Air in brake line

[Flyyak]

RV7 condition inspection completed a couple of months ago. Checked, cleaned and bled brakes. Have flown about 20 hours over the last several months. No issues that I noticed. Was checking a issue with wiring, looking under the panel and sub panel when I noticed air in the line for my left brake. No leaks located on the floor, along the line(s) or on on any of the brake master cylinders. No leaks at either brake assembly or around the brake reservoir on the firewall.

Re-bled the brakes, all air removed from the lines. Checked the brakes by pumping the brakes several times and noticed air in the lines and brake fluid leaking through the brake reservoir top nut. No leaks around any master cylinder, around brake assemblies or any brake line. Only air in the left brake line.

I have rebuilt one master cylinder about two years ago, not that hard but I am not as flexible as I once was and it resulted in a real mess in the cockpit floor area.

I am baffled since there has been no evidence of leaking fluid in or around the RV. Any suggestions on what to check other than removing and rebuilding the master cylinder?

 

[gasman]

RV7 condition inspection completed a couple of months ago. Checked, cleaned and bled brakes. Have flown about 20 hours over the last several months. No issues that I noticed. Was checking a issue with wiring, looking under the panel and sub panel when I noticed air in the line for my left brake. No leaks located on the floor, along the line(s) or on on any of the brake master cylinders. No leaks at either brake assembly or around the brake reservoir on the firewall.

Re-bled the brakes, all air removed from the lines. Checked the brakes by pumping the brakes several times and noticed air in the lines and brake fluid leaking through the brake reservoir top nut. No leaks around any master cylinder, around brake assemblies or any brake line. Only air in the left brake line.

I have rebuilt one master cylinder about two years ago, not that hard but I am not as flexible as I once was and it resulted in a real mess in the cockpit floor area.

I am baffled since there has been no evidence of leaking fluid in or around the RV. Any suggestions on what to check other than removing and rebuilding the master cylinder?

After you complete bleeding the brakes, suck out the fluid in the reservoir to lower the volume by about 3/4 of an inch from the top. If you establish the exact depth of the fluid after installing new pads, the drop in fluid (without any leaks) will show you the progressive wear of the pads.

If the bubble is before your left master, and you have dual brakes, then do a pumping test from the passenger side and watch for added bubbles in the line that goes to the pilot left master. The passenger master may be at fault. If rarely used, is subject to failure.

 

[Flyyak]

it was suggested by another A&P RV guy to cut the ferrule from my nylon line and replace the ferrule to make sure no crack/air leak at that point. At 28* it was a challenge to replace the brass insert in the end of the line. I can barely get under the instrument panel at my age much less squeeze/drive the insert in a nylon line at 28*.

Still had a leak of air. That necessitated removing the master cylinder and rebuilding with new o-rings. I had rebuilt this master cylinder in 2021. This time I polished plunger with 2000 grit to make sure the o-rings sealed good.

So far that did the trick. No leaks of air or fluid.

 

[jeffreyuh60]

For what it's worth, I had troubles bleeding my Beringer brakes until I discovered how to bleed them properly. Beringer brakes must be pumped with the cylinders horizontal. This may, or may not work for you, but it may be worth a try.

 

[lr172]

Assume these are matco or cleveland MC's. Further assume the air pockets are appearing in the upper input line. If you bleed them and then after some use they slowly develop air pockets in the upper lines, that means you have a failed seal(s) in the MC. It draws in a small bit of air via the upper oring on the down stroke and releases it into the input line on the up stroke. This can all happen without ANY leakage of fluid. Sorry, but time for another MC rebuild. The oring at the top of the MC MUST seal well to prevent this. The rod can get scratched up, especially if you put in helper springs, and it is critical that you put that rod in a drill and work with 400 or 600 grit sandpaper to get it polished before reassembly.

Not to insult you, but do you by chance have automotive brake fluid in the system? I have surprisingly seen it before and it will trash the rubber seals in a relatively short amount of time relative to the hydrocarbon based stuff. Glycol based fluids require a special type of rubber.

Larry