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Discussion Starter · #1 ·
Hi everyone.

Just wanted to post this and see if anyone had an idea what my issue is or has had the same issue with their SF. First off the brake light switch recall has been performed on my vehicle already.

At three different occasions, while on cruise control I pressed the brake pedal to deactivate the cruise, but the pedal was hard as rock. Felt like the brake booster was not funtioning. I then press again hard on the pedal and it goes in and then functions correctly.

It only happened on long trips when cruise had been activated for a while. I tried reproducing the issue on short commutes many times with no luck and the dealer can't reproduce either. The dealer said they have no similar complaints in their system and advised to bring the vehicle in when it happens again. (Sure I won't brake until I get to the dealer and their tech. can jump in the rolling vehicle LOL).

A friend mentioned that the booster and cruise might work on the same vaccum and that it might not be able provide enough vaccum (Small leak) to keep both in operation?

Thanks in advance!
 

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I doubt the cruise uses vacuum, it's a throttle-by-wire car so it'll be done electronically.

Even if you had a vacuum leak in the brake servo circuit, the servo has a non-return valve so you get one assisted braking before the pedal starts to go hard.
 

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Cruise is an added internal function of the PCM,, as the throttle is electric motor driven, all we do with cruise button is tell PCM we want it to control throttle when the "set" button actuated.. and cancel with brake light switch signal.
 

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Discussion Starter · #4 ·
ok so vaccum issue is out of the equasion. Any other ideas as to what could be causing the brake booster to fail while on cruise?
 

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Unrelated issues. As sbr711 notes, there's no interaction between the systems. My guess is that if you didn't use the cruise control, but spent a great deal of time on the road driving without using the brake for any reason, you'd discover the same issue with the hard brake pedal. It's just that it takes quite some time without using the brake to reach this state, and you only drive such distances without using the brake on cruise control.

ASSUMING you've never received an ABS light on the dash...

What you've neglected to mention (and it may be very relevant) is whether the brakes were effective or whether you had poor braking when you applied them 'hard'. If you just pressed them hard enough to knock down the cruise control, we don't know all we need to know.

Start with this check valve test:

Start the engine to allow vacuum to build up in the booster. Shut off the engine. Wait five minutes. Try the brakes to see if the booster is still providing power assist. If not, your check valve may be shot.

Then move on to booster tests:

Shut off the engine. Pump the brake pedal with the engine off to bleed off any vacuum that may still be in the booster. Then hold your foot on the pedal and start the engine. If the booster is holding vacuum as it should, the amount of effort required to hold the pedal should drop and the pedal itself may drop slightly. If you don't feel any difference, check the booster and vacuum hoses.

Beyond those quick checks, we're getting into some more difficult areas to DIY without a fair bit of experience.
 

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Discussion Starter · #6 ·
Thanks for your answer!
At first I slighly pushed to deactivate the cruise but the pedal didn't budge. So I pressed hard and the booster kicked back in after a second. It was basically like pressing on the pedal when car is off.
It failed the check valve test (turn off and wait 5 minutes and see if booster is still active) After 5 minutes the pedal was hard as rock.

The booster test results were good and everything seemed to be ok.

Now how do I go about getting the dealer to check this? (still under bumper to bumper) Is there another test I can do to confirm the check valve is the issue or is it pretty much a sure thing?

Thanks
 

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You could remove it and try blowing through it from each end -- it's a 1 way valve -- but I think that's not much fun, and something your dealer should become aware of in any case!

As for the test, try after 30 seconds or so to see if there is any difference. I'm unclear as to why you are only experiencing this after a long highway drive, and yet why the valve seems to be acting badly in your driveway.
 

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Discussion Starter · #8 ·
Tried after 30 seconds and it is hard. Will check with my coworker's SF next week to see if his is the same to be 100% sure when I go argue with the dealer.

Thanks again
 

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Understand that the drop you will feel in the pedal is VERY slight. You'll need to be paying close attention to detect it on these particular vehicles.
 

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Discussion Starter · #10 ·
Ok so I tested my coworker's SF and his booster is still effective 3 minutes after the vehicle is turned off. Might be even longer, but since my pedal becomes hard after 15 seconds I figured that was long enough to confirm the difference.

Any one else could confirm that brake pedal is still soft after a few minutes of vehilce being off? I just want to be 100% sure there is an issue so that the dealer doesn't give me the run around like they usually do!

Canderson: Just to confirm, you think the booster check valve is letting air seep into to the diaphragm chamber correct?

Thanks again
 

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The difference between your friend's SF and yours is indeed indicative a problem, though I am still very surprised that it occurs during a period of driving.

I'm wondering if there's both a leak that develops at the check valve, and/or whether it gets stuck, and the eventual leakdown of the booster over many miles leaves you with no power assist (vacuum eventually lost) and no way to 'replenish' it either due to a major leak or a stuck valve. Surely must be something right in that area, but it's not something I've seen before on a SF.
 

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For giggles,, this was an odd random problem on Genisis sedan... with engine at idle,, spritz some carb around the mating surface where brake master mates to vacuum booster.. there was issue with o-ring either missing, or damaged when master mated to booster.. long shot though.......
 

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Assume you'll be looking for slight increase in RPM due to the induction of the carb cleaner into the intake via the boost line?
 

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Looking for change related to vacuum leak due to ingestion of carb spray..
 

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Discussion Starter · #15 ·
Brought it to the dealer, they spent the day on phone with hyundai engineers! Haha, hyundai agreed to replace booster because they are concerned for my security, but there are no problems from the tests they performed.. Such BS. been waiting 2 weeks for the part to come in. Seem to be giving me the run around!
 

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Discussion Starter · #16 ·
Well was in today for booster replacement. Pedal is still hard as rock 15 seconds after I turn the car off. Will have to go on a long trip to find out if it still fails during long drives.

Will try carb cleaner and report back.
 

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Discussion Starter · #18 ·
Tried spaying carb cleaner and no noticeable difference in RPM. I will bring it to a different dealer when I have a chance. In the meanwhile would some of you mind doing a small test on your vehicle for me?

After turning off your vehicle, count to 15 and press the brake pedal. Do you still have a couple pumps (pedal travel) left? Or is the brake booster not active (pedal is hard)?

My coworker's SF still has vacuum for 5 mins after he turns it off maybe longer. The dealer on the other hand said they tried with 2 identical vehicles on their lot and that they acted the same as mine.

I might just go on a used lot and test drive one to see how it acts.

Thanks for all the help!
 

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Discussion Starter · #19 · (Edited)
Just did a little reading and found out that on some model the check valve for the brake booster is integrated in the vacuum hose. I did notice a "lump" in the centre of my hose. Would the SF use such a check valve? The dealer said the valve was integrated I the booster...and that's why they replaced it, but I have 0 trust in them.

I looked at oem parts online the hose is called "hose assembly" and costs 35$ US so I am starting to think it makes a lot of sense!
 

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Yes - the older SM models had a separate check valve, but on your vehicle, the check valve is built into the booster itself. Can't fault them for replacing the booster since it did sound very much like a wonky check valve. Your dealer may have a bit more troubleshooting left to do.

Right out of Hyundai's own tech docs for this, here's their recommended three step approach to this testing, and this is what you can expect from the dealer after the replacement. Again, try each and pay close attention to the results:

  • Run the engine for one or two minutes, and then stop it. If the pedal depresses fully the first time but gradually becomes higher when depressed succeeding times, the booster is operating properly, if the pedal height remains unchanged, the booster is inoperative.
  • With the engine stopped, step on the brake pedal several times. Then step on the brake pedal and start the engine. If the pedal moves downward slightly, the booster is in good condition. If there is no change, the booster is inoperative.
  • With the engine running, step on the brake pedal and then stop the engine. Hold the pedal depressed for 30 seconds. If the pedal height does not change, the booster is in good condition, if the pedal rises, the booster is inoperative.

If the above three tests are okay, the booster performance can be determined as good.

If one of the above three tests is not okay, check the check valve, vacuum hose and booster for malfunction.

On other Hyundai models, the 59133a check valve is still external in the vacuum line, but not for the '08 Santa Fe.
 
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