Brake bleeder up or down?

[atalla]

Hey guys maybe an AME AMP can answer this for me. Why are bleeder screws pointing down on aircraft calipers but cars always point up so pressure bleeding isn't necessary. Thoughts?

[BillL]

I have pondered the same question, and have no answer. But - -with the bleeder down, it will have to be pressure fed from the bleeder back to the master cylinder. I made a pressure bleeder for my car and it worked great. Finally, I got a store bought pressure bleeder to prevent contamination with the aircraft fluid. It has an orifice on the outlet, the fluid will just trickle through!?? I hate spending good money for a product compromised for the brainless (litigants) that reduce the products' effectiveness in its intended use. It does not bleed as easily as my DIY unit. Now it will have to be "fixed".

Back to the original question, probably (purely a guess) attachment to automotive history (convention) and the fact they were bled from the master down, not the other way around. The bleeders leak trying to reverse flow it and overflow the reservoir too. Why? Because it is easy to push a bubble up or down in a small diameter line or hose, but in the caliper cavity, a large cavity, the bubble will come to the top, because the bubble floating velocity is higher than the fluid going down to a bleeder opening.

FWIW, Personally, I don't like pushing the most heat cycled and contaminated fluid back up through the system on a car, I like it going from the master down.

[DanH]

Originally Posted by atalla

Hey guys maybe an AME AMP can answer this for me. Why are bleeder screws pointing down on aircraft calipers but cars always point up so pressure bleeding isn't necessary. Thoughts?

Convenience.

With bleed screws on the bottom, fluid is pumped into the system from the caliper, and flows up to the master cylinder, then onward to a reservoir. Aircraft systems tend to have a lot of vertical distance between components; moving bubbles upward is easy, while moving bubbles downward is more difficult.

Bleed screws on top means fluid is pumped down from the master cylinder. Ever seen a pneumatic brake bleeder? It applies a small pressure to the reservoir/master cylinder combo typically used on cars. Some even supply fluid. With the reservoir pressurized, just raise the car on a lift and walk around to the four corners with a hose, jar, and wrench, bleeding each caliper until the bubbles stop.

[Robert Anglin]

Yep, Dan is on the money here with a little twist form the peanut gallery. That's me in this case. I have done it thousands of times both ways and my "Pet Cooks" in the aircraft are facing up and I do bleed it from the top. The kicker is to think about air bubbles in the system and how to purge them as you bleed the lines. If you bleed from the bottom-up, you want the air to flow to the top and bubble out on top of the fluid in the highest point. If you bleed from the top down, you put a small hose on the Pet Cook at the top of the wheel cylinder. Then stick it in a small bottle add some fluid to the bottle and use the Master cylinder to pump the air through and out at the top of the small bottle with the brake fluid in it. The idea is to purge the system of air bubble no matter which way you approach it. With practice both ways work very well. It is easier for me to look at the little clear bottle and see the air bubbles coming out of a clear plastic hose, than it is to pump it in from the bottom and not be able to see the air bubbles coming to the surface of the fluid in a metal container, and I can do it with no help quickly with no spills and the fluid that is in the small bottle goes back in the can or if it is old stuff we get rid of it with the contamination in it. Just what we have done in the field for a lot longer than me and the guys before me and so on. On a part 23 aircraft, you do it by the book and the Pet-Cooks go as the book says. Hope this helps, Yours, R.E.A. III #80888

[F1Boss]

I have an 'overflow' line from the hyd fluid reservoir to the lower edge of the firewall that allows fluid to dump overboard when filling from the bottom. Of course I can catch the fluid for re-use as is appropriate. So adding fluid is easy with this setup, as is any bleeding air operations.

BTW Mobil 1 ATF is my choice of fluid in this system: available just about anywhere with no HAZMAT shipping charges.

Carry on!
Mark

[longline]

has been my practice for years. Never thought about the placement of the brake bleed fitting on top, however.
Mark: did you change out the orings to something different when you use the Mobil !?

[F1Boss]

No - the std O rings work well in any mineral fluid normally used in brake systems. Viton O rings are good up to 400F, so they will last on get in a system that gets hot from time to time.

The Mobil 1 fluid has a flash point virtually the same as the newest 5606 type fluid - about 200C if memory serves. The old 5606 had a flash point of around 100C (memory again).

Carry on!
Mark

[atalla]

thanks for all the replies! I work on many different types of brake systems in my line of work Snowmobiles, Atvs, MT bikes, Cars, trucks, HD trucks air brakes, HD equipment HYD release spring apply just about everything except aircraft and boat brakes...

Makes sense, one person can bleed with a syringe, a basic system no prob. only done this on sleds and mt bikes. I guess I'm just used to complex ABS valve bodies and was taught in school to never back purge brake fluid as the particles of dirt and garbage can plug small orifus and cost big$--- To this Cadillac use auto bleed via a scan tool, walk around with a wrench and press a button.

I do own a pressure bleeder for automotive use that has some dot6 (Racing hi temp fluid) that hooks up to tire pressure for operation so i would probably mod this to fit our reservoir. I'm sure a through fluid flushing will be in order.

If the pressure bleeder is used should the callipers not be flipped? it is possible however unlikely that air bubbles would be trapped at the top if the caliper bleeder swrew

What type of fluid is used in our kits? not dot 3/4?
Mineral oil?!?