Hi, How can I find the source of the air leak into the brake lines?
I have to bleed the right front and left rear brake lines about every week, as I keep finding air in the calipers when I open the bleeder screw. It is affecting the way the car drives and the left front brakes are running hotter and producing brake dust more than the right front.
I have replaced the caliper on the left front that is running hot, all four brake hoses, and the master cylinder.

 

Did you perform the master cylinder bleeding procedure in the FSM?

 

Bleed it, someone press the pedal while someone else listens for a hiss of air/fluid. Probably rubber hose right behind one of the tires

 

I did bleed the master cylinder using a bleed kit that came with it, before installing it. Anyways the problem existed before replacing the master cylinder so it's not like it created a new problem.

 

Ambient pressure air does not magically introduce itself into a closed system with a higher pressure. So, "anyways", I have no idea. Good luck.

 

Suppose I did not get all the air out of the master cylinder. This is possible as it is awkward operation for one man. That I still get air migrating to the calipers, it would either be coming from master cylinder and solve its own problem, or from somewhere else in the closed system that may experience temporary vacuum at a leaky point (just theorizing). I should add the brakes don't feel especially spongy but they do get a little harder after bkeeding, so it is not a lot of air but enough to cause problems.
Supporting my theory of air entering the system, what if brake fluid cooled from a heated state and there was some water in the system that vaporized, then a temporary vacuum would be created as the steam condensed.

 

Yes, that's why you are supposed to replace the brake fluid every few years.

 

that sucks. you don't feel any fluid leaking from any of the connections including the bleed valves and calipers?
i would say you didn't bleed the MC correctly but if there was non stop air before that doesn't matter.

 

I'm taking seriously the MC suggestion and I should try bleeding it on the car. After that I will get a pressure bleeder to look for leaks.

 

this is how i did mine for the bmw.
you must pump it very very slowly it took a long time to get everything out of it because
the little bubbles will stick to the inside. Even hitting the brakes and mc with a hammer works to knock them loose.

by https://www.flickr.com/photos/156811228@N08/, on Flickr

 

So, if it's the MC then it must be storing air where it doesn't get pressurized like not in line with the piston because the brakes improve a bit after bleeding and then degrade after a day or so.
By improve, I mean the drivability improves and I think the pedal is slight firmer but I could be mistaken. By degrade I mean the driving gets choppy and there is more heat generated at the left front wheel.

 

could be
it's worth trying maybe the booster isn't pushing the mc all way completely in where the bubbles are.
Take it off and do it on vice with a screwdriver. its a ***** but so is your brake situation
but remember to literally push it in in slow motion you will see little micro bubbles going up the line
where just pumping it doesn't get them out.

 

I could do it on the car with speed bleeders at the calipers because the lack of pressure will allow the piston to travel all the way. Up to now I have been using the 2x4 and power seat method building up pressure and then opening the bleeder screw.
I will put speed bleeders on both front calipers being a dual system, and slowly pump away a whole litre of brake fluid.

 

Seems as if air is getting in then fluid must be coming out. Unless it is in a non-pressurized area. I don't know how it could happen. I'm no brake pro but nothing can get in if nothing gets out.

 

I will do visual inspection for fluid and if no fluid then invest in a power bleeder to pressurize the system and look for leaks. But first I'm hearing on this forum that most likely there is air in MC from the time it was installed. .

 

Is it true that air in brake lines can cause caliper drag or am I chasing a red herring?
It is true I am having an issue with air in brake lines. I discovered an undertorqued brake line in the master cylinder corresponding to the side getting air. I can't properly bench bleed my master cylinder due to unremoveable plastic mesh in fill opening. It is messy and not completely gets air out. So I am driving and bleeding until the air works it's way down the lines.
I'm not convinced the caliper drag will be completely go away with full removal of air as generally that is not recognized as a cause, but in this case appears to be an aggravant based on my driving experience. Next step would be to question the ABS control unit or the proportioning valve or brake booster because I've replaced everything obvious.Perhaps I should disable brake booster just to test.