Brake failure in cold weather on Toyota 4runner

My son and I were coming down the mountain on I-70 near Georgetown, Colorado in a snowstorm in his toyota 4runner. We had just gone through the Eisenhower Tunnel. There was alot of traffic, and driving conditions were terrible. We were traveling approximately 10 miles per hour when my son told me that his brakes were not working. We downshifted, and then slid into the guardrail on the median (we were in the left lane). Luckily we were not hurt, and we did not hit anyone else. Once the car was towed back to Denver, the brakes worked again. My son is getting the car checked by a mechanic, but any ideas as to why the brakes gave out and how he can avoid that happening in the future? Was it related to the cold weather (approximate 10 degrees)? I did not feel that he was riding the brakes as we descended. The road was icy at the time. Thanks!

Are you positive it was actually the brakes that gave out, or did you just lose traction and skid on the ice/snow?

Did you try the emergency brake?

Brakes today are split into pairs so at least 2 wheels will brake if you have a mechanical problem.


I agree that it could have been a case of loss of traction, and not a loss of braking power.
However, it is also very possible that this is a symptom of a failing master cylinder.

You didn’t bother to tell us the model year or the odometer mileage of this 4Runner, but it is possible that the piston seals in the master cylinder are worn. Then, when you introduce very low temperatures and the seals shrink slightly, the result can be loss of braking power, as the hydraulic fluid leaks past the shrunken seals.

And, despite the “split” system that BillRussell referred to, this can take place on both hydraulic circuits, as the piston seals in both chambers of the master cylinder likely have the same amount of wear.

Have your mechanic carefully check the internals of the master cylinder.

My son (age 22) just yelled to me that his brakes were not working and what should he do. I did not think of the emergency brake, but said we should down shift into lowest gear - we did that, but then said he could not control the car (probably sliding on the snow). We are trying to figure out what happened because the officer who came to the scene 30 minutes later gave him a ticket, so we want to be able to tell the judge what happened. Thanks!

It is a 2001 Toyota 4 runner with over 100,000 miles. We had a mechanic look at it in June when my son bought the car, but since then my son drove it across the country to Denver. Thanks for the help, I will pass this on to my son as he takes it to the mechanic tomorrow.

A 14 year old vehicle with over 100k miles could certainly need to have the master cylinder replaced.
In fact, it is probably fairly likely.

When he drives it to the mechanic, please advise him to drive VERY slowly, or–even better–have the vehicle towed.

He already has one ticket, and if he causes an accident because of defective brakes he will wind up with a second ticket for “operating a vehicle with defective equipment”, and he could possibly take somebody’s life.

A tow is far cheaper than what could result from a second loss of braking power.

Thanks! I appreciate your help.

Were the brakes making grumbling noises, and was there a pulsing sensation coming from the brake petal that your son could feel in his foot? In very low traction conditions the ABS brakes can activate and essentially you have no brakes because your tires have no traction. If yes, then the only thing that could helped is winter tires. I had a similar experience in a '01 Toyota Sequoia, no effective braking due to no traction coming down a very slick hill. A very helpless feeling, but the brakes were working properly and still I wasn’t slowing down or stopping.

I will pass this on to my son. I am not sure about the grumbling noise and the pulsing sensation. He was so shook when it happened, and it happened so fast, I did not ask him these questions. I did not hear a grumbling noise, but we had the radio on. Thank you.

I too wonder if the brakes really failed or if traction was lost.

Does your son remember if the brake pedal suddenly went lower than normal?

Because of the ticket (and the probable hit to his insurance rate), you should ask a mechanic to inspect the master cylinder carefully and give you a written report if anything is wrong with it. He should also check the condition of the brake fluid. Speaking of that, when was the last time the brake fluid was replaced?

Was the pedal soft? Stiff? Normal? We need more information.

I will get more information.

My son said there was a bit of pulsing from the brake pedal at the beginning, and then it just stopped working.

My son does not think he tried to push it all the way to the floor.

You may have moisture in the system that froze.
Whatever else you do, you may want to have the system flushed with fresh fluid.

Yep, I suspect the same thing, @“the same mountainbike” . I had that happen to me last winter and it startled the hell out of me. Slamming my foot on the brake pedal broke the ice buildup.

Water can accumulate in the brake booster’s vacuum supply hose. Then when the temps drop, that water freezes and causes a loss of vacuum. Sometimes this problem goes away on its own, but sometimes you need to disconnect the hose and dry it out.

I’m going to say the master cylinder failed because of heat and not cold.

Metal expands as it heats up and rubber shrinks as it heats up.

After driving some distance that got the engine hot, you encountered slow moving traffic doing 10 MPH. As the vehicle was moving at 10 MPH this allowed the under hood temperature to increase. This then got the master cylinder hot enough where the bore in the master cylinder expanded and the worn seals shrank causing no hydraulic pressure.

You see this happen all the time with worn clutch and brake master cylinders when they get hot from the engine heat.

If the shop says that they can’t find anything wrong with the brakes, have them heat the master cylinder up with a heat gun and then see if there’s no hydraulic pressure.


Thanks for all these helpful suggestions!

“My son said there was a bit of pulsing from the brake pedal at the beginning…My son does not think he tried to push it all the way to the floor.”

Although I still think that this is a case of a failing/failed master cylinder, the statements above make me wonder if the OP’s son is one of those folks who doesn’t know how to use ABS correctly.

There are some folks who are not familiar with the pulsing in the pedal, assume incorrectly that it means some kind of malfunction, and then go into panic mode and fail to exert sufficient pressure on the pedal in order to be able to brake the car properly. In other words, the OP’s son may have gotten scared by the ABS kicking-in, and may have unconsciously caused the vehicle to brake poorly.

I’m curious as to the ticket. What was the infraction listed?