Vapor Lock- Legit or Excuse?

My problem is this: I was driving through hilly neighborhoods on a hot day (90’s) and my car had sat in the sun for hours. I noticed my brakes did not seem to be stopping as smoothly a usual. As I was coming down a long hill, I applied the brakes so I would maintain the speed limit of 30mph, however even with the brakes on I was still going 40. At the bottom of the hill, the light turned red. I put my foot to the floor on the brake pedal,and while there was resistance and my car slowed down, I was not able to stop all the way an had to pull my emergency brake to stop. After I pulled off the road, I then thought to move forward so I could be under an overpass for some shade. Although my car started, I had no power steering and was only able to coast. After waiting for over an hour to be towed, I found that my car started right up, and the brakes worked fine. The mechanic I took my car to says it was probably “vapor lock.” 1) is this a legitimate issue, or something made up because they can’t figure out what is really wrong with my car? 2) I was told that “this is just something that happens” and “there isn’t really anything that can be done”… I don’t know if I buy that… Am I limited to only driving when it is less than 80 degrees out? I lived in eastern Washington for 5 years with summers much hotter, and never had this problem with the 2 other cars that I had. 3) what can be done to prevent this from happening again?

Check for a sticking caliper. If a caliper sticks and heats up a rotor (sometimes red hot) the brakes will suddenly work very poorly, especially if the fluid boils in the caliper. Once it cools down it often works fine until it heats up again. If the caliper sticks intermittently it can be tough to diagnose.

Would a sticking caliper make any noise, like an intermittent rubbing sound coming from the wheel, when the car was running?

In addition to checking for a sticking caliper, I strongly suggest that the OP flush the brake hydraulic system. Brake fluid is hygroscopic, meaning that it absorbs moisture from the atmosphere, and, after a few years, this process can dilute the brake fluid to the point where the moisture in the fluid will actually boil when the brakes become very hot–as when braking on a long downgrade.

When brake fluid boils, the brakes become ineffective, bordering on useless, and it appears that this is what the OP experienced. Just for future reference, this is one of the reasons why downshifting to a lower gear is advisable when descending a long grade, as this will allow the brakes to stay cooler, while keeping the car’s speed in check.

I surmise from the screen name that the OP’s car is a 12 year old Focus. Even though Ford may not specify a brake fluid flush in the maintenance schedule, this is something that should really be done about every 3 years–especially if the car has ABS.

So, if–as I suspect–this car has never had its brake hydraulic system flushed, it is long overdue for this important safety-related service.

First find a new mechanic. Second have the brake system flushed and bled. Third have then engine checked. Having no power steering means there’s some other problem.

Vapor lock in brakes is caused by boiling fluid. It can’t happen just from parking in the sun on a hot day.

+1 for @texases. The “mechanic” was no mechanic.

+1 with all ^those guys. They know of what they speak. Go elsewhere. Brake fluid doesn’t boil by the car sitting in the sun.
I’d check the rubber hoses to the caliper. While they may look decent, they could do what you describe. When they get old, they can behave like they were check valves. They can collapse under the right circumstances, not allowing fluid to return back into the system properly, causing sticky calipers.
Steel braided lines are better - maybe a bit more expensive but worth it: They don’t bulge (causing fade or you’ll feel your brakes ‘loosen’ suddenly, as if you’re going downhill while sitting still or like someone is pressing your gas pedal while you’re sitting still, with the car wanting to roll forward) or constrict (sticky calipers).
You’re going to get the brakes bled anyway so changing rubber brake lines will only cost what you pay for the part and maybe one hour labor at very most.

Add my opinion to those already posted. You’be gptten some good advice here.

Riding the brakes on a hill on a hot day can cause the fluid to boil, especially if the fluid is long overdue changing and has absorbed moisture. I’d suggest learning to use your gears to help maintain your speed as well as learning to apply your brakes intermittantly to try to maintain your speed.

But for the shop to have just done nothing and sent you on your way is pathetic. At the very least the system should be flushed with fresh fluid and examined for evidence of damage such as a collapsed flex hose at the calipers. As a matter of fact I support Remco’s advice to change the lines.

I know I’m repeating advice you’ve alerady been given here, but it’s worth repeating.

+1 on 12 year old brake fluid boiling from driving on hills.

OP: was the engine running during the descent, and after stopping at the bottom?

From re-reading, it occurrs to me: the engine could have quit. Once the vacuum bled off, braking would have been much harder/less effective. Also explains no PS, too. It also explains the “vapor lock” daignosis.


  • did the check engine light come on?
  • were burning brakes smelled once stopped at light?
  • Did engine need to be re-started at bottom of hill?
  • How did the car run, driving away from light?

(Of course, boiling brake fluid is still the most likely explanatiom.)

We are guessing it’s a 2001 Focus with unknown miles…Is that correct? Boiling brake fluid? I don’t think so…Were you pulling a trailer or something else going on?

Define long hill. You should not be riding the brakes going down hill. That’s a good way to get brake fade, as you discovered. :wink:

Have the brake fluid changed now, and don’t be surprised if your master cylinder dies soon.

My guess is the engine died, can only coast no power steering, no brakes. You might have been able to restart the car and gain back all functions.

After reading MeanJoe’s reply, I reread the original post and everything suddenly made sense. I think Joe is on to something. I still think the fluid is overdue for a flush, but I think a stalled engine is a much more likely candidate for the overall problem. Whoever looks at the car should include a thorough look at the engine as well.

I have doubts about the engine stalling theory, due to these statements:

"I put my foot to the floor on the brake pedal"
Did the brake pedal go to the floor? Not so easy to do when the booster is empty of vacuum.

"Although my car started, I had no power steering…"
This one leaves me scratching my head.