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San Jose Mercury News auto columnist Brad Bergholdt.

San Jose Mercury News auto columnist Brad Bergholdt. (San Jose Mercury News/TNS)

Q: I just had my tires rotated at a tire store. When I got the car back the brake pedal felt lower than it should. I took it back and they checked it out and said there was nothing wrong and rotating the tires couldn't have caused this. I'm still convinced there's something wrong, and a friend said I should "bleed the brakes." Does this sound right? What do you think is wrong? It's a 2011 Acura TSX.

- A.P.

A: This is quite odd, as a low brake pedal doesn't seem like something that would have resulted due to a simple tire rotation. Let's look at some possible causes for a low brake pedal.

Your TSX employs disc brakes front and rear, so this rules out a brake adjustment problem. Older drum brake equipped vehicles sometimes suffered from a low pedal due the wheel cylinders needing to push further in compensation for greater brake shoe/drum clearance.

This steers us towards a possible hydraulic system problem. A leaking caliper or brake hose would result in fluid loss, possibly noticeable at the back of the wheel/tire, and low/depleted fluid in the master cylinder fluid reservoir. Symptoms would include a low/mushy brake pedal and more than likely an illuminated red brake warning indicator. This is a very serious condition requiring immediate repair!

Air can be accidentally introduced to the hydraulic system if the master cylinder reservoir fluid level becomes low/depleted due to neglect and/or very thin brake pads requiring extended caliper piston position. Since air is compressible, one would notice a spongy/low brake pedal that may improve somewhat if the pedal is repeatedly pumped. This is where the brake bleeding process would be employed, as the air is gradually drawn out of the system as new fluid is drawn in.

Brake pad knock-back, a rare circumstance, caused by large rotor thickness variations, can also lead to a soft/low pedal as the caliper pistons need to push out more than usual during each initial brake application. Upon a second pedal push, the pedal would be higher. It's unlikely this is occurring, as you didn't mention brake pedal pulsation or a shuddering effect when braking.

You might try this: With the engine not running, press down firmly on the brake pedal. It should be much firmer and higher than when the power brake system is active. Note the pedal height and firmness. If sponginess is felt, either there's air in the system or a brake hose is swelling, similar in effect and seriousness to an aneurysm in an artery. Try also pressing firmly for perhaps a full minute. If the pedal starts firm but gradually sinks it's likely due to a leaking master cylinder seal. This type of leak is internal, so fluid loss won't be noticed. Both situations scream immediate service is needed!

If you should find the master cylinder reservoir fluid level to be acceptable and the above pedal height/firmness checks appear good, I'm not sure what to suggest next other than to trust your judgment and seek a second opinion for a brake system inspection.

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ABOUT THE WRITER

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at bradbergholdt@gmail.com; he cannot make personal replies.