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Brakes - Air in line no leaks

We’ve been having a lot of issues with the brakes on a 2000 Dodge Caravan. Within the last year it has been taken into several mechanics for the brakes not working properly. A new master cylinder and a new brake caliper have been put in along with multiple bleeding of the brakes. When I looked at the car I very quickly found that there was air in the lines as the peddle went to the floor. I put it on jack stands and had another person press the brakes as I checked to see if I could spin the tires. Two of the tires, the front driver and rear passenger would spin with the brake pressed. When the brake was pumped several times the brakes would apply for both tires and I couldn’t spin them.

I bled the front brake and now the all four tires stopped (brake would apply) with one press. All seemed good and the brakes seemed to work 100%. A week later and the brakes are back to having air in the lines like the way it was when I started. The brake fluid was actually higher then when I left it which was interesting. There is no signs of any leak or brake fluid on the ground (which why should there be the brake fluid was higher than I left it).

My best guess is, air is getting in the line somehow but fluid is not leaking out. What would cause this? Has anyone experienced this? The system does have ABS, not sure if this would play a role.

Please help, I’ve worked on brakes for mall my life but never experienced anything like this.

Did you just replace one caliper?

Tester

Only one caliper was replaced. It was the caliper with air in the line.

Calipers are always replaced in pairs, or if applicable, all four.

What can happen is, the seal in the caliper piston doesn’t allow brake fluid to leak out, but when the brake pedal is released and the caliper piston retracts back into the caliper it can draw air back into the caliper piston bore.

You bleed the brakes. They work fine for awhile until you use the brakes for a while and air gets sucked back into the caliper(s) from a bad seal.

I’ve seen more than one so-called mechanic scratch their heads over this problem.

Tester

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It’s usually possible to pinch off the flexible hoses that feed each wheel to isolate which wheel is causing the problem. For example if you pinch off the right front flexible brake hose and the pedal goes from soft to firm, you know air is somehow getting into that location, so at least you where to start looking. The caliper bleeder valves can be a source of air getting in too.

Be aware that pinching off the flexible brake hoses can damage the hose, some risk involved.

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Are you sure the caliper you replaced is the right one, if they gave you a left instead of a right the caliper will fit but the bleeder will be at the bottom instead of the top.

This car would appear to be on a cross pattern with the master cylinder. The driver front and passenger rear are together. The driver front caliper was just replaced. So it sounds like it’s most likely the passenger rear cylinder is the one that needs to be replaced. Would you agree?

I’m not the person who put on the driver front caliper or who put in the master cylinder. Several weeks ago I purchased both rear cylinders and brake shoes and plan on putting them in.

You could pinch off the rubber brake hose for that wheel to test the theory. If you are correct the pedal will go from soft to firm.

Wouldn’t the air that’s coming in from the cylinder work its way up the line? I only bled the front driver brake and it seemed to fix the issue (for a short time). If there was air back passenger cylinder when I bled the front brake would it have pushed this air out or is the issue with the front line?

If bleeding just a single-side front brake caliper temporarily solved the soft pedal, but later the soft pedal returned, I’d interpret that as some problem with the front brake hydraulics, not the rear brake. In my diy’er experience air bubbles in brake lines tend to move towards the calipers/wheel-cylinders as the brakes are used for some reason.