Brake Failure At Low Speed

[dasha1]

I have a 98 LS400 with @ 133K miles. Yesterday when my wife was approaching an intersection the brakes decelerated the car as normal from 40 mph to about 3-5 mph. The brakes then failed to further decelerate the vehicle for maybe another 30 feet as it approached the car in front of it. The brake pedal was depressed very hard the entire time, but it felt perhaps softer than normal. Finally, after the car had "coasted" to within just 2-3 feet of the car in front of it, it jerked back and forth abruptly a few times (ABS?) and then stopped. There was no collision, thankfully, but it was very close. The problem has not repeated in several miles of driving since then. Also, as possibly a related issue, the car occasionally lunges forward at low speeds with the brakes applied. The front brakes pads and rotors were replaced only 4 weeks ago (w/ Lexus OEM parts) by a reputable independent repair facility and the brakes have worked great since the time of the repair. The rear rotors were turned and pads replaced by the Lexus dealer about 14 months ago before I bought the car from the dealer. The brake fluid has also recently been flushed and changed. Any thoughts on what might be causing this problem would be greatly appreciated. I plan to take it to the Lexus dealer, but I am afraid that they will not be able to diagnose it because the brakes work fine almost always and they won't be able to get the problem to repeat. Thanks.

[curiousB]

Wow sounds dangerous my first inclinations would be that the new pads were non OEM and maybe didn't fit the caliper properly. You said they were OEM though. Are you sure?

Other than that I would recheck brake bleeding. Maybe some air in the master cylinder that comes into play as you fully depress pedal. Here's the procedure for the master cylinder bleed. I'd go ahead and rebleed the entire system jst to be sure.

[steve2006]

It could be loss of vacuum to the brake servo/booster have a good look at the pipes and check the non return valve,the engine didn't cut out during braking when the fault happened did it as this could result in the same problem as the vacuum reserve in the brake servo/booster is used up.

To check correct operation of the brake servo/booster keep pressing the brake pedal with the engine off until the pedal becomes hard, keeping your foot on the pedal start the engine you should then feel the pedal move down slightly.

[dasha1]

Wow sounds dangerous my first inclinations would be that the new pads were non OEM and maybe didn't fit the caliper properly. You said they were OEM though. Are you sure?

Other than that I would recheck brake bleeding. Maybe some air in the master cylinder that comes into play as you fully depress pedal. Here's the procedure for the master cylinder bleed. I'd go ahead and rebleed the entire system jst to be sure.

Thank you CuriousB. The pads, rotors, fit kit and shim kit are all Lexus OEM bought from parts.com. I will have the dealer rebleed the entire system as you suggest. Aside from this one incident, the brakes have worked perfectly.

[dasha1]

It could be loss of vacuum to the brake servo/booster have a good look at the pipes and check the non return valve,the engine didn't cut out during braking when the fault happened did it as this could result in the same problem as the vacuum reserve in the brake servo/booster is used up.

To check correct operation of the brake servo/booster keep pressing the brake pedal with the engine off until the pedal becomes hard, keeping your foot on the pedal start the engine you should then feel the pedal move down slightly.

Thank you Steve2006. The engine did not cut out, but there must have been some loss of pressure or similar event that stopped the force on the brake pedal from being transmitted to the mechanical brake parts. I tried the servo/booster test you suggested and the pedal became hard. I then turned on the ignition and the pedal came down maybe a couple of inches. I will have the dealer check the pipes and non return valve per your suggestion. I am wondering if the problem is related to an ABS malfunction since the jerkiness my wife described before the car finally stopped seems like it could have been the ABS activating.

[steve2006]

Hi,

ABS faults normally but not always throw a fault code,even if there is no warning lamp on the dash maybe you could ask the dealer to read the codes while they have the car.

There could have been some spillage on the road perhaps diesel which although not visible could have made the surface slippery hence the ABS operated.