Mechanics

NISSAN MAXIMA BRAKE FLUID PROBLEM

1991 Nissan Maxima

The front brakes keep tightening up even after changing calipers, hoses, and pads. What am I missing?
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Tdhood2000
January 18, 2013.




By "tightening up", do you mean the brakes are applying by themselves? If so, next time they do that, loosen the steel lines at the master cylinder. If that lets them release, suspect the brake fluid is contaminated with a petroleum product.
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Caradiodoc
Jan 18, 2013.
Thats what I have to do is loosen the steel lines at the master cylinder, it does release them for a bit then it does it again. I have bled all the brake fluid and replaced it with new and it still does it. Any more suggestions, please?

Tiny
Tdhood2000
Jan 18, 2013.
First be sure the brake light switch is not misadjusted and is holding the brake pedal down a little.

If loosening the lines at the master cylinder releases the trapped fluid, it is being trapped IN the master cylinder. That is caused by contaminated brake fluid and simply replacing it is not going to solve anything. All of the rubber parts that contact the fluid will be swelled and mushy. You will see evidence of that on the rubber bladder seal under the reservoir cap. The two lip seals on the pistons in the master cylinder will grow and cover the fluid return ports. That prevents the fluid from releasing. As the dragging brakes get hot, the heat migrates into the brake fluid causing it to expand. That makes them apply even harder.

Every year I did a demonstration for my students to stress the importance of not getting any hint of wheel bearing grease residue from your fingers, or other petroleum products mixed in with the brake fluid. I put a wheel cylinder lip seal in a beaker with about two inches of clean brake fluid, then added one drop of engine oil or power steering fluid. Within one week that seal had grown more than ten percent and was soft and mushy.

Simply replacing the brake fluid will not address the contamination already in the rubber parts. That will leach out into the fluid recontaminating it. The only acceptable fix is to replace every part with rubber parts that contact the fluid, flush and dry all of the steel lines, THEN fill the system with fresh, clean brake fluid. This includes wheel cylinder lip seals, the combination valve under the master cylinder, front calipers, master cylinder, and all the rubber flex hoses. That also means your new calipers and hoses must be replaced again. The pads are okay. You can buy rebuild kits for the master cylinder, calipers, and wheel cylinders but in this case that is not the right approach. Master cylinder rebuild kits usually cost more than a rebuilt unit with a warranty. The calipers and wheel cylinders are made of cast iron which is porous. Some contamination will leach back out if they are just rebuilt. Also, in the last 15 years or so, rebuilt calipers have come down so much in price that they don't cost much more than the rebuild kits. Wheel cylinders are also very inexpensive.

Caradiodoc
Jan 18, 2013.
Thank you so much for your help I'll give this a try and get back to you.

Tiny
Tdhood2000
Jan 19, 2013.

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