Question:

Brake fluid.?

Why is it that the brakes on cars need to be bleadI know the answer is to get rid of the air within the brake system.My question is how does air enter the brake system when it is enclosed and there is no way air can get in?What causes the air bubbles?

Answer:

Air does not enter a system without opening a line. What you are referring to is moisture in a system. To elaborate on this you need to distinguish what type of fluid you have. You will only have what you are describing happen with DOT 3 brake fluid. It is a mineral based fluid that absorbs moisture. With DOT 5 brake fluid it is a synthetic based fluid that does not absorb moisture. DO NOT MIX THE TWO unless you want to replace every component in your brake system. Hope this answers your question.
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It is moisture not air that isd making it need bleeding. Air cannot get in. Moisture may be introduced into your brake system, and brake fluid will absorb it to prevent corrosion: but this also makes the fluid less effective. Bleeding the brakes should be done every other year, or any time you suspect diminished brake performance and have verified the remainder of the system is in optimal condition.The only way air gets in is if you let it in manually by messing with the bleeder valves letting the fluid tank go below the line.
Air can get into the system if your fluid level gets too low because the brake pads are wearing. As the pads wear the more fluid must go into the lines. Many people do not pay any attention to their brake fluid level until the brakes need replacing, and by then they can have air in the lines. Also, if the brake cylinder seals are worn and leak fluid when you apply the brakes, they will also leak air in, when you take your foot off the brake pedal. This air pocket will build up in the lines where they connect to the cylinders.

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