Upgrading brake o-rings

22 Jul 2006 Upgrading brake o-rings

There have been many brake fires on RVs and other aircraft. These are usually caused by the type of brake fluid used in many aircraft. The most common fluid is MIL 5056, which has a flash point of around 250°F. This is a very low flash point, and if any fluid leaks on the brakes during braking action, it will catch on fire.

There are other hydraulic brake fluids available with a higher flash point, but they will all burn at some temperature. This is a problem that has been solved in the automotive world, and I wanted to apply this solution to my RV. That solution is to use automotive brake fluid, like DOT-4. There are two problems with using this fluid. The first is that it requires different seals in all the brake components. The other is that the fluid, being based on glycol, is hydroscopic. That means that it will absorb water. This is a good thing, but it means that once the fluid has been saturated with water, it needs to be replaced. I plan to replace the fluid every annual, or perhaps every two years to eliminate this problem.

Changing the o-rings in the brake components was made much easier by RV-8A builder Charlie Kuss, who went out and found the right sizes of o-rings, and created a little kit that he made available. Here’s his announcement from the Yahoo RV8 group:

Listers,
Back in January, I started a thread on the main RV
List regarding alternate brake fluids. I have
researched this and have decided to use automotive DOT
4 style fluid. My main objection to the 5056 mineral
oil fluid used in certified small aircraft is that it
BURNS! And it’s flash point is a mere 250 degrees
Fahrenheit. A number of RVs have burned because of
brake fluid being sprayed on a hot brake rotor (failed
caliper O-ring or failed line). This recently happened
to RV-6A owner Ed Anderson. Ed only lost his tire,
brakes and wheel pant, because his son had the
presence of mind to snuff the fire, using some
antifreeze he had in his car. At least 3 other RVs
have burned to the ground over the years because of
this.
Automotive brake fluids do not burn and have much
better performance (hot and cold) characteristics when
compared to MIL 5056. However, automotive brake fluids
are not compatible with the Buna N O-rings in our
brake parts. Auto brake fluids require the use of EPDM
(ethylene propylene) O-rings. I recently purchased the
needed O-rings to convert my master cylinders, Matco
parking brake vale and Cleveland brake calipers. To
obtain the highest quality O-rings, I had to purchase
in quantities of 100. Therefore, I have a limited
number of brake “kits” to offer for sale for $12
(shipping included). This will include enough parts to
convert both master cylinders, the parking brake valve
and both calipers. I will also include enough spares
to rebuild all items twice.
If you would prefer to buy these items yourself, I’d
be happy to email you the part numbers off list. I’ll
be at Sun N’ Fun, in case some of you would prefer to
pick them up there.
Charlie Kuss

Here are a few pictures I took while changing the o-rings.

This is the brake caliper.

To remove it, I just blew a bit of compressed air into the blue fitting. The puck popped out.

The new o-ring is on there, and ready to be installed. There have been cases of these pucks being installed backwards, with the ring closer to the brake pad. This can cause a leak. Some pucks are stamped with “this side out”, so it makes it pretty easy to get it right.

Here is the matco parking brake valve.

I’ve already done the upgrade, but I forgot to take a picture of the inside. Not much interesting, unless you are a parking brake valve engineer, just three o-rings to be replaced.

Here is the left master cylinder with the brake insides removed.

Here are the insides.

Another shot after replacement. You can see the three o-rings. One is difficult to see, its inside the puck on the left, and seals the master cylinder shaft.

Here is the master cylinder reassembled, and ready to go back on the aircraft.

I need to buy some DOT-4 brake fluid, and install it. Then the brakes will be ready for action. Thanks again, Charlie!

Update: 2008-08-02 – I need to replace the hoses that connect the master cylinders to the brake system (brake valve). Charlie suggested these:

Pegasus has Teflon lined SS hoses with AN fittings. For an 8 or 8A, you want 21″ long with straight fittings on both ends. ($15.99 each) See: http://www.pegasusautoracing.com/productselection.asp?Product=4