Do Brake Pistons Wear?

[Vlad]

I am replacing a lot of brake components at 3,000 hours. New discs, hoses, o-rings etc. I am wondering do caliper pistons wear?

[gasman]

The piston should outlast the bore. The bore should be fine if you can keep it from pitting.

[az_gila]

I wondered the same thing working on the Cleveland brakes on my Tiger.

Very little help in the Maintenance Manual - The main snippets -

Inspect pistons for nicks or burrs. Check the piston tail for damage, and replace piston if damaged beyond repair. Remove nicks or burrs by hand polishing with 600 grit emery paper. Thoroughly clean part.

Installation of Pistons
Installation of pistons with an internal guide is the same as for other pistons. Care must be taken to insure that no damage to parts occurs during installation. If considerable effort is required, remove piston and inspect bore and pilot bore area for damage. If bore is damaged, check the corresponding area of piston for damage.
(a) Place piston in bore and rotate to seat friction spring (if applicable), and insure that piston and o-ring are in proper alignment. Press piston into bore by hand. Tap the piston squarely with a wooden or plastic mallet (if required) while rotating piston.

Lots of wear limit dimensions for the brake disks, but no numbers for piston or bore wear.

[Vlad]

Originally Posted by gasman

The piston should outlast the bore. The bore should be fine if you can keep it from pitting.

Warren what would be the defense against pitting? Other then flying nearly every day.

[DanH]

I recalled an excellent discussion which approached the matter from a different angle. The OP thought he had a piston-to-bore clearance problem, but it turned out his mechanic installed the pucks bass ackwards:

http://www.vansairforce.com/communit...d.php?t=104995

(1) Cleveland wouldn't supply official dimensions when asked.
(2) There is nothing in the Cleveland manuals beyond "remove burrs and damage with 600 grit".
(3) Reported clearances in the field were from 0.002" to 0.010" (the wide range may be a matter of measurement accuracy.)
(4) If we assume not more than 100 lbs of leg pressure, a Shore 75 viton or nitrile o-ring should be safe at 0.010" clearance.

The piston really sort of floats in the bore, and should not wear much at all. If the bore is cylindrical, damage free, and diametrical clearance is 0.010"or less, I'd would think a good cleaning and new o-rings are all you need.

[Vlad]

Nice find thanks Dan. I guess we don't pay attention till it happens to us. Everything is here in VAF archives.

[rightrudder]

Vlad, how often are you replacing brake pads and rotors? I'm curious to see how much my brakes have worn when I take them apart at the first annual inspection (Jan. 2017).

[gasman]

Originally Posted by Vlad

Warren what would be the defense against pitting? Other then flying nearly every day.

Wear your brake pads out as fast as you can.......

If you don't use your brakes, the pistons will stay deep in the cylinder and keep the cyl walls exposed to the environment.

There is really nothing you can do.

[ty1295]

From my automotive background, I believe some pitting also can be tied to moisture in the system. Just like a car, sitting is probably the worst issue our planes have. Probably just like all of us also as we age, ever notice in general those that live long and happy lives tend to not sit around the house?

[DanH]

Originally Posted by ty1295

From my automotive background, I believe some pitting also can be tied to moisture in the system.

That's certainly possible for those who are using automotive brake fluid, which is hygroscopic, i.e. attracts water from the air.

MIL-5606 and 83282 fluids are not hygroscopic, which is why the MIL spec says it may be stored in closed or open containers, and why we don't need sealed caps on our brake reservoirs.

To Vlad's question (pitting), I'd venture the real risk is atmospheric and water-borne corrosives finding their way into the annular space outboard of the o-ring. An example might be salt water, given Vlad's irresistible attraction to beach landings