Need Brake Bleeding Education

[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👍

[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]

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

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

[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.

[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

[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]

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!