Brake failure on SW2 ... or WAS it ??

[Hidalgo]

Strange happening yesterday ...

The wife left for work, drove 3 miles to the interstate, then 7 miles to her exit. She said that when she started to brake for the exit ramp, the brake pedal felt "spongy". Then when she pressed it again, she said it didn't do anything to slow the car. She ended up going to the shoulder and using the hand brake to stop the car.

No ABS on this car, btw.

She called me to assist her, and when I got there I immediately checked the brake fluid. FULL. I looked under the car for fluid. NONE. Then I got into the car and cranked it, and pressed the brake pedal. The brakes worked as well as ever.

My wife is 52 years young, and has been driving since she was 14. She is a very careful driver, but can also handle a car on the track (she has road raced in the past). She knows very little about mechanics, but can drive a vehicle with the best of them.

What do you guys think happened here?

If there were no fluid leaks, and the fluid reservoir was full, then I'm a little stumped at this happening. I have driven the car about 100 miles in a stop-and-go manner with no ill effects.

My theory: She had pushed the floor mat up under the brake pedal and stopped it from going down. I've noticed at times that she has a tendency to push the mat far forward in the floor.

Any suggestions / theories ??

[fdryer]

Get rid of the floor mat for at least a week or two. Think over if it really bunched up under the front - if you have to, either get rid of it if it slides easily because its a hazard or make grommet holes to tie it in place from the back. My L300 doesn't need expensive carpet covers for the cheap OEM carpets so I made a custom set in the front with stair runners and snap buttons that snap to the back corners and tucked under the plastic trim on the sides to keep it in place. I unsnap it once or twice a year for the vacuuming. Never had "an incident" with floor coverings sneaking up and jamming the pedals, ever.

The way to test brakes is with the engine OFF. Press the brake pedal, slowly, and see if it sinks to the floor. If it does you have a worn master cylinder; either rebuild it along with a complete flush of brake fluid or replace the master. The slow pedal application allows a weak seal in the pistons to leak brake fluid around them resulting in the sinking pedal. Apply the brales several times as you're using up the reserve vacuum assist; as soon as several applications have gone by you'll notice less pedal travel (in a prefectly good brake system) where you're using only the hydraulics for braking. Remember when brakes were that way before power assist? The short pedal travel, about a third of the way, is perfect for normal brakes. There's never a need for the brake vacuum assist when testing just the brakes. The vacuum assist is only needed for driving and is tested separately. All you're doing is trying to feel for a worn master cylinder or leaking anywhere in the system. With the engine running, you're using vacuum assist with the brake booster unit to help with brake application; not what you want if attempting to isolate a brake system problem. A thorough check would be to remove each wheel to inspect each brake caliper or wheel cylinder for leaks.

When done testing, start up and allow the engine vacuum to build up the vacuum in the boost unit, apply the brakes carefully; sometimes the pedal will travel to the floor until vacuum is built up. This won't take more than a minute before braking returns to normal for everyday driving.