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Another brake question

But more practically - the stock brakes could easily lock up. Why on earth would you want to apply more pressure than that? If you just apply needed force, even a huge power brake cylinder should not affect the expander tube.

That said, the original diaphragms worked exceedingly well, and with the proper screws and tools I could change a diaphragm in ten minutes, including seat lacing. Save the boosters for disc brakes. Opinion.
 
I pumped up the brakes and not impressed. My first 2000 hours of cub time was in a PA11 and don’t remember the brakes being so crappy. Maybe just spoiled with Clevelands and brake boosters.
 
I just bought a 90hp Super Cub with original 800X4 wheels and brakes. Got a set of North River brake boosters I was gonna give away but thought I might use them to replace the Scott diaphragms. How tough are the expander tubes? Will the booster blow them out asap?
The original expander tubes require a certain volume of fluid to expand enough to apply the brakes. The original master produces that volume. This is done at a maximum pressure of 350 psi.

The booster masters put out a small volume at a much higher pressure. They are intended to be used with brake cylinders which move very little so in turn require a small volume of fluid. Since they are small they require more pressure to exert their clamping force.

The issue arises when you install a large diameter tire on the original brakes. The large diameter has more mechanical advantage working against the brakes. Thus the brakes need to be able to apply more resistance force to the wheel rotation. The original brakes have difficulty doing this. Thus people started using disk wheel brakes with the original masters. While it was an improvement, the original masters couldn't put out the amount of pressure which the disks required to function to their best ability.

The master cylinders need to be matched to the wheel cylinders taking into consideration the tire diameter. Low volume = high pressure. Higher volume = low pressure.

Kase, does your 90 Cub have parking brake valves? If so, You can set the parking brake, add a few drops of fluid and release the parking brake valve. This will give you a harder pedal.

The North Rivers were designed to be an inexpensive solution to the issue. Yes they are susceptible to temperature changes due to them being a closed system. This can be a nuisance. A simple fix is an added reservoir as I've shown in post #6. Years after the North Rivers were on the market other people designed vented systems which solved this issue. Since they were more complicated to manufacture they are more expensive. All of the replacement masters are designed to put out higher pressures with lower volumes to operate on high pressure disk brakes.

The original diaphragm can fail suddenly giving no brake due to it's design. All of the others are less likely to have a sudden total failure since they are pistons with "O" rings.
 
I have a stock 150hp model and only recently noticed it has the parking brake valves. However, the levers from both are missing. Are these available to buy or would anyone have them lying about spare?

Thanks.
 
I would contact Univair as a possible source to purchase since they make new parking brake valves. Maybe they will sell you just the arms.
 
Might ask yourself why the levers are missing. We have a Super Cub with valves safety wired open. We did not do that arbitrarily; there was a reason.
 
Might ask yourself why the levers are missing. We have a Super Cub with valves safety wired open. We did not do that arbitrarily; there was a reason.

There is a safety issue with the park park valves that CubCrafters uses on EX and FX models. They completely isolate the brake master cylinder from the wheel slave cylinder. Start the engine with the parking brakes set and, if there is insufficient trapped pressure, the airplane will start moving. No pressure on the brake pedals will help as they are isolated from the brakes.

The Scott parking brake valves I have flown with on several different airplanes (including to the best of my recollection the PA-18 ) release as soon as pedal pressure exceeds the trapped pressure. No safety issue that I know of. I suppose if you worked hard at it you could set the parking brakes in flight.
 
It has happened way too many times around here - once resulting in minor damage. The Super Cub is in a flying club, and while the pilot group with access is carefully selected, the brakes got partially locked on a seriously regular basis.

I would have removed them completely.

Old timers in San Diego and Grants Pass will remember Don Marks. He was a mechanic’s mechanic, and my mentor. The two of us converted a Stinson to dual brakes in an afternoon! He hated lightplane parking brakes! It was long after he escaped to Oregon that I discovered why.
 
I have Steve's boosters on my Cub, once you figure out how to bleed them it couldn't be easier.
I can go years without having to add any 5606 to the system.
 
I recently installed the Dakota red top master cylinders and linkage, we bled the system as directed and have a pretty solid pedal when brakes are applied. The problem I am having is the amount of travel the brake pedal travels before the brakes reach a firm pedal, we adjusted the Dakota linkage as directed. I flew the North River system for 40 years and got used to having little or no travel in the brake pedals when brakes were applied. Do vented systems like Steve's and Dakotas require extra pedal travel or can they be adjusted. I noticed a set screw on the master cylinder that could be used to move the master cylinder arm forward, or can the linkage be shortened to move the master cylinder arm forward without effecting the braking action? I have the linkage adjusted to allow the master cylinder arm approximately 1/16" inch of play, I even put new pads on to try to shorten the travel!
The workmanship and customer service are great from the Dakota office in South Dakota, no complaints there.
 
I recently installed the Dakota red top master cylinders and linkage, we bled the system as directed and have a pretty solid pedal when brakes are applied. The problem I am having is the amount of travel the brake pedal travels before the brakes reach a firm pedal, we adjusted the Dakota linkage as directed. I flew the North River system for 40 years and got used to having little or no travel in the brake pedals when brakes were applied. Do vented systems like Steve's and Dakotas require extra pedal travel or can they be adjusted. I noticed a set screw on the master cylinder that could be used to move the master cylinder arm forward, or can the linkage be shortened to move the master cylinder arm forward without effecting the braking action? I have the linkage adjusted to allow the master cylinder arm approximately 1/16" inch of play, I even put new pads on to try to shorten the travel!
The workmanship and customer service are great from the Dakota office in South Dakota, no complaints there.

I am not familiar with how Dakota vents their master cylinders. However, if they are anything like mine they would use a small amount of pedal travel to close off the vent before making pressure. My brakes need about 1/8 inch movement at the shaft. Which is about 3/16 inch or so at the pedal end. That travel is the movement needed to close off the static seal to the back of the piston at which point the piston will start to move and create pressure to the calipers.

Brian.
 
I am not familiar with how Dakota vents their master cylinders. However, if they are anything like mine they would use a small amount of pedal travel to close off the vent before making pressure. My brakes need about 1/8 inch movement at the shaft. Which is about 3/16 inch or so at the pedal end. That travel is the movement needed to close off the static seal to the back of the piston at which point the piston will start to move and create pressure to the calipers.

Brian.

Thanks again Brian!


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Make sure theyre well bled, and pump em up from below. I used an oil can with a piece of tygon tube to the bleeder fitting to get a bit of preload.
 
What is the best method to remedy soft brakes with the original Scott brakes on a 1973 Supercub?
If they just got soft over time try adding fluid, put the top back on and pump a few times and repeat. It is a normal thing with the original Scott master cylinders. Make sure the gasket under the plug is a black fiber like substance, I have seen cooper gaskets and all kinds of other things over the years that did not work correctly. Univair has them. You might also determine when the master cylinder diaphragms were changed. They get old and bust. Then look at the parking brake valves for leaks in the end of the stem and the calipers for leaks around the pistons and you should be good.
 
Steve the master is full and I have some brakes but not very good brakes, I bet the diaphragms have not been changed. A couple years ago,they were so tight I let fluid out because I couldn’t push it around by hand. Now I have to stand on the brakes and make sure the tailwheel is straight behind me or the right brake won’t hold during run up. Are the diaphragms the place to start?
 
Also the diaphragm can deform resulting in excess pedal travel. That's why pumping from below can be effective.
 
Got them pumped up from below on the calipers, rock solid now! I was using the wrong wrench. Thanks for the help and the great, knowledgeable fellows on this sight. Many thanks again.
 
On this subject of brakes. I have just ordered a pair of Steves Vented Brake Boosters for my 1976 PA-18.It has standard Cub gear with Clevelands and Goodyear 26" fatties. My question is concerning the high pressure brake lines needed for the install. Custom made by my A & P in shop or does someone make these lines for sale?
 
I custom make them when installing in my shop. Personally I like stainless steel braided PTFE hose.

Unfortunately I would have to be a certified hose shop in order to make and sell them nationally. Aircraft Spruce has a pretty good custom hose build system through their website.

Brian
 
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