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Discussion Starter #1
Hi guys,
I know I'm not the first or last to have this issue.... so here goes... a few weeks ago I drove the car and the brake pedal went real soft to the ground. After driving for I'd say 15 min. the brake pedal came back to normal. Pressurized back as it's supposed to be. I went for a quick little drive... came back home. Turned off the car and again the pedal went soft again to the ground. So I figured I had air in the system. How or Why i don't know because I hadn't worked on the brakes.

I pulled it out of the driveway yesterday to get it on the lift and sure enough it's not building any pressure whatsoever like mentioned previously.

so I put it on the lift and started bleeding from the furthest line to the closest. I used my Motive pressure bleeder to pressurize the system so it's under pressure. I didn't spot any leaks anywhere so I started bleeding and noticed black tiny particles coming out of the brake lines... like dirt was trapped in the lines. (Yes I opened both inner and out brake bleeders for all the brakes.) Perhaps I didn't bleed enough? I opened the lines 1 time each.

ok moving forward car still has the problem. I started thinking I must have a vacuum leak somewhere. I pulled out the front trunk liner again... brake booster and master cyl look fine. I checked the booster check valve. Nothing is hissing air out so i figured the check valve isn't cracked.

So now I worked my way back into the engine bay and unplugged the hose that is on the engine left side (Driver side) and turned on the car. So you hear hissing coming out of the pipe from the engine. I got in the car and the brake pedal was super hard. I put back the hose and the brake pedal went back to super soft w/ very little bite to the floor again.

I'm wondering what you guys think it can be. no check engine lights. no brake lights.

If you guys have an idea please share because I'm thinking I need to use an EVAP machine and blow smoke in the system to see if I have a vacuum leak somewhere that's not showing to the naked eye.

Please let me know what you guys think.

Thanks everyone :D

Charlie
 

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Discussion Starter #2
Any ideas guys? Aside from taking it to the dealer LOL
 

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Based on what you have done so far my first thought would go to a failing master cylinder. The results of your brake booster test doesn't sound out of the ordinary per se but the pedal should feel the same with the engine off as it would with the vacuum line disconnected. If the booster was not functioning correctly the pedal would be harder to push.
Does the pedal get hard if you pump it a few times and if so will the pedal hold steady once it is firm or does it sink to the floor? If it sinks that is usually a sign of a leak or bad master cylinder.
 

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Discussion Starter #4
Thanks for the response. I had a feeling this would be the case. I'm wondering if Audi sells the master cylinder by itself. Trying to track down the part number that is shared. To remove the entire booster seems like a PITA. Especially if it's not broken why replace it.

The pedal feels hard when the car is off. If I pump it a few times it stays the same pressure and doesn't sink to the floor(Car is off). When I turn the car on the pedal instantly becomes soft and goes to the floor. Pumping it w/ the car on it builds a bit of pressure but barely anything and still going to the floor.
 

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I agree that it is almost definitely a bad master cylinder. You could rebuild the master cylinder with new seals to avoid buying one. Usually the labor is such that it is better to buy, but perhaps not with Lamborghini parts prices. When you push the brake pedal down, you create pressure in the hydraulic system, and that pressure must go somewhere. It either pushes the brake pads to the discs (normal operation), compresses trapped air, or leaks fluid. Once you eliminate those, you are left with a bad master cylinder. In this case, the seals in the master cylinder have failed and are not allowing you to build up pressure in the first place. Your pedal sinks to the floor with no pressure on your pads. The vacuum circuit is separate from this and would not play a role, although that does not mean there is not another problem with it. The engine vacuum provides a good deal of assist, so the pedal might seem firm with the engine off and soft with it on because you are now applying enough pedal pressure to get the fluid to leak past the seals in the master cylinder. The presence of black particles is another confirmation of deteriorated seals, although you will also get that with failing rubber brake hoses. My Gallardo has teflon brake hoses with the ceramic brakes, and perhaps they all do. I have not worked on steel brakes on a Gallardo.

You need new seals in the master cylinder or a new master cylinder. A very thorough brake fluid flush, perhaps 2-3 times as much as normal to get these particles out. Unless you have teflon brake hoses as I believe we all do, you would also replace your brake hoses. Brake fluid should be flushed every two years at a minimum. I do annually because I also track the car.
 

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Discussion Starter #6
Thanks brothers for the input. I'm fully agreeing. Seeing as I need to replace the master cylinder the price I'm finding online includes the booster. Which seems ridiculous. 2 bolts to remove the master and just a few metal lines and that sucker comes out. The lousy part is I'm trying to track it down on the Audi parts catalog. I found the reservoir that's ridiculously cheap from Audi but the master i'm having a bit of a difficult time.

I must admit I hadn't flushed the fluid in 4 years so definitely agree w/ you gents. If any of you know the part number to this unit I'd greatly appreciate it.

Below are the part numbers I have found but none are for the master by itself.

4E1 612 105 A (brake booster) Used on 2004 VW A8 Brake booster link
3D0611301B (brake reservoir) Used on 2005 VW Phaeton 3000cc Quattro / Audi R8 2008 - 2010 4.2L and 5.2L


for the life of me I can't find the master cylinder the correct part number for it. Some are close but not the same.
 

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Discussion Starter #7
Made some headway guys... found the part for the master cylinder but not the support bracket that is the second part of the master cylinder.

Our part number for the Master cylinder is 3D0-611-021 (Master Cylinder) Used on 2008 Audi R8 (about $150 ) difference in price.

Still need to find the part number that equals to 400611329A (support Manual Gear Box) that Audi uses on another car. Most cars don't use this part number including the R8.
 

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You shouldn't need to replace the support bracket as it is a removable part of the assembly. You may consider replacing the grommets that go between the bracket and master cylinder but those typically slide apart relatively easy and are pretty resilient. New master cylinders often come with the grommets.
 

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Discussion Starter #9
Awesome! Then this will be an easy replacement. Ordered the part yesterday evening. Thanks so much.

Curious do I have to bench bleed the master or if I install it can I just pressurize the system and push any air out the lines once everything is bolted back?
 

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I would follow a bench bleed procedure, it is not that hard. When dry, there is too much air to simply push or pull through the system as it has to go through all the channels of the antilock braking system.

I sounds like your booster/vacuum system is fine. I would do some extra bleeding to make sure you get out those particles you saw.
 

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Discussion Starter #11
thanks brother. I'll go that route. makes perfect sense.

Will do. I'll report back on Monday. The part comes in so I should have it wrapped up in the evening. (y) :D
 

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Discussion Starter #12
Well boys. You were correct. I replaced the master cylinder and all is back to normal. If any of you need the part number from Volkswagon. I listed it above on Post No. 7. Direct replacement.

Thanks everyone :D
 

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Glad you got it sorted. Thanks for reporting back and letting us know on the fix.
 

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Well boys. You were correct. I replaced the master cylinder and all is back to normal. If any of you need the part number from Volkswagon. I listed it above on Post No. 7. Direct replacement.

Thanks everyone :D
Thanks for the update and VW part # info. This type of information needs to be in the sticky thread at the top of this forum. There was a part number cross-reference chart here at one time. What happened to it?
 

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Discussion Starter #15
I don't remember where it went but I have it saved on my local. Absolutely it needs to be added. I listed part numbers for all the parts that we use on our car that is used on the Audi and VW so in case we need any of those part numbers :D
 

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Discussion Starter #17
I would say so. Nothing really changed in that department between both versions.
 

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I don't remember where it went but I have it saved on my local. Absolutely it needs to be added. I listed part numbers for all the parts that we use on our car that is used on the Audi and VW so in case we need any of those part numbers :D
Can you upload that file back onto the forum? I recall that it was loaded with good info on common parts and was a great reference. I can't find that chart in the sticky DIY thread at the top of the page.
 
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