Need Brake Bleeding Education

[robcaldwell]

Grove 66-152

I thought I knew what I was doing... Bleed the brake from the bottom and watch the reservoir until filled. Go to other side, repeat.

It's been almost 3 weeks now and I have spent every single day trying to figure out what is wrong with my brakes... The right brake was grabbing at moderate taxi speeds. I've replaced the right side masters and changed o-rings in the caliper pistons. At Bob's suggestion, I have added 1/16" shims between the back plates and the caliper housing... Still the right side is dragging and grabbing. Yesterday, I changed a suspicious brake line and bled the brakes again. Went for a taxi, and now BOTH right and left brakes are dragging until the plane will not move. Period. Both sides are locked. I literally have to loosen the back plates to get the plane roll just to get her back in the hanger.

I'm thinking:

1) I am not bleeding properly because occasionally I cannot push fluid in. It just won't move.

2). I should take off each line one at a time and blow air through each of them. Reassemble.

Or, 3) I need to replace all of my lines...

I have a new caliper and rotor coming from Grove because I think they just flat are out of suggestions. I have my doubts about replacing a single caliper and everything just suddenly works...

If you have any suggestions, please let me know. I don't care how brutal the thoughts are. I could care less anymore. The weather has been absolutely gorgeous here with blue skies and no wind for over a week now. Mother nature is thumbing her nose at me and I SHOULD BE FLYING!!!!! Instead, I'm stuck on the ground playing with my brake system, daily.

Signed,

EXASPERATED!

[gregc]

1) If you are looking at the caliper/brake pads and someone in the plane applies the brakes, do the pads grab and then cleanly release?

2) Bleeding bottom up is usually better but you could try top down. You need someone to repeatedly apply the brakes (basically they are pumpling fluid through the system) while you watch for fluid/air bubbles being pushed through the caliper.

[robcaldwell]

1). They grab initially, but never "cleanly" release.

2). I've been doing the brake pumping myself. I see very few bubbles.

[BravoGolf]

Hey Rob,

you have a video posted in March 2018 that I viewed to get an idea of your brake installation. I came away with two thoughts. Do you still have the loops in the brake lines, those seem like great air bubble catchers? Second, is it worth isolating the parking brake valve? Is it possible the pressure is building as you taxi and brake and the parking brake is not allowing the pressure to release?

Scott

[robcaldwell]

Hey Scott,

I have modified the 2018 iteration of the brake lines. Here's where I indicated the change (scroll to the bottom). https://bearhawk4place.blogspot.com/...ake-lines.html

I have considered isolating the parking brake. Just cannot see how that is the problem when I tested it profusely during installation. EDIT: I will bypass the PB today and see what happens.

[Nev]

Went for a taxi, and now BOTH right and left brakes are dragging until the plane will not move

Hey Rob,

Sorry to hear you’re having these issues.

Following on from what Scott was saying above - the parking brake is designed specifically to do exactly this, so I too think it may be worth isolating it.

Perhaps get someone to work the brake pedals and park brake cable, and watch the callipers to see what happens. If the brakes are off (check the callipers), and the park brake valve is off, and you then select the park brake knob to ON (usually pulling the knob out) without pressing the brake pedals, I think you can still taxi. But then as you press the brake pedals, they will “lock” at the last depressed position, essentially making it harder and harder to taxi with each application until fully locked. So I wonder if each time you bleed the system it is (coincidentally) releasing the brakes, but the park valve is remaining ON.

Not sure what brand you have. I installed a Grove park brake recently and I made a “note to self” when I connected it that it wouldn’t take much for the park brake to lock on inadvertently. Mine came with two valve arms, so it can be installed at 90 degrees, or 45 degrees. I don’t know which one I need yet until I add fluid and test it.

I suspect that if the cable housing was to slip, it could result in mine locking on, perhaps while airborne. Someone suggested that I used lock wire to secure the cable housing to an Adel clamp.

I also noticed that it would be easy for the valve arm to go over-center, and not release.

Just a few thoughts, and hope you get it resolved.

Nev.

[robcaldwell]

Thanks, Nev! I'll have to figure out how to "isolate" the parking brake. On my system, this would require completely by-passing it all together and I would need replacement (longer) lines to make that happen. Maybe I can use a pair of unions.

Just FYI, the parking brake is a Matco and I have experimented with it quite a bit. From what I can see, it is either all on or all off. Here's a schematic. https://static.veracart.com/matco/it.../document1.pdf

[quadra]

I don't know your system well enough but at their simplest brakes are single direction hydraulic systems, something either mechanical or pneumatic is storing pressure. Try having someone apply brakes while you crack bleed and close before they reach full stroke..keep the master reservoir topped up and do that 5 or 6 times each side.. I have done this with a mityvac on the bleeder and had big air bubbles show up after 4 or 5 strokes. I have used top side positive pressure systems and far end vacuum systems on lots of vehicles ( none were aircraft ) but I have no idea how I would pressure fill a system from the brake end reliably without a bleed screw at the high end of the system.. Especially if it was a field repair.
Hopefully you get this sorted soon, we are all looking forward to your flight videos👍

[Mark Dickens]

Do you have return springs on your pedals so that they fully release when you take your feet off the brakes? Just a thought.

[robcaldwell]

I mentioned using return springs when I was speaking with Grove. They replied that something would need to be wrong with the master cylinders if a spring was necessary and that would only serve as a band-aid to more latent problem.

[Mark Dickens]

Yeah but they are only making a comment in re their master cylinders, not the entire assembly including your brake pedals. If the pedal is pressed down only a little, it will block the flow. Definitely get someone in the cockpit and pump the pedals to make sure that everything moves freely. I helped a friend on his RV-8 brakes and he had to add return springs.

[robcaldwell]

Good point!

[Steve W]

Get a floor jack and lift the bad side. Have someone apply your brakes. When released you should be able to freely move the wheel. If that works OK, see if you can easily push the piston back into the calliper by wedging a screw driver against the pad plate. Push them back out with the masters. Do this several times. If there is a firm resistance, then you have something wrong with your masters. Just a brain fart here, but are your parking brakes actually on when your thinking their off? Some you can pump up after selecting “on” but they won’t release. You should be able to freely push the piston in, moving the fluid back up into the reservoir. If you can do this repeatedly, then I have no idea what’s happening there. Something is acting like a check valve. All I can offer.....

[bkyser]

Having trouble (sometimes) pushing fluid from the bottom, is a clue. If fluid can't be pushed through the system from the bottom, the same thing is happening when you release the brakes (the fluid isn't being allowed to return causing the brake(s) to drag).

A couple of things that I can think of that could cause this:

1) The master cylinders aren't returning fully. When you're having issues grab the pedal and pull it backwards to make sure it is fully returned. You might also try disconnecting the pedal from the master cylinder in case there is something wrong with the geometry of your setup that isn't allowing the pedal to return all the way.

2) The parking brake value. You might consider eliminating this from the system until you get the problem identified.

3) A flap in one of your flexible lines. When assembling flexible hoses, it is possible to partially shave off a piece of the interior rubber creating a flap that acts like a check valve. Since you've seen the problem occur on both sides, I'm thinking 1 or 2 is more likely.

Air in the system will cause spongy/less effective brakes, but wouldn't cause the dragging/lockup problem you are seeing.

[svyolo]

Loose wheel bearings keep popping in my head, especially after adding the shims.

[quadra]

can you post up a few pictures of the current system? I was looking at the 2018/10 pictures and those loops in the lines have me wondering if there is a chance you have some air in the system? if you have someone to help you bleed try cracking the bleeder and letting them get to half travel before closing it back up.. do that a couple times each side and if there is air in the system it will decompress... after that if your brakes are working properly when cold but have a spongy pedal i would be looking for air in the system.

[Nev]

I'll have to figure out how to "isolate" the parking brake.

Rob, just try to figure out if the park brake is in fact completely OFF when it should be.

The Matco looks to be very similar to the Grove. I had issues determining if it was ON or OFF, and I think it’s very easy to have the arm out by 45°. Presently I don’t know if mine is working correctly (knob into the dash = OFF) or if it is the complete opposite (knob in = ON). I won’t know until I fill the reservoir. Then I may need to adjust the cable position so that it’s not just partly on / partly off.

Steve W made a good suggestion above, raising a wheel off the floor and get someone to press the brakes / operate the park brake, while you try to spin the wheel.

Matco have troubleshooting procedures on their website, but all these troubleshooting procedures seem to make the assumption that certain items such as the Park Brake are already working properly.

[Bdflies]

I'll take a swing at it.

Your diagram indicated that the pilot masters are plumbed to the top of the passenger masters, which feed lines to the calipers. Is it possible that pressure from the pilot masters is pushing the passenger piston down, but nothing is making the passenger piston return to the uppermost position? If this were happening, you would indeed have proper braking (from the pilot's side), but the 2nd piston not returning up - would effectively block the path for depressurization of the caliper? couple of things could contribute to this scenario: adjustment of the passenger brake pedals might not allow the rod/piston to fully retract. The spring in the master cylinder isn't strong enough to push the piston against the pedal mechanism. There IS a spring, either inside the master, or external on the rod? Without some sort of spring, to retract the piston to it's uppermost position, the return path will certainly be blocked.

As an aside; I fully understand why folks plumb one master - through the 2nd master - to the calipers. Makes for a neat, easy installation. The part that puzzles me (somebody please splain it to me) is why go from the reservoir, to the pilot, to the passenger, to the caliper. If a leak develops or nobody ever checks the reservoir, the first cylinder to suck air (and lose brakes) will be the pilot. The passenger will still have fluid and thus retain braking, while the pilot has a dead pedal (or 2). If it were plumbed to the passenger side first, the pilot would have brakes even when the passenger side goes flat. Additionally, if a passenger rests his big old feet on his brake pedals, the system could lock up, as Rob is describing. Just a thought.

Bill

[robcaldwell]

can you post up a few pictures of the current system? I was looking at the 2018/10 pictures and those loops in the lines have me wondering if there is a chance you have some air in the system? if you have someone to help you bleed try cracking the bleeder and letting them get to half travel before closing it back up.. do that a couple times each side and if there is air in the system it will decompress... after that if your brakes are working properly when cold but have a spongy pedal i would be looking for air in the system.

Loops were eliminated around November 2018. Scroll to the bottom of this page to see the 2nd iteration. https://bearhawk4place.blogspot.com/...ake-lines.html

[robcaldwell]

PROBLEM SOLVED.

I eliminated the Matco Parking Brake and I can now watch the calipers compress and decompress as I work the toe brakes.

Good Lord!!!

I cannot say that I endorse the Matco PB. Pretty scary to think about pressure in the brake lines on short final. I'll be buying a nice set of wheel chocks...

[whee]

Yay! Now we all expect a sneak peek flying video!