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Brake pedal displacement

ndotreppe

Member
Hi, My RV7 is flying for three years, and I have noticed for a while a difference between both brake pedals: when I apply full brakes, the top of the left pedal is displaced by ½ an inch where the top of the right one is displaced for more than an inch. I did not remember this at the beginning.
I understand that the brakes are « self-purging » so a bubble should end up in the transparent tubes on top of the brake pedals and should not stay in the brake piston? there is no visible bubble on the line. I have a single brake installation so I cannot compare with the other side.
Any suggestion on how to tune the brake pistons to have them balanced? Should I replace the brake pads? I cannot see a difference in brake wear.
Thanks, Nicolas
 
Hi, My RV7 is flying for three years, and I have noticed for a while a difference between both brake pedals: when I apply full brakes, the top of the left pedal is displaced by ½ an inch where the top of the right one is displaced for more than an inch. I did not remember this at the beginning.
I understand that the brakes are « self-purging » so a bubble should end up in the transparent tubes on top of the brake pedals and should not stay in the brake piston? there is no visible bubble on the line. I have a single brake installation so I cannot compare with the other side.
Any suggestion on how to tune the brake pistons to have them balanced? Should I replace the brake pads? I cannot see a difference in brake wear.
Thanks, Nicolas

Are both pedals hard when pressed down equally? Assuming they are, then there is no air in the right line. I would not call them "self purging".

If it is not hydraulic, then the pads/piston could be pushed back more on the right requiring more fluid to make full contact.

Things that could do that are: sticking pins not floating freely, loose wheel bearing, or a very warped rotor.

If not the caliper piston retracting too much. Hmmmm???
 
Sounds like air in the system to me. Probably a small bubble somewhere. The brakes can be a pain to bleed. Bubbles like to get trapped in the upward loop of the brake lines from the pedals to the firewall. I like to lift the rudder pedals up near horizontal while I pressure blead from the bottom. My pedals are as hard as a rock and move very little during application.
 
The system is NOT self-purging. You have likely introduced air into one of systems. Several potential reasons for that.

Larry
 
It's unlikely you'd get enough uneven wear in the pads or rotors to induce this much difference. Is there fluid in the reservoir? If so, it's likely air. Pressure bleed them from the bottom. If the fluid is down (and even it's not) check very closely for leaks. Leaky seals, cracked lines, etc... Uneven pedals were exactly the symptom I was getting about 5 hours before a line let go. We went around faster than you can say "ground loop".
 
Hi, Thanks for your advices: I will try again to bleed the right brake. The fluid level is at the maximum for the last three years, I did not loose anything. With maximum pressure applied on both pedals and aircraft stationary I have a much biger displacement on the right pedal than on the left.
 
Wear on the pads of a properly function brake system will have no influence on how much the pedal strokes when applying brakes because the system is self adjusting.

There is two main causes of excess stroke

Air in the system - the air compresses as brake is applied... this requires a lot more stroke to get an equiv pressure at the brake pad.

A warped brake disk - the wobble in the disk spreads the pads father apart when not braking so that the when the pedal is applied it has to move the caliper piston further.

On the first, the pedal usually feels a bit spongy even with hard braking because of the pressurization of the air (depending on how much air there is)

For the second, it will feel solid / hard under braking but will just require more stroke of the pedal.
 
Dear Scott,
That's very clear: I will look first to the warped disk as this looks exactly the symptom. Thanks for your help, Nicolas
 
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