Break Bleed Questions

phapp · #1 09-08-2022, 04:34 PM

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.

D-Dubya · #2 09-08-2022, 05:25 PM

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.

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.

gasman · #3 09-08-2022, 07:48 PM

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.

Kyle Boatright · #4 09-08-2022, 08:31 PM

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.

rocketman1988 · #5 09-08-2022, 09:30 PM

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.

lr172 · #6 09-09-2022, 08:23 AM

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

bjdecker · #7 09-09-2022, 12:06 PM

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)

Taltruda · #8 09-09-2022, 12:26 PM

Quote:

Originally Posted by bjdecker

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!

LouFly · #9 09-09-2022, 01:18 PM

Quote:

Originally Posted by Taltruda

I love Locktite 567.. it’s the high temp version of 565. I use it in all tapered pipe threads.

I have been using 567 and wondering if there was a downside to that in lieu of 565.

rockwoodrv9 · #10 09-09-2022, 04:55 PM

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