bleeding brakes

[Jerry Pressley]

I realize this may have been covered a number of times before but I cannot find it.  Having trouble bleeding the brakes on a 68 F.   Tried the bottom up using oil can to push upwards. pressure pot didn't work. Tried the top down as per bleeding car brakes. Get a good pedal but 10 minutes later pedal goes to the floor.  no leaks out the bottom anywhere. Other than this the plane is ready to sell. 

[Tom]

When is the last time that the brakes were known to operate effectively?  What changed since then?  When were the cylinders last individually serviced?

If the bottom up method was done correctly the only reason I can see that the pedal would go to the floor would be spent rings in the master cylinder.  Not a fan of the top down approach, but the same ring problem would give the same symptom. Assuming there are no line leaks.  With respect, I assume that your bottom up method was inadequate for some reason, giving you less (short term) performance than your top down method.

FWIW numbskull hangar elves have been known to substitute DOT fluid which can cause multiple problems, including ring issues. 

[MooneyMitch]

1 hour ago, acpartswhse said:

I realize this may have been covered a number of times before but I cannot find it.  Having trouble bleeding the brakes on a 68 F.   Tried the bottom up using oil can to push upwards. pressure pot didn't work. Tried the top down as per bleeding car brakes. Get a good pedal but 10 minutes later pedal goes to the floor.  no leaks out the bottom anywhere. Other than this the plane is ready to sell. 

Do you have pilot and co-pilot brakes?  If so, I’d suspect shuttle valve O-ring leak.

[RobertGary1]

Anyone who has done it has a lot of sympathy. There is no magic formula. I spent some time on the phone with Lake Aero and they kind of laughed and said its always a struggle. Even when you think you have it right a bubble will migrate and the customer will come back. They often disconnect the lines at the master cylinders and bleed it with a cap on the line to create back pressure.

What works best for me is to go fly it, pull into a 20 degree pitch up and pump the brakes and the parking brake. Then fly around with full rudder both directions and do the same. Eventually it seems to find the bubble. 

But be wary of anyone here who says "oh just do this" because even the pros say its not straight forward.

-Robert

[Yetti]

I have found both sides after replacing the flexible brake lines and rebuilding the master cylinder on one side to be straight forward.      Suck it down from the top.   Pump the brake pedals some.   Then with oil can and rapid pumping with someone watching for bubbles.   We had to pump the oil can 20 times which will usually do it for one side and 40 times on the other side.

 

 

Edited July 23, 2019 by Yetti

[Yetti]

 

[N201MKTurbo]

2 hours ago, RobertGary1 said:

Anyone who has done it has a lot of sympathy. There is no magic formula. I spent some time on the phone with Lake Aero and they kind of laughed and said its always a struggle. Even when you think you have it right a bubble will migrate and the customer will come back. They often disconnect the lines at the master cylinders and bleed it with a cap on the line to create back pressure.

What works best for me is to go fly it, pull into a 20 degree pitch up and pump the brakes and the parking brake. Then fly around with full rudder both directions and do the same. Eventually it seems to find the bubble. 

But be wary of anyone here who says "oh just do this" because even the pros say its not straight forward.

-Robert

I have found this to be true also. I’m not sure the maneuvering does anything, I have always thought it was the altitude. The lower pressure causes the bubbles to grow and migrate easier.

Whatever is going on, flying and pumping seems to finish the job.

[Tom]

12 hours ago, RobertGary1 said:

Anyone who has done it has a lot of sympathy. There is no magic formula. I spent some time on the phone with Lake Aero and they kind of laughed and said its always a struggle. Even when you think you have it right a bubble will migrate and the customer will come back. They often disconnect the lines at the master cylinders and bleed it with a cap on the line to create back pressure.
...
But be wary of anyone here who says "oh just do this" because even the pros say its not straight forward.

This is misleading.  The military and airlines don't do a provisional bleed, send the aircraft up to bounce around, then finish the job on the ground with a second go.

There are numerous A&P articles and threads on the internet on this subject.   What is straightforward is that, if you have a healthy cylinders, valves, and connections, you cycle air-free fluid from the bottom-up, preferably at a higher rate of speed (pressure pot or pump device) to push/drag air up, such that the air should be evacuated.  By cycle I mean technically overflow the brake system with a catchment system off the reservoir with several quarts going up and through.  A helper jiggling/tapping along the line and connections helps.

This is ordinarily a bit more complicated, requiring a bit of contraption set up, than a "fill up till full, pump 'em a few times, seems to be holding" approach that many an A&P will do the first time touching the brakes.  Often that's all that is needed.  But when it comes back, you'll hear "you know, them brakes, they can be finicky."  My first plane was a high speed homebuilt with a castoring nosewheel.  Zero tolerance for "come backs."

It's a simple hydraulic system, not rocket science.
 

[MB65E]

I’ve had good luck retracting the gear and then bleeding them. Little harder but it works.

Piper retracts with right side brakes and that stupid parking break we’re always a challenge.

Recently had bad luck with a 182 that I couldn’t get air out of. Found a pin hole in my pressure pot too. Dang!!

-Matt

[DonMuncy]

My admittedly ignorant take is that there could easily be a bubble in the brake system that migrates rather than being pumped out. If one is pumping in from the bottom at a high enough rate (ie. the right pressure), it might push the bubble faster than it can migrate. I would certainly try increasing the pressure. Someone suggested a pressure of 15psi.

[Shadrach]

9 hours ago, Tom said:

This is misleading.  The military and airlines don't do a provisional bleed, send the aircraft up to bounce around, then finish the job on the ground with a second go.

There are numerous A&P articles and threads on the internet on this subject.   What is straightforward is that, if you have a healthy cylinders, valves, and connections, you cycle air-free fluid from the bottom-up, preferably at a higher rate of speed (pressure pot or pump device) to push/drag air up, such that the air should be evacuated.  By cycle I mean technically overflow the brake system with a catchment system off the reservoir with several quarts going up and through.  A helper jiggling/tapping along the line and connections helps.

This is ordinarily a bit more complicated, requiring a bit of contraption set up, than a "fill up till full, pump 'em a few times, seems to be holding" approach that many an A&P will do the first time touching the brakes.  Often that's all that is needed.  But when it comes back, you'll hear "you know, them brakes, they can be finicky."  My first plane was a high speed homebuilt with a castoring nosewheel.  Zero tolerance for "come backs."

It's a simple hydraulic system, not rocket science.
 

What’s funny here is that 1) you’ve never bled a Mooney brake system.  2) You’ve made comparisons to the braking systems of military and transport category aircraft. 3) And then finished up with the textbook method for bleeding brakes...

To be honest the post comes off as a touch pedantic. I assure you that one can bleed the brakes with a pressure pot from the bottom up into an overflowing catch can and still end up with a soft pedal even after repeated attempts

It’s my opinion that the design would have benefited from more judicious use of hard lines. I think that over time and use, the soft lines develop high and low spots in between the calipers and the brake cylinders. 

@acpartswhse  now that it’s been bled from the bottom up.  I would recommend reducing the fluid level in the reservoir to about 2/3rd full and apply vacuum to the reservoir with the vent sealed. A MityVac is probably easiest for the job but other tools could be modded to work.  A coolant vacuum purge kit (rentable from most auto parts stores) may work as well but look at it before renting to see if you can make a good seal. Once a vac is attached, draw system down to ~ -18 to 20inHg and have someone actuate the brakes and parking break for a minute or so. This should eliminate any stubborn air bubbles from the system. 

Edited July 24, 2019 by Shadrach

[EricJ]

8 hours ago, DonMuncy said:

My admittedly ignorant take is that there could easily be a bubble in the brake system that migrates rather than being pumped out. If one is pumping in from the bottom at a high enough rate (ie. the right pressure), it might push the bubble faster than it can migrate. I would certainly try increasing the pressure. Someone suggested a pressure of 15psi.

Automotive calipers put the nipple at the top of the caliper, so that any air in the caliper sits there and if you open the nipple the air is the first thing to come out if you bleed, or at least start to bleed, from the top down.    I don't know why, but GA calipers put the nipple at the bottom of the caliper, so it's much harder to work air out of the system from either direction, top down or bottom up.

In my experience with race cars one reason to not use a pressure (bottom up) bleed is that any debris that has settled into the caliper gets blown back up, potentially into the master cylinder where it can damage the seals.   On a car you're also worried about blowing crap back up through the ABS system, but that's not so much of an issue with GA airplanes.

So, yeah, it's harder to get air out of a GA airplane brake system, as simple as they are, because the nipple is on the other end of the caliper compared to a car.

 

[Jerry Pressley]

tried flying today holding just off stall with gear up then down. Steep climbing turns, straight and level with gear up and down,  when cramps sat in from pumping, landed and breaks goos but will they still be good tomorrow?  will add then.

 

 

[RobertGary1]

4 hours ago, acpartswhse said:

tried flying today holding just off stall with gear up then down. Steep climbing turns, straight and level with gear up and down,  when cramps sat in from pumping, landed and breaks goos but will they still be good tomorrow?  will add then.

 

 

Try some uncoordinated flight too. It might help or at least help rattle out any loose screws hiding under your panel.

-Robert

Edited July 26, 2019 by RobertGary1

[FastTex]

I'm facing a similar issue with my left pedal. I need to push my foot all the way down in order to break. Based on you guys experience is overhauling a master cylinder an option? or don't bother and "just" buy a new one? It has been bled multiple times...

[Yetti]

It's 3 or 4 standard O rings depending on the cylinder you have and a hour for R&R

[Shadrach]

5 hours ago, FastTex said:

I'm facing a similar issue with my left pedal. I need to push my foot all the way down in order to break. Based on you guys experience is overhauling a master cylinder an option? or don't bother and "just" buy a new one? It has been bled multiple times...

The main reason to overhaul is a leak. If it's going to the floor, it has air in the system, overhauling the brake cylinder is not going to make bleeding easier.

[FastTex]

27 minutes ago, Shadrach said:

The main reason to overhaul is a leak. If it's going to the floor, it has air in the system, overhauling the brake cylinder is not going to make bleeding easier.

Can the air get in because the master cylinder is defective? O-rings have been changed and bleeding has been performed multiple times...

[Shadrach]

15 minutes ago, FastTex said:

Can the air get in because the master cylinder is defective? O-rings have been changed and bleeding has been performed multiple times...

If fluid is not leaving the system under braking, I cannot envision a scenario where air is entering. Try applying vacuum to the reservoir for a few minute while actuating the brakes. 

[Yetti]

1 hour ago, FastTex said:

Can the air get in because the master cylinder is defective? O-rings have been changed and bleeding has been performed multiple times...

Pull the panel behind the nose wheel.   That is where the brake fluid goes if you have a master cylinder leaking.   It's the hardened black stuff vs. the gooey engine oil.

Read though @Marauderthreads on brakes.  There are three of them

Step one is to see what the level is in the reservoir

 

[Bolter]

7 hours ago, FastTex said:

I'm facing a similar issue with my left pedal. I need to push my foot all the way down in order to break. Based on you guys experience is overhauling a master cylinder an option? or don't bother and "just" buy a new one? It has been bled multiple times...

 

1 hour ago, FastTex said:

Can the air get in because the master cylinder is defective? O-rings have been changed and bleeding has been performed multiple times...

Follow the goo trail.  I had a slow loss of brake pressure over months.  Turns out I had multiple intermittent  leaks, including the master cylinders.  O-ring's nicked in a master cylinder.  Replaced the o-rings, polished the actuator rod, and they worked like new (after replacing 2 lines, as well).  My point is that overhauling the master cylinder is easy but you can have other leaks to chase down.  O-rings in the brake cylinders at the wheel, included.   

-dan

[N201MKTurbo]

OK, I lucked out. I just changed my slave cylinder O-rings yesterday. I capped off the brake lines when I took the calipers off. I filled the cylinders with fluid before I put them back on and reattached the brake lines with little loss of fluid. I pumped them up and they were soft. I gave 30 pumps with my oil can on each brake and they are both solid as a rock! 

So, sometimes it is easy!

[PT20J]

1 hour ago, N201MKTurbo said:

OK, I lucked out. I just changed my slave cylinder O-rings yesterday. I capped off the brake lines when I took the calipers off. I filled the cylinders with fluid before I put them back on and reattached the brake lines with little loss of fluid. I pumped them up and they were soft. I gave 30 pumps with my oil can on each brake and they are both solid as a rock! 

So, sometimes it is easy!

I did the same thing. My mechanic said to just let the fluid drip from the brake line for a while through bleeder.  I noticed that it was discolored (fluid had been replaced 2 years ago) and let it drip until it was the color of new fluid. Closed the bleeder and pumped the brakes a couple of times and all was good. Mechanic says sometimes this works (sometimes not) but it’s good to try it first.

Skip

[RLCarter]

I use an oil squirt can as well, works just fine but keep it full or you'll pump more air into the system. I use a clear line between it and the  bleeder so I can see if air is being introduced 

[RobertGary1]

1 hour ago, RLCarter said:

I use an oil squirt can as well, works just fine but keep it full or you'll pump more air into the system. I use a clear line between it and the  bleeder so I can see if air is being introduced 

I’ve done that but it depends on how clear the lines are. If you have old fluid in there you want more pressure to push it through as it’s a bit gelatinous. 

-Robert