Trick for bleeding brakes

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hankrausch

Pre-takeoff checklist
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I had a brake piston overtravel due to worn linings 600 miles from home. Lost all hydraulic fluid because the o-ring came out. The shop where I was at had new linings and I got those in ok, but they would not touch the rest, they told me it would take 8 hrs to bleed them.
So I flew home with no brakes and brought it to my mechanic who is somewhat of a God in this area, here is what he did: I had been trying to bleed by pumping up the bleed valve back to the reservoir, what he did was pump up the pressure in each side using the hand brake, then relieved the pressure by loosening the nut of the feed pipe at each master cylinder and he kept doing this til there was nothing but fluid and both sides were hard. This was for a PA-38, I understand some master cylinders have proper bleed valves.

Just sharing this in case anybody else gets stuck far from home and needs to know how to do it!
 
Must be a Cherokee. Once air gets into the system they are a bear to bleed.
 
Yep that why I posted this trick with the handbrake it could not have taken more than 30 minutes!
 
I hook up a pressure bleeder to the bleed valve on the brake cylinder and push the fluid from the wheel back up to the master reservoir. Any air tends to go up and back to the reservoir where it won't cause problems. Not familiar with hand brakes in airplanes since most I've worked on were old and not pipers.
Glad you did ok without the brakes. My first instructor hit me everytime I tried to use the brakes...he said "there will be times you need them that they won't work....figure out how to do without them now instead of when you're dependent on them and they aren't there...."


Frank
 
Good training, wish I had had an instructor like that.
 
BiffJ said:
I hook up a pressure bleeder to the bleed valve on the brake cylinder and push the fluid from the wheel back up to the master reservoir. Any air tends to go up and back to the reservoir where it won't cause problems. Not familiar with hand brakes in airplanes since most I've worked on were old and not pipers.
Glad you did ok without the brakes. My first instructor hit me everytime I tried to use the brakes...he said "there will be times you need them that they won't work....figure out how to do without them now instead of when you're dependent on them and they aren't there...."


Frank
Click to expand...
Prolly shouldn't try to pressure bleed Piper brakes. An expanded or ruptured reservoir may result. Grumble, grumble, grumble. I never have confronted the A&P about it but I know what happened to cause the bulged reservoir. I never did ask him again to bleed the brakes...

Edit: the vent tube is quite small diameter and apparently easy to exceed its capacity to flow brake fluid. Maybe removing the reservoir cap may help...
 
Clark1961 said:
Prolly shouldn't try to pressure bleed Piper brakes. An expanded or ruptured reservoir may result. Grumble, grumble, grumble. I never have confronted the A&P about it but I know what happened to cause the bulged reservoir. I never did ask him again to bleed the brakes...

Edit: the vent tube is quite small diameter and apparently easy to exceed its capacity to flow brake fluid. Maybe removing the reservoir cap may help...
Click to expand...
The Mechanic ---> Should have removed the filler cap.. many of the pipers have a small can as a reservoir it does bend easy.

I have modified a garden sprayer to fit a small clear plastic hose that can be attached to the brake cylinder and simply back filler the reservoir. that brings the air with the new fluid. 3-4 psi is all it takes. we catch all over fill of course.
 
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Tom-D said:
Should have removed the filler cap..
Click to expand...
Duh! What did I type? Remove the reservoir cap if attempting to pressure bleed.

Of course there is no need to pressure bleed and pumping the air out is fine. The real problem with Piper brakes is the seals in the master cylinders and the ancient hoses. Replaced all the cylinder seals and all the hoses and the system has not required bleeding since.
 
BiffJ said:
I hook up a pressure bleeder to the bleed valve on the brake cylinder and push the fluid from the wheel back up to the master reservoir. Any air tends to go up and back to the reservoir where it won't cause problems. Not familiar with hand brakes in airplanes since most I've worked on were old and not pipers.
Glad you did ok without the brakes. My first instructor hit me everytime I tried to use the brakes...he said "there will be times you need them that they won't work....figure out how to do without them now instead of when you're dependent on them and they aren't there...."


Frank
Click to expand...

Exactly the way I did it. Had a tube coming off the reservoir cap and into a drain bottle. I will add that while the pressure is on, all the brakes including the hand brake are moved back and forth to bleed all the air out. Even then, it will take some time and two people. One in the cabin to work the brakes and the other to keep the pressure going and check for overflow at the reservoir.
 
Timbeck2 said:
Exactly the way I did it. Had a tube coming off the reservoir cap and into a drain bottle. I will add that while the pressure is on, all the brakes including the hand brake are moved back and forth to bleed all the air out. Even then, it will take some time and two people. One in the cabin to work the brakes and the other to keep the pressure going and check for overflow at the reservoir.
Click to expand...
There is a procedure to connect lines from the bleeder valves to the fluid reservoir so that one person can bleed Piper brakes. A few feet of plastic tubing, some tape, and some safety wire is all it takes.
 
Tried it and you end up with 12' of tube with fluid in it that you have to disconnect and drain somewhere. Read: It makes a mess.
 
Timbeck2 said:
Tried it and you end up with 12' of tube with fluid in it that you have to disconnect and drain somewhere. Read: It makes a mess.
Click to expand...
That's easy, gotta have a bucket handy.
 
Would a suction bleeder work?

I ask because I bought one from Harbor Freight when I was having a terrible time getting all the air out of BMW motorcycle brakes, and it worked like a charm. Around $35 IIRC. You do need an air source.

19467809441_b5810827f4_z.jpg
 
Capt. Geoffrey Thorpe said:
Gravity bleed doesn't work?
Click to expand...
Piper brakes started as a simple single master system. They became a system with up to five master cylinders with inter-connections. The hoses which connect the cylinders have many high spots. Getting the air out requires moving at least the system volume when all the seals are good. When some of the seals start letting air in it gets to be a nightmare if you are trying to use the individual master cylinders as the pumps to bleed the system.

Long story made short once air starts getting in the system through cylinder seals just bite the bullet and rebuild all the cylinders...may as well replace the 40 year old hoses while they are off. Chances are they are rather stiff.
 
Clark1961 said:
Piper brakes started as a simple single master system. They became a system with up to five master cylinders with inter-connections. The hoses which connect the cylinders have many high spots. Getting the air out requires moving at least the system volume when all the seals are good. When some of the seals start letting air in it gets to be a nightmare if you are trying to use the individual master cylinders as the pumps to bleed the system.

Long story made short once air starts getting in the system through cylinder seals just bite the bullet and rebuild all the cylinders...may as well replace the 40 year old hoses while they are off. Chances are they are rather stiff.
Click to expand...

If the master/slave cylinder seals are letting air in, wouldn't they also be letting fluid out? I ask because my Comanche is a beeatch to get a good bleed. I've rigged a system to circulate the fluid back to the reservoir, but by the 200th pump it get tedious. Master seals have been changed, slaves haven't but no evidence of leaks. two separate masters, share common reservior.
 
Chip Sylverne said:
If the master/slave cylinder seals are letting air in, wouldn't they also be letting fluid out?
Click to expand...
The answer is apparently not. I never observed the why/how/where air was entering the system but the reservoir was kept full while pumping and there were no oil leaks. The final bit of evidence is that I haven’t had to bleed the brakes since having the cylinders rebuilt and hoses changed. I was having to do it monthly before the rebuild.

Saying it another way, I’d bleed the brakes until they were good and firm using circulation method. A month or two later the brakes would be mushy again and there was never any evidence of an oil leak. Air was getting in the system during normal use and no oil was lost.
 
cross_section_1.jpg
lip-part-angle-air-suction-direction.jpg


Hydraulic pressure on the right side will push the sealing edge against the shaft improving the seal. Any suction on the right side will tend to reduce the pressure on the seal and let air suck in.
 
Several years ago I had the left side brakes fail. I pulled the two left brake cylinders and replaced the O-Rings in them. I bought a bunch of AN plugs and caps and plugged or capped every how I took off. Once everything was back together a friend helped me bleed them. We did it the old fashioned way and it took a while to get it done. Definitely using the parking brake to pump things up helps since it is on both brake circuits. Haven't had any problems since.
 
Tom-D said:
The Mechanic ---> Should have removed the filler cap.. many of the pipers have a small can as a reservoir it does bend easy.

I have modified a garden sprayer to fit a small clear plastic hose that can be attached to the brake cylinder and simply back filler the reservoir. that brings the air with the new fluid. 3-4 psi is all it takes. we catch all over fill of course.
Click to expand...
Corrected for Clark :)
 
Clark1961 said:
Long story made short once air starts getting in the system through cylinder seals just bite the bullet and rebuild all the cylinders...may as well replace the 40 year old hoses while they are off. Chances are they are rather stiff.
Click to expand...

I repeatedly bled brakes on a couple of aircraft until I learned this. Now I'll give them one shot at bleeding, if they need to be bled again shortly after that I'm going to rip every cylinder out of the brake system and replace or reseal them. Doing so has cured almost all of the impossible to bleed brake problems I've experienced since.
 
Timbeck2 said:
I made my own for about $20 including the on/off ball valve with a garden sprayer from Home Depot.
Click to expand...

Me too. Took all of 10 minutes to bleed the brakes when I built the aircraft, and 2.5 years later they're still fine.

We shouldn't wear our brake pads excessively, but....it's poor caliper design when highly worn pads allow the piston's O-ring to pop out of the caliper bore. With every automotive application I can think of, you can wear the brakes down to the backing plates and the pistons won't pop out/O-rings remain seated. ("Full metallic brakes" at that point. ;))

Of course, with proper annuals you're going to catch worn pads before the rivets start to gouge the rotors.
 
SoCal RV Flyer said:
Me too. Took all of 10 minutes to bleed the brakes when I built the aircraft, and 2.5 years later they're still fine.

We shouldn't wear our brake pads excessively, but....it's poor caliper design when highly worn pads allow the piston's O-ring to pop out of the caliper bore. With every automotive application I can think of, you can wear the brakes down to the backing plates and the pistons won't pop out/O-rings remain seated. ("Full metallic brakes" at that point. ;))

Of course, with proper annuals you're going to catch worn pads before the rivets start to gouge the rotors.
Click to expand...
Just a word to the wise. If ya git the brakes so hot that ya can smell them such as an aborted take-off on a short runway, inspect them afterwards...not a lot of meat on the pads...
 
I didn't mention removing the cap but its always a good idea. On the other hand if your mechanic blew out your reservoir its because the vent was plugged. That could have been your problem from the beginning. I normally have someone inside watching the masters to make sure we don't overfill and pump fluid all over the floor. Pressure isn't high and doesn't need to be. If the reservoir can't handle 3-5 psi it needed to be replaced anyway.

We've "pressure" bled piper bladder brakes like those on the J3 but we use a small handpump which allows some feel and still low pressure. No need to blow the bladders out which can happen. I hate those brakes......

My 41 Taylorcraft had chinn cable actuated brakes and they were more pilot pacifiers than actual brakes. You could normally use them without worry....they wouldn't flip the plane if you over did it but sometimes would hold the plane for runup. I kind of like them....no or low maintenence since they didn't really work anyway......

Frank
 
My bleeder bomb and return hose.
Cessna singles have a 1/4" pipe plug in the top of the master cylinder my return tube screws into that port, normally the filler port. the steel fitting is the adapter for the piper single cylinder type brake master cylinder it is 5/16 x28 thread.
my pressure sprayer came form ACE it is just a 1 Qt size. It pumps up really easy. and works for spraying down the engine.
 

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