Brake Master and Slave Cylinders

Introduction

Properly maintaining the brake system on Grumman American aircraft is extremely important because the brakes are the only way to steer the plane on the ground at low speeds where the rudder is ineffective.

The hydraulic brakes use a Mil Spec fluid MIL-H-5606G, commonly referred to as "5606". This is a red petroleum-based fluid, as opposed to the Glycol based automotive fluids, and the system is not compatible with any other fluid.

If hydraulic fluid leaks have been found at the brake caliper on either main wheel, the likely causes are worn o-ring, pitted wheel cylinder or cracked wheel cylinder casting at the inlet or bleeder fitting.

The Cleveland Wheel and Brake Maintenance Manual is available on-line. The FARs require the use of the manufacturer's current maintenance instructions, and the Grumman maintenance manual gives very little detail about the wheel and brake system.

The Wheel Slave Cylinder

Mechanical Difficulty:

To remove the caliper the 1/4" bolts that hold the outer brake pad retaining plate in place must be removed. The inlet line should then be removed by untorquing the AN 818-4D line nut. The line nut should require no more than 1/8 turn with a wrench before being free enough to spin off with your fingers. If the threads require a wrench to loosen them further, careful inspection of the line nut and inlet fitting threads is in order.

As soon as the line nut is loosened, 5606 fluid will begin leaking out. A small catch pan is a good idea to prevent a spill and an unsafe condition with a slippery floor. A well prepared mechanic will have an AN 806-4D plug ready to seal the line and prevent losing all the fluid in that side of the brake system!

With the brake line removed the caliper can be pulled inboard and removed from the mounting bracket. Exploded views of the caliper can be found in the Cleveland Wheel and Brake Parts Catalog for easy reference to part numbers and assembly details.

With the caliper off the mounting bracket, slide the inner brake pad and backing plate off the mounting pins and remove the piston. A common method is to wrap the caliper assembly in a rag and apply air pressure to the inlet fitting to drive the piston out of the bore. There is a small amount of fluid that remains in the caliper and eye protection is suggested to prevent a chemical irritation should the remaining fluid spray in the direction of your face.

After the piston is removed, a thorough cleaning of the caliper housing and piston is next. A solvent rinse (mineral spirits or an automotive style "brake cleaner") to degrease, followed by soap and water with a mild abrasive like Scotch-Brite to remove stubborn dirt and oxidation, completed by a fresh water rinse and air drying should be sufficient to allow a good inspection.

Evidence of pitting in the caliper bore is quite common on older components, especially those that have been operated in a corrosive environment. If the pitting is very light, and only at the outer edges of the bore, it may not cause a leak under normal circumstances. The o-ring that seals the piston to the caliper bore is set back from the edge of the piston, so the actual seal takes place deeper in the bore. However as the brake pads and the rotor wear, the piston can move further out and reach a point where the o-ring may leak.

The minimum thickness of the rotor and the minimum thickness of the brake pads are designed such that the piston will not travel too far out of the bore when the minimum thicknesses are reached, and the o-ring will still seal if the bore is in good shape. Allowing the brakes to deteriorate past the point of the recommended minimum thickness can cause the piston to move far enough that the o-ring pops out of the caliper bore, and all brake effectiveness is lost without warning.

After cleaning and inspecting the caliper and piston, I suggest an acid etch and alodine, just like the pre-treatment of structural aluminum in preparation for painting. A coat of zinc chromate or an epoxy primer on the external surfaces, followed by a color coat to protect against future corrosion would be a good idea. The caliper has a data plate on it that identifies the manufacturer (Cleveland), the model and other info like the date of production (month and year) in most cases. If the data plate is missing or unreadable, then the part is technically unairworthy. The good news is, that if you contact Cleveland support at the number in the on line manuals, and you ask nicely for replacement data plates, they will send them to you AT NO CHARGE. The current production data plates are laser printed on a plastic based label, and just peel off and attach to a clean, smooth surface.

After all the surface prep, primer and paint, the caliper is ready to assemble. Install a new O-ring. Its important to use the correct part number that is made of the right material, and not just some "hardware store" o-ring. The early planes use a 1.5" piston and the later models use a 2" piston. The Mil-Spec is an MS28775 in either -222 or -224. It is important that the o-ring not be twisted in its groove. Lube it and the bore of the caliper with a generous amount of 5606 fluid, and insert the piston with a rotating motion. Once lined up correctly, it will slide into place with a quick motion and be ready to wipe off the excess fluid and install the assembly on the plane.

Refer to the section on bleeding the brakes for instructions on how to purge the air from the system.

The Master Cylinders

Mechanical Difficulty:

On most Grummans there are 4 Master Cylinders, a pair for each pilot. They are plumbed such that the pilot's side master cylinder output in hooked to the co-pilot's side master cylinder inlet, and the co-pilot's side master cylinder output is connected to the brake caliper by flex and hard lines.

This means that low fluid in the pilot's side master cylinder can cause a loss of brake effectiveness and a soft pedal, and the co-pilot's side brakes can continue to work.

Removing the brake master cylinders requires disconnecting the flex line from the output, and on the co-pilot's side, also disconnecting the flex line to the input. The master cylinder must be disconnected from its bracket on the floor, and from the brake pedal. The installation hardware can vary from year model to year model, with some having Clevis Pins, some Clevis Bolts, and some AN3 bolts. In all cases the greatest difficulty is in reaching the hardware. The master cylinders are on the firewall side of the rudder pedals, so its awkward to reach the hardware.

Some AN806-4D flare plugs are handy to seal the flex lines as they are disconnected. Unless the fluid has all been drained from the brake system, several ounces of 5606 fluid will leak out when the lines are disconnected.

Once the line(s) and attach hardware are disconnected, the passenger side master cylinders can be removed. The pilot's side, on the early models may also have a mechanical parking brake attached. Some have a ring like a keyring that can be removed from the actuator arm, and some have had safety wire substituted for the link between the chain and the actuator arm. Disconnect the parking brake chains, and remove the pilot's side master cylinders.

The early master cylinders had notches in the rod that engage the parking brake actuator. These notches weaken the rod, and often lead to bends that cause the brake to not actuate fully or possibly to stick. There are no new replacement parts for the Gerdes cylinders, and the answer is to replace them with Cleveland master cylinders, or make replacements as "owner produced parts" as allowed by the FARs. A suggestion for material to use in making a new rod for the master cylinder is a long AN bolt of the correct diameter.

To disassembly the master cylinders, there is a clevis with a jam nut at the top of the rod, and an internal snap-ring that holds the top on the cylinder. It it not always necessary to remove the clevis, and if it is going to be removed, it is important to measure the distance from the clevis to the master cylinder, or to count the turns on the threads as it is being removed, so that it can be assembled to the same length when re-installing.

After the snap-ring is out, the rod may be pulled out, bringing with it the "guts" of the master cylinder.

Be prepared for some fluid to leak, even when the cylinder has been drained. Eye protection is encouraged, as anyone that has gotten a drop of 5606 in their eye will tell you it is a strong irritant.

After the rod and piston assembly are removed from the bore of the cylinder, clean the cylinder housing and bore with a solvent suitable for removing petroleum based residue. Some mineral spirits, "brake clean", etc. works nicely.

With the bore clean, look for vertical scratches or scoring. Such marks can allow the fluid to by-pass the piston and o-rings, causing a loss of brake pressure. The scoring may also damage the o-ring, such that a new o-ring will only temporarily solve the problem, until it too is damaged.