Q-talk 129 - A Lesson on Airheart Brakes

Several months ago I did the LaRue brake mod and at the same time I decided to rebuild the calipers and the hand controlled master cylinders. Using the bronze bushings suggested by Jim Patillo (I think), the mechanisms functioned very smooth. There were some problems encountered aligning them with the disc and I received sound advice from David Hyatt. He suggested using a layer of flox under the mount then applying the brake and letting the flox harden.

With the new setup the brakes worked very nice at least in the beginning. Then things became odd.

After flying above 12,000 feet, the left brake was hanging up. I turned to the group to see if anyone had encountered this sort of a problem before. No help. I pulled the caliper back off and assured myself that it had been rebuilt and was functioning properly. I spent several months scratching my head trying to figure out what was going on. I thought at one point that maybe the new hard pucks (metal flakes embedded) were expanding after heavy application of the brakes. But this was eliminated because the brakes remained dragging even after they were cool.

At some point I realized that if I opened the bleeder on the calipers that the brakes released full. So, the problem was in the master cylinder. Progress, maybe. I pulled the master cylinder to check that the relief port was not plugged. It was clear and was re-assembled. I checked the port again and it was blocked so back apart once again. The port was clear.

So, the piston or the cup must be too long. The brakes had worked ok for nine years so, there could not be a problem with the piston. The cup must be the problem. I decided to cut the cup down to clear the port and this solved the problem. Now I became aware that the right brake had the same problem. I ordered new master cylinder kits from Airheart thinking that the wrong cup (DOT 5) had been supplied and they had expanded. Airheart was contacted, but offered no insight as to what might be wrong. Finally, after the Jean fly-in, I pulled the right master apart and the new cup was a 1/16" shorter in the new kits than the ones in the old kit. With the new cups installed, the port was clear and the brakes now function properly at last.

Lessons learned:

1) Check new parts against your old parts before replacing them. On casual inspection they looked the same.

2) Make one change at a time if possible.

3) Red Herrings abound. (12,000 Feet)

4) Science works if I follow through and think things through.

5) If it ain't broken, don't fix it. The masters and calipers were working fine before the rebuild.

The kits that were bad were purchased in 2002 so, if anyone has kits that they bought in the past and have them setting in ready be sure to check the cup length. The new kits were fine. Airheart has been sold a few times so who knows where or when these bad cups showed up in the kits.

You can order a printed copy of Q-talk #129 by using the Q-talk Back Issue Order Page.