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Brakes locked after sitting two weeks

RVFan671

Well Known Member
Wanted to see if anyone here has an idea on my root cause of the left brake completely locked on (Rv4). I'll share the events leading up to discovery and what I've tried.

I park on the ramp in the PNW and it didn't really rain this year until during the 2wk stretch since my last flight when all was normal and picked up the tail to pull it backwards into its spot. The other odd phenomenon was the first decent wind storm of the year for us.

Jumped in after a pre flight and it feels like my tail wheel is tied down to the ring still but I clearly removed my strap in pre flight. I holler back to my 6yo passenger "did you happen to put the strap back on the plane". He claims "no" but then says he forgot, so at this point I have too much throttle in for a normal taxi with full tanks and then it pivots as if only left tire is frozen.

Hop out and left tire won't move forward or back. Totally frozen. Pull the wheel pant and then tap the master cylender a little and it frees so I think it was just pads temporarily rusted to the rotor due to rain. I didn't fly that day and decided two days later I should go.

Same locked wheel can't get it to move and tapping doesn't fix it so I take pads off and it frees. Put pads back and it's locked. Used a c clamp on the piston and doesn't really seem to move.

At this point I've pulled the caliper and rebuilding the piston via scotch bright and anew o-ring in a week when it shows up. I'm not sure this is the fix for good so ideas on root cause are welcome. On a182 I used to own the piston gave a warning. It would start to have a dragging (not at all locked up) brake.
 
After you rebuild the caliper, you may find that you can't bleed the line without pulling back on the rudder pedal for that wheel, to pull the piston away from blocking the port. Many here install external springs to help this process. Make sure your pedal is free to move.

Pads can and will stick to the disc if surrounded by rust. But, you should be able to reach down and place your fingers between the disc and rim and with your thumbs, push on the caliper and retract the piston and pad a little.
 
It's not terribly uncommon for the brake cylinder itself to be the problem rather than the pads/piston. If the brake pedal does not return FULLY then the system remains closed and can be slightly pressurized in that configuration. Many owners have installed heavier (or additional) springs on the pedals to assist in returning them fully when the brakes are released.
 
Caliper pistons need clean/rebuild

Pretty common on RV's and certifieds...My RV-4 has done it when I was not flying for a bit. Takes very little build-up of corrosion or dirt to freeze up the piston.As your pads wear, the exposed part of the piston goes deeper into the caliper. Once it starts, it wont usually fix itself. Take the caliper apart and pull the piston out. may need to carefully blow it out with air, lightly clean the outer surface with scotchbrite to clean-up the corrosion. Clean everything and put back together with new O-rings (VANS or ACS have these ). Of course, you'll need to service and bleed them.
 
It's not terribly uncommon for the brake cylinder itself to be the problem rather than the pads/piston. If the brake pedal does not return FULLY then the system remains closed and can be slightly pressurized in that configuration. Many owners have installed heavier (or additional) springs on the pedals to assist in returning them fully when the brakes are released.

That happened to me on my RV6. The right brake felt like it was locked up - couldn’t push it back into the hangar after a flight. I reached in and pulled aft on the top of the brake pedal and it released. I exercised the brake pedal back and forth several times and it seems to have cured it, but I got a new set of Matco master cylinders to replace the installed Cleveland cylinders. I have a set of extra springs for the Marco’s. The Cleveland’s have springs, but the cylinders are probably 20 years ‘new’. I bought the kit used and have flying since September. I’ll probably get a seal repair kit for the clevelands so I’ll have a spare.
 
Same locked wheel can't get it to move and tapping doesn't fix it so I take pads off and it frees. Put pads back and it's locked. Used a c clamp on the piston and doesn't really seem to move.

If you can't move the piston back into the calipar with a C clamp (while still connected hydraulically), that is a pretty good indication of a problem with the master cylinder. Our MCs must be FULLY retracted to allow the fluid to reverse direction (i.e. let the fluid out of the calipar). Stopping just a 1/16 or 1/8" from full retraction is enough to act like a parking brake. The springs in the MC (Matcos at least) are really just strong enough to do this, so if something like a failing o ring is in there creating drag, they may stick while retracted just a bit. Many of us add external helper springs to the Matco MCs to avoid these issues.

First pull back on the brake pedal to release it and test again. MC overhaul kits are like $12 and probably worth the effort in this case to overhaul them.

I don't really see how a piston could sieze in the calipar in our case (all Alum parts) at least not to the point that it wouldn't move with a C clamp. A cocked piston could do this, but should be visible to the naked eye.

Larry
 
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Pulling back on the master

I'll pull back the pedal after the rebuild (just waiting on an oring to finish the job). But what is suspect is that I wheeled the plane back before parking it. However, I could see that after hopping in to fire it up during warm up I obviously held on the brakes and maybe that is the trigger point.
 
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