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Brakes fail after 30 minutes

I have a 2003 VW Passat V6 Wagon. About 6 months ago, I had some work done on the car including Brake Fluid flush. A month later, I had to park the car because I suspected the front bearings went bad because of a loud moaning sound that just got much louder with higher speed.

Anyway, after 5 months of the car just parked in the garage, I had to jump start the car and drove the car back to the shop (a reputable mechanic) to handle the wheel bearings. He replaced the bearings and replaced the battery. And on the way home, after about 30 minutes of driving, I stepped on the brakes and it all the way down (very soft) with no stopping applied. I immediately let go and stepped on the brake pedal again and at that time the brakes were applied. I had to do this everytime I need to apply the brakes for the 5 minutes to get home. The next few days, the brakes behaved properly because my trips were less than 30 minutes long. But anytime the car has been running continuously for over 30-40 minutes, this happens again; brake pedal is soft for the first pump and sort of normal on the second pump.

Brake fluid is clear, full on the reservoir. All brake pads are over 75%, I don’t see fluid leaks around the lines on the calipers. When engine is off, I can pump the brake pedal three times and it is hard and stays up, it will release when I start the engine, so I’m thinking the Master cylinder is OK. My suspicion is air in the lines but shouldn’t that manifest symptoms more regularly rather than only after 30-40 minutes of driving?

Any help is appreciated.

Sounds like there may be moisture in the fluid which is vaporising as the system heats up.

Bleed them out anyways. I’d be surprised if an air bubble or two did not come out.

By the way, the test you performed with the engine off was of the booster, not of the master cylinder.

I’d be inclined to have the brake system flushed out with fresh fluid and bled by a different shop. It sounds like it wasn’t properly bled, but that would leave the brake work as a whole suspect.

Drive the wagon for 20 minutes. Pull off the highway. Use the back of your hand to feel the radiant heat of each brake disc (don’t actually touch). If one disc is considerably hotter than the rest, suspect the fluid is boiling in that caliper. You also could feel the wheel and/or hub to determine which is running hotter.

If one caliper is dragging overhaul the caliper and check the flow of that wheel’s flexible hose.

Hope this helps.

Has the brake fluid ever been replaced? If not, that what I would do.

The brake master cylinder may be leaking internally.

As the vehicle is driven and the engine gets hot, the heat also heats up the master cylinder. As the master cylinder gets hot the bore inside the master cylinder expands. If the cup seals in the master cylinder are at all worn they can’t seal against the bore while the master cylinder is hot. This doesn’t allow for any hydraulic pressure to be produced and you have no brakes.

You can easily test if this what’s happening by carrying a bottle water in the vehicle.

The next time the brake pedal goes to the floor, pull the vehicle over, open the hood, and pour the bottle of water over the brake master cylinder to cool it off. If the brake pedal returns, the master cylinder is leaking internally and requires replacement.


“The brake master cylinder may be leaking internally.”

That is my theory, also.
I really hope that the OP gets this fixed a.s.a.p. because that “pumping routine” may not work much longer if it is actually an internal leak in the master cylinder.

I agree with @VDCdriver here. At some point…that’s rapidly approaching…the master cylinder will fail. I’ve had that happen and when it does…it’s all hands on deck…because your feet will be useless in stopping the vehicle unless you have a foot operated emergency brake. Get that Passat repaired as soon as possible. Funny…but the master cylinder that blew out on me was in my Dad’s Type III VW Fastback.

This has the classic symptoms of a bad brake master

I think it’s just coincidence that it failed soon after the brake fluid flush

If ReBleeding The Brakes Does Not Remove Any Air Then It Could Be A Master Cylinder.

I think it’s possible that a master cylinder could leak internally following a flush and for a reason other than coincidence. If the pedal is used for bleeding, the piston(s) is/are made to travel where they haven’t been in a long time and who knows what lurks in the bore in the new neighborhood? I saw this “coincidence” frequently back in my old car dealer shop days.


I would certainly hope that if the customer pays for a brake fluid flush, the shop uses the diaphragm brake bleeder or a vacuum brake bleeder, versus the two man method

Big assumption. The quality of work out there in the world varies wildly.
I’m inclined after reading everything to also suspect the M/C as a good possibility.

If it only happens after driving for some time, then one or two calipers are hanging up for whatever reason.Like Researcher said above. The brake is on and the fluid boils, it needs to stay liquid to work,other damage could be done by the high heat also,Caliper piston seals can leak inside the dust boots.The rubber flex hoses could also be damaged and even the cause of the line-lock condition.

Perhaps the “Flush” dislodged some rust flakes or other crud that is now lodged in the master cylinder occasionally causing a major internal leak allowing the pedal to go to the floor…If bleeding the brakes does not cure it, I would replace the master…

I’ll have the brake fluid bled by another place and if that doesn’t fix it, I’ll replace the Master cylinder. Thanks for all your input. I’ll post results after I have these done.

Maybe a master cylinder. But I’m with researcher, it’s one or more caliper frozen. From sitting, old age etc. A brake is dragging boiling the brake fluid. Feel the wheels rims after highway driving some time.

I agree. I would not just bleed but replace all the fluid and have system entirely checked for leaks including “within” the matter and slave cylinders

No, it’s what I told you.

Liking the bad caliper or rubber brake line theories for an old post.