, but those stainless balls can eat into a casting with impurities. Polish the cavity and put some Lubriplate in there.

It sounds to me like you have done a good job of checking stuff and you brake function is normal. I deal with this issue twice a year, when I go to skis I run a smaller tire with different wheel and rotor. This rotor is newer and a bit thicker, so I have to remove the caps and let some fluid out as I tighten down the rotor's. When I swap back I need to add fluid. With a disk brake the pucks are not pulled back into the caliper they are pushed back by the rotor so as you spin the wheel you should hear/feel a bit of drag until the wheel has had enough rotations that any wheel bearing slack comes into play allowing the rotor to move the pucks back even more. Skywagon's advice to add only to the bottom of the brass adapter will give you more pedal play if you like that. DENNY

Simple solution: Buy a set of Steve's Brake Boosters.

Problems (all of them) solved.

MTV

Buy Steves brake boosters and never worry about constantly topping them off again!

I got your private message Mike. Sounds like Steve’s boosters are the way to go. Any headaches with fit in the -12 with a stock seat?

Kodiakmack said:

I got your private message Mike. Sounds like Steve’s boosters are the way to go. Any headaches with fit in the -12 with a stock seat?

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Don't throw in the towel too soon. You are not far from having good brakes.

I got sick of fiddling with my brakes all summer and bought Steve’s brake kit. Whether it was pedals being uneven or needing to top them off, it was way more often than I want. Can’t wait to upgrade my Scotts and have even pedals and minimal servicing moving forward.

It’s been a spendy year already, so we’re going to spruce these up the best we can and ride them for another year. Next year we’ll throw Steve’s boosters in. If this was a matter of the brakes totally not working that would be one thing, but the budget doesn’t support a $850 nuisance stopping purchase. I’ll report back after we clean everything up and fly after annual.

Pay attention to Skywagon8a. North Rivers are his invention. I loved mine. Set them up right and they’ll serve you well. Spend your mod money on something you’ll
Like more.

stewartb said:

Pay attention to Skywagon8a. North Rivers are his invention. I loved mine. Set them up right and they’ll serve you well. Spend your mod money on something you’ll
Like more.

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Yes, this. I've flown both Northrivers and Steve's boosters and both are very effective.

My Northrivers work great! One thing that helps is I keep my brake fluid (same stuff I use in the ski pump) in a old school squirt can with small flex nozzle. Every 25 hours or so just pop open the cap and top it off (pull the pedal back before you do) just takes 5 min. Gabe at airframes has that small hard washer for under the cap. Save your money for AOSS now that is worth getting MOREBETTERFEVER!! DENNY

bob turner said:

The original Scott cast pedals can wear out and bind against the piston rod end. Not a problem with the Scott plunger, but those stainless balls can eat into a casting with impurities. Polish the cavity and put some Lubriplate in there.

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What Bob says. That wear is not obvious to the casual looker. Also place a dab of grease between the plunger and the piston. The plunger wobbles slightly on the piston. Only one O ring is needed for proper operation. The second dry O ring is just used to help keep the dirt away from the operating O ring. You could spray some light oil or WD-40 in the outboard end of the cylinder to reduce any light friction.