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Mushy Brake

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GreggC

I'm New Here
After more than 20 years and 2000 hours I developed a mushy right brake on my RV-6. No worries, bought a rebuild kit and overhauled the Parker 10-30 master cylinder, bled the brake from the bottom up, and had a nice, tight brake.

After a couple days and a number of flights, the right brake gradually started to get mushy again. After a week or so, no brake at all. Checked the lines and fittings and couldn't find a drop of brake fluid. Pulled the right brake line and found it to be clogged. Hmmm, okay, removed and replaced the brake line, bled the brakes - nice and tight...for three days. Same problem.

At this point I'm scratching my head. I've read many previous posts on attach points, sideloads, top versus bottom bleeding, scratched push tubes, etc. yet can't figure out what's going on.

When I get in the cockpit, hold pressure on the brake pedal with my hand, and observe the reservoir line I do see small bubbles rising in the line, which seems to indicate air is entering the system somewhere, but I'd think there'd be associated signs of brake fluid at those air-entry points.

Any ideas on what to troubleshoot? Rebuild the caliper? Replace fittings? Screwed up master cylinder overhaul?
 
Unless you clean up up the scratches in the bore and on the shaft, what you describe is exactly what will happen.
 
GreggC said:
, hold pressure on the brake pedal with my hand, and observe the reservoir line I do see small bubbles rising in the line,
Click to expand...

This is exactly what happens when the stato-seal or top o-ring goes bad. Either something in the MC didn't go back together correctly or, as Bob said, you have scratches on the piston shaft that let in air during the downstroke.

Larry
 
Last edited:
Thanks Guys,
I took apart the suspect master cylinder under the watchful eye of another A&P mechanic and confirmed the rebuild was proper. However, taking heed to Bob's advice, we closely inspected everything and found what I previously thought were minor scratches on the piston rod. Those were buffed out with 1200 grit paper, cylinder parts cleaned and reassembled, bled from bottom up and put back together.

After a week and 4 hours of flying the brake is solid and holding fine.

Problem solved, greatly appreciate everyone's help.
 
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