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Air in Brakes

I changed out my brake fluid 6 weeks ago but made the horrible mistake of not keeping the reservoir full of fluid. As my wife was pumping the brakes, she said the last pump was squishy.

After that, I filled the reservoir and tried to bleed all the air from the system with the pump the brakes method (spent about 2 hours and a couple of bottles of brake fluid) and still had a soft peddle and little amounts of air in the R front and L rear.

Then I bought a vacuum pump and tried to remove the rest of the air. It seemed to do the trick. The brakes were firm and stopped well.

After a few days, the brakes became softer and spongier. I bleed the brakes again and noticed the L rear and R front were fine but the R rear and L front still had some air that came out. I bleed it out and the brakes were fine.

We went on vacation and same problem. But I could only reach the L front bleed valve, so I only bleed it. The air came out and the brakes were good for another few days.

I have been repeating this cycle for the past few weeks. Bleed the L front (lots of air comes out with 3 pumps), firm brakes for 4-5 days, then the brakes become progressively softer and need to be pushed further down.

Is this still residual air in the lines from that 1 pump without enough brake fluid 6 weeks ago? Will the rest of the air eventually work itself out? Whenever I bleed the brakes, we pump until no additional air comes out. I haven’t made the mistake of letting the reservoir run low again. We have probably spent 4-5 hours pumping and bleeding these brakes. Any help would be appreciated.

The air may be getting into the brake system thru the caliper piston seals.

When the brakes are applied the hydraulic pressure forces these seals to expand to seal the caliper piston to the caliper bore. When the brakes are released the caliper piston retracts slightly back into the caliper bore. If the caliper piston seal is worn it can draw air past the seal into the caliper. Then after a few days of operating the brakes, enough air accumulates in the caliper causing the low brake pedal, and the brakes require bleeding again to remove the air.