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Break Bleed Questions

phapp

Active Member
Patron
Hi,
-8, Matco masters and Cleveland calipers. I know about the check-valve. Same both sides.

At home before moving my RV-8 to the airport, I had the brakes bled and a firm pedal (pumped fluid up from the calipers). At the airport a year later I cannot get pressure build up at all even though I can see no air bubbles in the clear reservoir lines. I do have the tail up on a stand and the fuse level.

Is it possible to have an air leak without any fluid leaks? Could stuck caliper pistons be the issue (really, both?) ? Should I be tail down? Might I need to pull and rebuild the masters if they can leak air and not fluid?

Looking for the Occam's razor solution here even if it's the most labor intensive.

I was hoping to upgrade the insurance to "in motion" and do the first start and fuel flow test (and taxi) before snow flies. But not without brakes.

Thanks for any advice.
 
Need Some Clarification

When you say you can't get any pressure build up, what is your actual indication of that? Is the pedal spongy? Do the brakes not hold when you push it around the taxiway?

To address some of your questions, I would think if you had an air leak, you would see fluid. And it shouldn't matter if the tail is up or down. Unless you have the spinner in the dirt. You have much bigger problems if that's the case.:eek:

I believe I have the original Cleveland master cylinders that came with the kit, and their operation is a little different than the Matco cylinders. So I may not be too much help here. But surely the group can figure out something for you to start looking at.
 
The only time you will get air in the system after the initial bleed, is to wear out both pads and have a fluid leak at a fitting. Air can't leak in, only fluid can leak out. If you don't have a fluid puddle.... you don't have a leak.

There is no check valve in the brake system.... so I don't understand that statement.
 
I had an extremely frustrating time bleeding the brakes on my -10. I was using a pump up sprayer to force fluid in from the bottom up. I kept getting bubbles in the lines.

Turned out that the way my pump-up sprayer worked, when you pumped it, it created air bubbles in the fluid in the tank. The more you pumped up the tank, the smaller the bubbles were (the old PV=NRT thing). When you pumped the fluid into the lines, the bubbles expanded and made things a nightmare.

Going forward I'll use a syringe or pump oiler to bleed brakes.
 
Mity vac

I used a mighty vac attached to the reservoir. Put a jar of fluid at the bleed screw, open the screw and the fluid moves from the caliper to the reservoir with zero bubbles. Easy peasy.
 
Air can get introduced in different ways or it may not be air at all. A defect or failed seal in the master cylinder can also prevent the brake system from building pressure, without fluid leaking out of the system. Not sure there is a magic bullet answer for you here. You are going to need to dig in and figure out the issue. If it were me, I would repeat the bleeding process and see what results. While you can see the upstream side of the system, everything downstream from the MC is not visible so you have no idea what is in there.

Larry
 
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Sealant

Check all those NPT (or ANPT) fittings for the correct sealant.

Do not use EZ TURN - FUEL LUBE on NPT fittings (kicking myself repeatedly). It is an anti-seize/lubricant compound and will NOT seal NPT threads.

Only use Permetex #2, Locktite 5xx series (565 is what Van's recommends, but 567 and 592 are ok as well)
 

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Check all those NPT (or ANPT) fittings for the correct sealant.

Do not use EZ TURN - FUEL LUBE on NPT fittings (kicking myself repeatedly). It is an anti-seize/lubricant compound and will NOT seal NPT threads.

Only use Permetex #2, Locktite 5xx series (565 is what Van's recommends, but 567 and 592 are ok as well)

I love Locktite 567.. it’s the high temp version of 565. I use it in all tapered pipe threads. It’s amazing how many people still use fuel lube and think they are using the correct thing.. eventually the fuel lube WILL leak. Like you said, it’s a lube, not a sealant!
 
Bleeding the breaks is one of the things I really hate. I have not had much luck and it has taken me days to get them correct. Having the clear tubes may help or not. I’m not sure. Maybe the braided lines hide some of the bubbles that probably don’t matter!

I had best luck with drip from the top and vacuum pressure from the bottom.
Good luck
 
Bleeding

I think the biggest issue with bleeding the brakes is leaking connections. Use Loctite 567 or similar on all NPT threads.

If you pull a vacuum on the reservoir (high) side, bubbles with naturally migrate there (from High to Low). Providing a vacuum at the top, or even the bottom, is using atmospheric pressure to move the fluid. Consider a 4" diameter container full of fluid. Now calculate the surface area of that fluid exposed to the atmosphere. Multiply that area by the atmospheric pressure. Draw a vacuum on the other end of the system and the atmospheric pressure, acting on the exposed surface area of the fluid will force the fluid towards the vacuum. Pretty simple, fluid goes where it is supposed to and any air bubbles migrate to the low pressure area.

If you cannot get a bubble free stream of fluid, you have a leak somewhere.

It took about 15 minutes to bleed the brakes on my -10 and there is no spongy feeling in the pedals at all...
 
Using a technique I learned here on VAF, I pump up from the bottom. Just use an old-fashioned oil can pump with some silicone rubber tubing to the bleeder valve, and make sure it seals ok.

Then I connect a hose to the reservoir with an NPT to hose fitting and run another silicone hose down to a catch can, and pump until there are no bubbles.

This keeps the system under pressure.

I'm using DOT5.1 with all the appropriate EPDM rubber o-rings and seals, and need to replace the fluid regularly due to its hygroscopic nature.

http://www.rv8.ch/upgrading-brake-o-rings/

The main negative of DOT5.1, besides the o-ring change, is that it does not like paint, so you have to be very careful when working with it - just like with your car.
 
I'm using DOT5.1 with all the appropriate EPDM rubber o-rings and seals, and need to replace the fluid regularly due to its hygroscopic nature.

http://www.rv8.ch/upgrading-brake-o-rings/

The main negative of DOT5.1, besides the o-ring change, is that it does not like paint, so you have to be very careful when working with it - just like with your car.

Hey Mickey -- what drove you to use DOT5.1 -vs- MIL-PRF-83282 or other? Availability or ?
 
+1 for drawing a vacuum from the top. Clear tube into a jar of clean fluid at the caliper, crack the bleeder (make sure the threads are sealed) and start sucking. Mity Vac, siphon solvent sprayer, shop vac, air nozzle blasting downstream of the open hose, or even by mouth... Does not take much. Just do it long enough for no bubbles at the top and you are done. Easy, easy, easy.
 
Solved

Vacuum from the top did not work for me. I went back to pumping from the bottom and now have both pedals hard and brakes working.

For the next person:
1) It takes a lot of patience.
2) It takes a lot of fluid. My A&P suggested I might use all of a quart. I used about 1/2 - 3/4 quart. Mostly waste and due to how many unsuccessful attempts I went through.
3) A lot of pumps. My pump oil can has a very small orifice, but a moot point since it's about the same size as the bleed nipple. It took about 100 - 200 pumps. The reason I counted is because I couldn't see the reservoir overflow tube I set-up. So I checked every 25 pumps.

Thank you all for your help.
 
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but...

"...Vacuum from the top did not work for me. I went back to pumping from the bottom..."

I would be curious to know why the vacuum didn't work...in reality, either method does exactly the same thing.

The only difference is the motive force; pumping a tiny diaphragm 200 times or using atmospheric pressure applied over a much larger surface area.
 
Agree. The vaccum from the top "has to" work as long as you don't introduce more air between the jar on the floor and the reservoir at the top of the system. This air path is often the bleeder screws themselves. They only seal with a mechanical cone/seat and only when tight. The instant that cone comes out of the seat, there is an air path through the threads. The way to combat this is to seal the threads with teflon or something. When pressure bleeding from the top, this leak path is irrelevant, but when trying to draw a vaccum, this has to be air tight, even with the bleeder off the seat.
 
When pressure bleeding from the top, this leak path is irrelevant, but when trying to draw a vaccum, this has to be air tight, even with the bleeder off the seat.

This is why I prefer pressure from the bottom vs vacuum from the top. The former forces fluid past the threads, where the latter suck air in through the threads. I made a bottle with a cheap pressure regulator and 10 PSI seems to work well and not enough pressure to blow the tubing off the bleeder.

Larry
 
leaks

I put loctite 567 on the bleed screw threads and had absolutely no issues. Brakes have been rock solid for over two years...
 
2 Thumbs Down on the PermaTex

Check all those NPT (or ANPT) fittings for the correct sealant.

Do not use EZ TURN - FUEL LUBE on NPT fittings (kicking myself repeatedly). It is an anti-seize/lubricant compound and will NOT seal NPT threads.

Only use Permetex #2, Locktite 5xx series (565 is what Van's recommends, but 567 and 592 are ok as well)

I had leaks that wouldn’t stop with the PermaTex #2.
Ended up draining brakes, removing/cleaning all fittings, master cylinder threads, caliper cylinder threads and reassembled the system with the 567 sealant. This was way early when the KAI recommended Fuel Lube and or “sealant” without being specific.
 
I used a mighty vac attached to the reservoir. Put a jar of fluid at the bleed screw, open the screw and the fluid moves from the caliper to the reservoir with zero bubbles. Easy peasy.

Yes!!! I tried the garden sprayer method and it failed miserably. Although Harbor Frieght makes OK stuff like this, the garden sprayer ended up failing with the brake oil. I think the seals were not designed for it. Anyways, I went to my backup of my mityvac I use for EVERYTHING else that has hydraulics. Did the regular method of a reservor on top with fresh fluid in it and the vacuum on the bottom at the bleed screw. Guess what? It worked just like it would in a car, truck or rv. Turns out just because it's an airplane doesn't mean you can't use the methods used on vehicles.
 
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