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Can't get air out of brake system

Hello all,

1991 Subaru Loyale 4x4 wagon

I just did a bunch of front-end work that required removing the brake calipers. After hooking everything back up, I keep getting air in the brake system at all four wheels when I bleed the brakes. I bleed them until there's no air, then come back a couple of minutes later and there's air again. I bled them diagonally, as the shop manual instructed. I haven't bled the master cylinder yet because the bolts on the brake lines are a major PITA to get loose and, more importantly, the more I work on this, the more convinced I am that air is getting in.

The thing that has me thinking this is that I have bled enough air out of the system over the last three days to fill the entire system at least once, no kidding, not to mention the quart of fluid I have put through it. I can't imagine that much air could be trapped in there. My understanding of hydraulic systems is that air can get trapped and compromise the system, but I have a continuous supply of air bleeding out. At some point the air has to stop coming out if air is not getting in, even if an air pocket remains trapped in the system.

Two thoughts: 1) I replaced the copper sealing washers on the banjo bolts where the brake lines connects to the front calipers. I couldn't remember for sure, but I thought that one washer went on each side of the banjo bolt. Any chance that could be wrong? 2) Maybe it's just a strange coincidence and the master cylinder has a leak. I haven't found any visible leaks so far. Any thoughts or, better yet, does anyone know how to test this? I'm curious to know what other people think.

Reply to
Rick
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|Any chance that could be wrong? 2) Maybe it's just a strange coincidence |and the master cylinder has a leak. What often happens in brake bleeding, using the classic 2-person "PRESS... HOLD. OK, Pump again! (repeat)" is that the pedal goes all the way to the floor with the inside person pressing hard, and the MC piston bottoms out, in the part of the bore that isn't made for it. Rubber seals get damaged. Then every time you try to bleed (using the MC again) it bypasses on you and lets air in.

I generally use gravity or vacuum bleeding, just to save the MC.

Rex in Fort Worth

Reply to
Rex B

I've never seen this happen, though I can't say that it couldn't. It just doesn't seem that a MC would be designed as such that bottoming out would damage something.

Anyhow, to the OP, I've normally never changed the copper washers on the banjo connectors. If there was air getting in at those connections, you would also see brake fluid coming out when the pedal is pressed.

It sounds like you haven't bled the system enough. Just keep filling the MC and allow the fluid to continue to bleed out. Follow the correct sequence (farthest from the MC first to closest to MC) and soon enough, you should be rid of the air in the system. If the brake fluid in the MC is churning the brake fluid too violently, air will get in it and it'll get sucked into the system.. Maybe you're pumping up the pedal too quickly?

Lastly, make sure the assistant in the car that's pumping the pedal is holding the pedal to the floor until you tell them to pump up the pedal again. If they don't, it'll just suck air right back up into the system.

Good luck,

-Bruce

Reply to
Bruce Chang

|> the part of the bore that isn't made for it. Rubber seals get damaged. |> Then every time you try to bleed (using the MC again) it bypasses on you

|I've never seen this happen, though I can't say that it couldn't. It just |doesn't seem that a MC would be designed as such that bottoming out would |damage something.

I would agree, it would be poor design. More likely is that travel exceeds the ordinary, and gets into a part of the bore that has been undisturbed and perhaps has corrosion.

|Anyhow, to the OP, I've normally never changed the copper washers on the |banjo connectors. If there was air getting in at those connections, you |would also see brake fluid coming out when the pedal is pressed.

If you re-use the washers, you should anneal them by heating to red-hot, then allowing to cool.

|Lastly, make sure the assistant in the car that's pumping the pedal is |holding the pedal to the floor until you tell them to pump up the pedal |again. If they don't, it'll just suck air right back up into the system.

Recommend a 1" wood block for a pedal overtravel stop. Rex in Fort Worth

Reply to
Rex B

If your really having that much trouble then consider having it pressure bled. It's fast and it works great.

Reply to
Childfree Scott

Very common on old MCs that have not seen regular fluid changes. I'm actually quite surprised that you haven't seen it, I'm just a "weekend warrior" and I've seen it quite a few times.

I agree, but I would change them on principle.

nate

Reply to
Nate Nagel

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