Tuesday, August 08, 2006

Leaky brake fixed, and I'm flying with my pants off again

I may have mentioned before that I noticed a bit of brake fluid on the inside of the left wheel fairing after a recent flight. Last week, I decided to fix that as long as I had the fairings off for finishing. It was really a pretty simple operation. Removing the caliper is something of a habit these days considering how many times they had to be removed and replaced while fitting the fairings, so the only new wrinkle this time was that I had to also remove the brake line. That's a simple matter of removing the AN fitting, and having a cap handy to plug the line before a significant amount of fluid can escape. Allowing the fluid to drain out would not only create a kitty-litter clean-up job on the hangar floor, but it would also introduce a bunch of highly compressible air into the line, and that would require that the brakes be bled. There's apparently a way to do that solo, but I've never done it and given that avoidance is the easier path, that's what I did.

Once the caliper was off the plane and cleaned up, I walked it down to the FBO to have the A&P glance at it. The caliper is actually a very simple piece, and pretty much the only thing that can cause it to leak is a age hardened or damaged O-ring. The A&P had the appropriate part in bench stock, and even with his mark-up and sales tax, it was only $1.09. I gotta tell ya, I'm getting used to this "fix a problem for a dollar" maintenance plan I've been on ever since selling the store-bought Tampico!

The A&P showed me another little trick that would hopefully negate the need for brake bleeding: he filled the reservoir behind the caliper puck (the only moving part on the caliper) with brake fluid, then put the puck in just far enough to hold it in place. I took it back down to the hangar and re-installed it on the plane. I then tightened up the brake line AN fitting just enough to hold it on the caliper fitting but not so tight that it would hold the fluid, and pushed the puck back in. That forced the fluid in the well behind it to squeeze out of the AN fitting and keep the brake line free of air. It's not guaranteed to work, but it did for me. I jumped into the cockpit and worked the left brake pedal. After a little excessive movement as the caliper puck moved back out into contact with the brake pad, the pedal firmed right up. No bleeding required!

The pants are both off now, and have a coat of gray primer on them. This first coat of primer seems to be primarily intended to show all of the pinholes and faults in the fiberglass that need to be filled. I mixed up a batch of epoxy and thickened it with micro balloons to give it enough consistency to stay put in some of the larger areas that needed filled. If it was just pinholes I had to fill, I think I might have gone in the opposite direction and thinned the epoxy rather than thickened it. That way I could have more or less just poured it into the pinholes or relied on capillary action to "suck" the epoxy in. As it is, I suspect I covered the pinholes more than I filled them. Hopefully that won't make much of a difference to the final finish. Tonight I need to go back and sand all of the epoxy and primer off, and get ready for the second coat of primer.

UPDATE: Well, after a couple of hours of hard sanding, the wheel pants have been successfully transitioned from having annoying, unsightly low spots to having annoying, unsightly high spots. Ok, just kidding. It worked out pretty well, actually. I was able to sand away the micro-balloon epoxy without sanding into the harder epoxy/glass of the pants, and it looks at first glance like the filling worked pretty well. I won't know for sure until I spray on another coat of primer, but any depressions that show up then will be very small and easily filled.

1 comment:

  1. ...I suspect I covered the pinholes more than I filled them. Hopefully that won't make much of a difference to the final finish.

    Don't worry, if you did just cover them, sanding will uncover most of them again