Around the Pattern

Ramblings about flying for fun and profit.

Globe Swift Brake Master Cylinder Rebuild

When I finished the rebuilding of the Swift landing gear struts, actuators and downlocks I thought I was finished with the hydraulic system for a while. Not so.

While I was in the wheelwells cleaning things up for the re-installation of my newly rebuild parts, I took out the aluminum hydraulic lines and polished them up a little just for asthetics. Little details like clean aluminum lines just give the plane a better look overall. Anyway, the lines I removed included brake lines running through the wheelwell. That required ‘bleeding the brakes’ in order to remove air in the lines between the brake master cylinders and the brake pucks on the wheels. Air is compressible, while hydraulic fluid is not and any air in the lines in place of fluid will cause a ‘spongy’ feeling when the brakes are applied. Enough air and the pedal will just ‘go to the floor’ and no braking action will be available.

My wife was nice enough to come out to the hangar and do the inside work of depressing and holding the brake pedal while I opened the bleed valve at the brake. This forces fluid and air down the line and out the valve. While the pedal is kept depressed, I would then shut the bleed valve and when the pedal is released the action of the master cylinder then draws more hydraulic fluid into the cylinder from the reservoir. Depressing the pedal then forces that fluid down the lines while the valve is reopened and the cycle is repeated. Eventually all the air will be forced out of the lines, signified by fluid without bubbles being expelled at the bleed valve and a firm feel to the brake pedal when you press it with the bleed valve closed. As long as you remember to periodically refill the hydraulic reservoir so you don’t drain it and then draw more air into the system, the process proceeds fairly quickly. And that’s how it worked with the right brake. Unfortunately, when we tried to bleed the left brake we had no fluid movement at all and the brake pedal had almost no resistance to it’s movement. That meant that the seals in the master cylinder were not doing their sealing job and fluid was just moving back and forth in the master cylinder without being forced down the lines when the pedal was depressed.

So, I pulled out my trusty seal kit SK-151 from the Swift Parts Store trying not to pull a muscle patting myself on the back for having one on hand, and started researching the process. Once again, I’ll throw up the disclaimer, just in case.

DISCLAIMER

I am writing these maintenance articles to make it a little easier for individuals to rebuild the Swift hydraulic components. This is NOT intended to be comprehensive or step-by-step advice about how the rebuild of these components should be completed. In order to accomplish this work you must hold either an A&P or AI certificate or you must be supervised by an individual who does and you must have in your possession the manuals and tools necessary to accomplish the task. I do not discuss all of the steps shown in the hydraulic manual. This is not considered to be preventative maintenance.

I looked in several books I have on hand to try and find a parts breakout or cut-away drawing of the brake cylinder, but was only able to find a generic diagram in an A&P airframe maintenance text. As a result, I decided to write this post to provide a little information about the project. At the same time that I did this work, I also complied with Special Airworthiness Information Bulletin CE-08-34 (pdf file) that deals with an inspection of the brake pedal link rod that connects the brake pedal to the brake cylinder.

Removal of the brake cylinders (one for each brake, left and right) is fairly easy, though in a cramped area under the instrument panel and above the brake pedals. The aft end of the cylinder (toward the instrument panel) is connected to a lever arm with a clevis pin and cotter pin. The forward end of the cylinder extends through the firewall and has a hydraulic (AN) fitting on the engine side of the firewall that connects to the aluminum hydraulic lines running down the firewall and to the brakes. Another AN fitting is located on the top of the brake cylinders that connects to a small manifold and then runs forward through the firewall and to the bottom of the brake fluid reservoir. The bottom of the brake cylinders has two ‘legs’ that each have a bolt, firmly attaching the cylinder to the airframe structure under the instrument panel. So, the process involves removing a clevis pin, two bolts and two hydraulic fitting to remove each brake cylinder from the plane.

Have plenty of rags and an easily-held container to catch hydraulic fluid as you disconnect the lines because the hydraulic reservoir will immediately drain down your arm without preplanning that first fitting removal. Once out of the aircraft the cylinders look like this:Globe Swift Brake Master Cylinder

The plunger actuator arm is pulled forward from the cylinder and the rubber boot disconnected to give a little better view. It is a good idea to take a photo or make a diagram of the orientation of the fittings for ease in the reassembly process. You will be removing both AN fittings and loosening the three small screws that hold the gray housing on the right end of the cylinder. The gray housing has a small arm that activates a parking brake, however that function had been disabled in this plane. I may get around to fixing that at some time in the future. Once these parts are removed, you will see a plunger inside the cylinder. The easiest way to remove it is to provide just a little air pressure to the upper fluid fitting. This will force the plunger out of the cylinder.

You will find two small rubber o-rings, a larger o-ring and a rubber cup in the rebuild kit for each cylinder. Swift Brake Master Cylinder Parts

The larger O-ring is located on the plunger that works against a spring when the brake pedal is compressed. The plunger is held in the cylinder with a metal clip, just visible between the parking brake housing and the rubber boot. Be very careful when rmoving that clip as the spring will easily shoot the clip half way across the hangar if you are not careful. I put the whole assembly vertically in a drill press and used pressure on the actuator arm to depress the plunger while I removed the clip. It’s then just a matter of cleaning up all the parts, lubricating and installing the new seals and reassembling the parts. Then you can reinstall the master cylinders in their proper locations, reconnect the hydraulic lines, service the fluid reservoir and restart the brake bleeding process (with far more air to expel this time).

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1 Comment

  1. Rick

    Great article.. Very clear and consise.

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