Right Brake Failure

67pileit · #1 10-09-2010, 09:16 PM

I had a complete right brake failure taxiing in after landing. Both right brake pedals (left and right seats )went completely flat. After less than one hour sitting, without changing anything, the brakes worked normally. There is no sign of leaking anywhere. Resevoir was low but not empty. Brake lines at the caliper are metal.
Any ideas what happened??

sbarger24 · #2 10-09-2010, 11:45 PM

My first thought was one of your two master cylinders on the brake pedals is leaking by internally. Possibly a bad O-ring on the ram? I don't know why it would go back to working though, but this theory would support your indications of no external leaks.

Steve Barnes · #3 10-09-2010, 11:52 PM

Years ago I had a similar problem on my RV4. The coil that you put on the aluminum line just before the slave cylinder, had preload on it. With vibration the force from the coil would drive the piston back in the cylinder of the slave cylinder, and leave a gap between the brake pads and the disc. It doesn't need much of a gap before your pedal will bottom out.

Steve Barnes "The Builders Coach"

Bob Axsom · #4 10-10-2010, 12:48 AM

Is this an airplane that has been flying for a while on is it relatively new? Low brake fluid is a concern - where did it go? That suggests a leak. A few years ago there was a notice (probably a SB) that said Cleveland assembled some brakes with the pistons backward so that the O-ring was nearer the opening and was subject to leaking as the pads wore down. Once the pressure is lost your peddle "goes to the floor" as the fluid is lost. The right brake is FAR more prone to pad wear and piston travel because of the tendency to apply right brake along with right rudder during the takeoff roll. Another possibility is in one of the master cylinders. If the O-ring is defective (I had one scalloped from the factory) it will allow fluid to travel past its piston into the top of the master cylinder and prevent the spring from returning it quickly to the "armed" or "ready" position. Over time the spring will force the piston back up in the master cylinder so it will work again. If you have an "A" model, brake failure is probably more serious than the conventional gear models. I have experienced the right brake failure twice in my RV-6A - once after engine start before starting my taxi (leak past the puck) and once as I turned off of the runway after landing (brake line failure at the rear of the tube flare - possibly due to my field work trying to fix the earlier failure) - and my recommendation is thoroughly investigate the problem and don't fly right away because the problem seems to have fixed itself.

Bob Axsom

molson309 · #5 10-10-2010, 02:44 AM

Had you been braking hard or at length before the failure? Chances are you heated the caliper/fluid to the boiling point. When this happens the brake is unusable until it cools to the point where the gas bubbles condense.

I've had this happen with the rear brake on dirt bikes when I rode the rear brake all the way down a steep hill. Near the bottom the brake pedal would go full travel without any perceptible effect. After a while it would work again once the caliper cooled down.

Use of a higher boiling point fluid will delay this somewhat but the real cure is to not use the brakes as much - sometimes this is unavoidable.

67pileit · #6 10-10-2010, 11:03 AM

Thank you for these suggestions.
My RV is a straight 6 with 460 hrs TT.
I didn't think it would be a master cylinder since both right pedals failed/went flat. That would mean 2 cylinder failures at the same time.....No ?? I suspected that it might be boiled fluid at the brake/wheel end since it happened after a long taxi. I have pulled the pads off both sides and although they still have some life remaining they will be replaced.
Steve I'm not sure how the spring action works that you mentioned but I'll have a look at that too.

Thanks guys