bleeding brakes

[jetdriven]

We just did the annual on our 77 M20J. The brake fluid was old and gummy so we drained it all out overnight.  Changed to MIL-H 83282. The next day it was impossible to get the airplane to take fluid from the brake cylinders. We ended up using a pressure pot @ 10 PSI and disconnecting the line on the master cylinders to get it to take fluid. 

 

The brakes still feel terrible. There must be plenty of air in them because there is way too much travel and it is impossible to skid the tires. By design, the master cylinders sit horizontal and the fittings appear to come from the side, not the top. 

Does anyone know how to bleed the brakes on these airplanes to get a good firm pedal, like we had before?

[KSMooniac]

I used a pressure bleeder from the calipers to push fluid all the way to the master cylinder.  I don't have co-pilot brakes.  My pedal was "softer" than before for quite a while, but eventually the firmness returned.  I can't explain why it didn't immediately firm-up, but I can tell you I pushed a LOT of fluid through the system.  Maxwell said the washer seals in the master cylinders will slowly pass fluid when they're old, and I suspect that is what my problem was.  I didn't change them, and it seems good enough for my needs now.  The seals are cheap, but of course a PITA to replace... 

[kerry]

I take a small oiling can with manual pump( I bought one from walmart $5) and attach vinyl tubing(also walmart from fish department).  Hook it to the bleeding valve on clevland brake and pump it up until your resevoir has fluid in it.  Then do other side.

[jetdriven]

Yeah we tried the hand pump thing, it would not take fluid at all.  The pressure pot forced some fluid in there and got us brakes again, but they are really awful.  Perhaps put the pot to it again and crank the pressure up?  What do the washer seals do?

[N601RX]

Try lifting the front wheel up so that everything will be up hill from the calipars to the master cylinders and bleed by forcing fluid in the calipars. This will help release any air that may be trapped in the dips and rises in the lines.

Be sure the parking brake is not set while bleeding.

[KSMooniac]

If it won't take fluid, then you've got something wrong inside the system, I would imagine.  The pot I used was a big stand-up cylinder from Mityvac that could be used to pump or suck using the built-in bicycle pump.  I'm not sure how much pressure it generated, but it had no trouble pumping fluid in through the bleed screws.  (Make sure you suck out the fluid in the master cylinder too!)

I don't have a diagram of the master cylinders handy, but you might be able to google it up.  I believe the seals were part of the actuation system of the cylinders, and any leakage past them contributes to a soft pedal.

[Piloto]

If the brakes still feel spongy after pumping from the caliper up it may be because you may stiil have air trap when you closed the caliper valve. The valve has to be closed while applying pressure. If still spongy apply pedal pressure and open the caliper valve momentarily to let any air trapped at the caliper escape. Pump again and open the valve but close it before the pedal hits bottom.

José

[jetdriven]

Jose, I am not sure how to flush air from the caliper (top down) when the valve is on the very bottom of the caliper.  We have the Cleveland brakes

 

[OR75]

I am sure you did a google search on hw to bleed brakes ?

http://www.vansairforce.com/community/showthread.php?t=33493

http://www.eaa439.org/newsletters/Jan09%20NL.pdf

 

 

 

 

[Vref]

did you try the bottm up horse syringe style method..? I used this method on my Cleveland brakes equiped motorglider...with the oil can I couldn't provide enough pressure...

[jetdriven]

So, update, we used the nuclear approach.  It takes fluid readily now.  I charged a pressure pot to 60 PSI and ran two quarts of fluid through from the bottom up.  I even loosened up the master cylinder and bled some fluid out there. We are getting NO bubbles now, but the pedals still travel quite far and I dont think its possible to lock up the brakes.  I am at my wits end with this thing.    Any ideas?

[nels]

Since I'm new to aircraft I'm not sure if this would be a possiblilty but if brake fluid for aircraft is of the synthetic or silicone type, it would have a tentency to hold very small bubbles of air in suspension for a long time. If you would leave the plane set for a couple days and just rebleed at the master cyl if possible, this may purge the trapped air that has finally worked its way up to the master cylinder. Sometimes several dozen slight motions on the brake pedal will allow accumulated air to bubble out the intake orifice in the master cylinder and this will do the bleeding for you, just don't go in and pump the heck out of it, just minimal pedal motion.

As I said, not familier with the aircraft brakes but I would assume all brake systems are similar.

 

[Bolter]

Quote: jetdriven

So, update, we used the nuclear approach.  It takes fluid readily now.  I charged a pressure pot to 60 PSI and ran two quarts of fluid through from the bottom up.  I even loosened up the master cylinder and bled some fluid out there. We are getting NO bubbles now, but the pedals still travel quite far and I dont think its possible to lock up the brakes.  I am at my wits end with this thing.    Any ideas?

[jetdriven]

When we took the plane to the shop, the left pedal was hard and the right has a slight bit of mushiness.  We drained the fluid out and let it sit draining overnight, both sides. Refilling the system was a chore, and the new pads made it difficult to stop.  After breaking in the pads, the pedals move forward about 3" at the tops.  With pedal exensions, the geometry makes you extend your toe like a ballerina.  Bleeding and bleeding hasnt changed the situation.  It is not getting worse either. Both pedals are even.  I just hate to make a mechanical pedal adjustment when I know there must be air in the system somewhere.  Maybe I need to take the master cylinders out, re-o-ring them, and flush them on the bench.

[N601RX]

Do you have copilot brakes?  If so  you have to alternately pump the pilot and copilot pedals while bleeding.

[Awful_Charlie]

Don't know if this will work on aircraft brakes, but used to do it frequently when I used to race bikes (when the brakes would get stripped & rebuilt several times a year).  It does rely on air not getting into the system through leaks, but simply leave pressure on the actualting cylinder (ie on the pedals) overnight or longer by using a bungee or similar.

The logic in this is that small amounts or air will eventually get past the seals whereas fluid won't

On my Bravo I've not had to bleed them from dry yet, but every annual pump a couple of strokes through each cylinder out of the bleed nipple, keeping the resovior topped up: this draws in some fresh fluid and chucks out some old - and it's the fluid in the calipers that has the hardest life due to the heat.

Good luck

Ben

[jetdriven]

No copilot brakes. The system looks stone-simple, but we have spent more hours on it than rebuildng the nose gear.

[kerry]

Byron

I previously have had my right brake work great and my left one was a bit mushy.  I always had to pump the left brake 2 or 3 times to get it to work.  I always figured it was air in the system.  Trying to rebleed the lef

[kerry]

Byron

I previously have had my right brake work great and my left one was a bit mushy. I always had to pump the left brake 2 or 3 times to get it to work. I always figured it was air in the system. I could never make the left side better by rebleeding. I replaced the O-rings on both calipers and now both brakes work great. If your going to replace master cylinder o-rings I would also recommend you replace the caliper o-rings.

[nels]

Don't know how old the system is but could the flexible brake hoses be kinked or swollen closed with age. Also, is the push rod in the master cylinder returning all the way to allow air to get out of the system and also fluid into the system from the reservoir?

[jetdriven]

Single brakes. We even tried pulling the pedals up and the master cylinders extend fully. it passes fluid, just still spongy after repeated attempts at bleeding.

[kellym]

Quote: jetdriven

Single brakes. We even tried pulling the pedals up and the master cylinders extend fully. it passes fluid, just still spongy after repeated attempts at bleeding.

[OR75]

Byron...ever solved those spongy brakes issue ?

[jetdriven]

nope, next try is jacking up the nose to bleed them with a syringe-slowly.  Next step after that is rebuild the master cylinders and bench bleed them thoroughly.

[N601RX]

I recently added copilot brakes and remembered this thread from a while back.  These pictures are of the internals of the master cylinder and show why fluid doesn't want to flow in the reverse direction.  Although these seals were still in good shape, there was still a lot of accumulated gunk and sediment that needed cleaning out.

Here are a couple of pictures of the spring loaded shuttle valve inside the master cylinders.  It is spring loaded and will prevent fluid from flowing from the wheel cylinders back to the resevor when bleeding.  It can be unseated by pumping the brakes which will allow fluid to backflow.