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2003 Volkswagen passat. Brakes not working when cold

Please help. I have a 1.8 T, 2003 vw passat with 186,000 miles on it. I’ve had this problem for 3 years and nobody can figure it out. If I start the car and begin driving immediately I have no brakes. However, if I let the car warm up for about 2-3 minutes (maybe less) everything is fine. I have replace the brake booster. If anyone has an idea I would really appreciate your help.

Does the brake pedal move normally? Have you taken it on a safe, wet street and activated the ABS after this problem started?

Yes, the brakes work (ABS) perfectly. It’s almost like they pressure up and then they are fine. It doesn’t have to be freezing cold, just not warmed up.

To recap: brake pedal is hard when I first start the vehicle.
After a short 1-2 minute warm up everything is back to normal

The booster was replaced, but was the check valve replaced? Replace the brake booster check valve, and have a good close look at the vacuum line.

There are 8+ check valves my car guy has replaced most. I think there maybe 2-3 left to change. I will have him recheck the vacuum line. It’s just so strange that it only happens when the car is cold. It’s been going on for 3 years, I’ve taken it to five different mechanics.

Ok - I’m just going to say up front that I am not intimately familiar with the '03 Passat. But a brake booster has one check valve. There’s a huge vacuum line that runs from the intake to the booster. On the booster is the check valve and that’s where the vacuum line attaches. These can stick in the cold and not allow the engine to pull the vacuum. So I don’t what what “check valves” your guy is changing, but I don’t think it’s what I’m talking about. If this person is just replacing check valves in the vacuum system and didn’t replace the booster check valve - well, then you need a new mechanic.

Thank you!!! You are correct ( at least I think you are). I think he has been changing the valves in the system and not the actual booster valve. I will post a follow up when this has been confirmed.

Thank you so much.

A vacuum gauge can probably be tee’d into the brake booster vacuum input to see if it is getting the proper level or not. It’s is probably the check valve as mentioned above. Some systems I think have some filters involved in the path too. To prevent stuff that shouldn’t from being sucked into the intake manifold. There could be a problem w/the vacuum source too. Like something wrong w/the engine when it is cold and it is not making enough vacuum. You’d think you’d notice that as a drivability problem though. Another idea is you have a leak in the diaphragm of the brake booster, and it is taking a while to build up the necessary vacuum. Some cars have a vacuum tank to keep some spare vacuum around, and if you car has a vacuum tank, maybe that tank is leaking.

Just to try and boil it down, there is absolutely no relationship between the engine temperature and the brake system, except for the vacuum side of it. Otherwise there is no connection.

Here’s my guess.

The way the brake booster works is that it is a canister with a diaphragm inside that’s pulled forward by engine vacuum and assists you in pushing the “brake rod” that pushes the pistons in the master cylinder when you press the pedal. It’s designed such that when you’re not pressing the pedal the entire canister, including both sides of the diaphragm, are under equal vacuum. When you pus the pedal, valves inside the booster seal the forward chamber of the canister and vent the rearward portion, allowing the engine vacuum to affect only the front of the diaphragm and pull the rod forward, pushing the pistons in the master cylinder. When you release the brake pedal, the rod comes back, the valves inside the booster canister change position, and the vacuum again acts equally on the front and rear of the diaphragm.

The check valve mentioned by Cig allows engine vacuum to the front of the diaphragm, but prevents flow in the other direction. That maintains vacuum in the canister when the brakes aren’t in use. By preventing the flow of air INTO the canister, it also maintains vacuum when the car is off.

If the check valve is failing, it will allow air to enter the canister when the engine is off, causing loss of vacuum. You’ll have no braking assist when you first start the engine until the engine has a chance to recreate a vacuum in the canister. That is what I think is happening in your case.

I recommend you follow Cig’s advice and test or change the check valve to the brake booster canister. As Cig said, there’s only one. I’ll bet lunch that’ll solve your problem.

By the way, your shop should know this. Hint hint.

Interesting. Is there a way to test for that, or is the easiest method just to replace the valve and if it fixes the problem you know the valve was the cause. Otherwise, if it doesn’t fix it, at least you have a new valve. ???

I suppose it’s feasible to “T-in” a vacuum gage and watch for loss of vacuum when the engine is shut off. A leak in the booster itself would result in failure of the system in operation, and I don’t see that in the OP’s post. All I read into the OP’s post is a loss of vacuum in the booster when the engine is off.

One could also jig up a test station on the workbench. Since the check valve is only about twenty bucks I’ve never tried to test the valve. Perhaps someone else here can suggest a test.

There is a quick test for the booster that might help. If the OP would, with the engine off, press the pedal it should be hard and risen. While still pressing the pedal, he could start the engine and the pedal should sink and become softer. That’ll verify that vacuum is being lost in the booster canister. Since the brakes become normal once the engine is started, it’s logical to conclude that the leakage is via the check valve and not internal to the booster.

Parts are in. I’ve got a new check valve for the booster and new check valve for the vaccum line. I will update when my car guy installs them. Thank you all for your comments.

Update: changed the brake booster check valve and a the puck valve. Also, changed out all check valves in the vacuum line. This did not fix the problem. Brakes still do not work after the car sits for an extended period of time. They do work perfectly after the car warms up.

When it’s cold enough that you are reasonably sure that this will happen, sit in the car with the car off. Pump the brake pedal up and down a few times and hold it. It should feel stiff. Start the car with your foot on the brake. Once the engine is running, that pedal should sink - not all the way to the floor. Just to it’s normal braking feel. If it doesn’t happen right away, just hold that pedal until things warm up and see if it ever sinks. Report.

Is this a diesel? (TDI)