Tom Rathjen

Hello. I have just begun having a strange problem with my 04 Cayenne's (V6) brakes, and wanted to see if anyone else has expereinced this.

When I first start the car cold (after sitting overnight or for many hours), the brakes feel as if they are operating without any power assist. They work, but I have to push the pedal really hard to stop the car....especially when rolling backwards down the driveway. The problem quickly goes away after driving a just a short distance, and the brakes then operate and feel fine.

I have looked for something obvious, like a brake fluid leak, low brake fluid, or a disconnected vacuum line, and everything looks fine. The next thing am planning to try is bleeding the brake lines thoroughly. (Although I did that when I replaced the front brake pads less than a year ago, so it will surprise me if that is the problem.)

Thanks in advance for any ideas!

deilenberger

Rust on the rotors?

working4it

Any update? Did you bleed the brakes?

mcbit

Sounds like a possible leak in the servo diaphragm to me. A good diaphragm will generally hold a vacuum and when you get in the car the brake pedal is 'soft' before starting the engine. After running the engine for a while, the vacuum is probably reaching an acceptable level.

What you have definitely sounds like a problem in development and like most of these things, failure is likely to occur at the most inopportune moment. If you don't know what it is, get it sorted.

Tom Rathjen

No, I haven't bleed the brakes yet. Had to leave town for work suddenly, and won't be able to get back to it for a few days.

But I do know the problem isn't rust on the rotors. I looked at that.

INURGRL951

have you had the brake booster fault light yet ? it could be the hard plastic line that is under the engine cover on the passenger side that have a crack in them.

mcbit

I had that one but it didn't affect my brake performance at all.

INURGRL951

It made a big difference in my braking. How may miles are on your cayenne it could be the booster itself going out or simply a vacuum problem. Have you made any discoveries yet?

Tom Rathjen

Howdy. Just got back to town tonight. Now, of course, I have to work the rest of the week so it will probably be Saturday before I can work on the Cayenne. Plus my wife annouced her Expedition brake squealers have announced I need to replace its rear pads. Grrr. Better do that first, since I have another car to drive.

Cayenne has about 65K on it. No, I haven't had a brake booster fault light yet. The only brake related faults I have had were when the pads needed replacing (front one twice, and rear once since new).

I will look for that hard plastic line. I couldn't see a vacuum leak when I was searching for one. But...I need to look again. Maybe I missed it.

Tom Rathjen

OK, I got back to town finally, and got my wife's brakes taken care of on Saturday. And then I had some time to poke around more on the Cayenne. The suggestion of looking more closely at those hard plastic vacuum tubes on the passenger side for the engine was excellent. I found a complex tube assembly which has a Y-shaped check valve sort of thing which had spit open. The "cap" end of it had separated from the body of this Y-shaped piece. It was still sort of sitting there partially inserted into the body, so it was hard to see. But definitely would have been a vacuum leak. I searched on-line, and found the exploded parts diagram for the intake systems. And the whole tube assembly is called the "Venturi Tube". Part is on order. About $90. So....when it gets here, and I get it installed, I will be able to test it and see if this was the problem. Fingers crossed!

Thanks!

Slow Guy

Good luck, hope that's it.

Worked at a garage in the mid 70's (on mostly American iron), had a car come it that had good power brakes the first time you pushed the pedal but by the 3rd time you pushed it the "power" in power brakes was gone. Wound up being that the vacuum line that runs from the intake manifold to the brake booster had collapsed internally. No sign on the outside of the 1/2 - 5/8" hose that there was anything wrong with it. Had to remove the hose from the engine and the valve/adapter from the end and look thru the hose to see it collapsed. Strange. Only remember seeing that once.

Tom Rathjen

Success!! The new "venturi tube" arrived in the mail, and was relatively easy to install. And all is back to normal. I did bleed the brakes for good measure while I was waiting for the part to arrive. But I didn't see bubbles and the fluid still looked relatively new. So....I'm sure the problem was the broken tube assembly.

Thanks again for all the suggestions, and especially to Costas who steered me to look more closely at the tubes on the passenger side of the engine.

INURGRL951

Anytime Tom glad I could be of help.

Vidyashankara

hey, any advice on how to install on the Venturi tube? I have a big crack in mine and ordered the replacement which will get here soon. How easy is it to replace the tube?

googone20

Does this Venturi tube also apply to the 957 Cayenne's with the 3.6L motors?