Replaced AOS....Now What?

What a great DIY resource. Bought AOS and parts from Pelican and dove in (literally) last Saturday. Took me about 7 hours to remove and replace the AOS. My Car: 1999, 996, manual, 70k miles. It has been showing what I think is high vacuum and has been causing the brakes (pedal) to stay down and engage the brakes during engine decel. No white smoke like others I have read. But the high vacuum was my reason for replacing the AOS and the dealer mentioned that this is what they would do first. The dealer checked the brake booster for water and there was none. I do clean out the little drain holes from time to time. If you remove the vacuum hose from the brake booster the brakes go down and come back very quickly. Now, when you drive you literaly have to put you foot under the brake pedal and pull it back up. Or the brakes drag. Weird. When you are just sitting there with the engine running the brake come back up, but a tad slowere than I remember. What else manages the amount of vacuum pressure in these cars? Anyone else have this issue? Maybe I have been lucky but I have had zero issue with this car and its been a real pleasure to drive. Any suggestions or links would be much appreciated. I have some decent mechanics skills and was thinking next to buy an extra oil filler cap, drill and tap for attaching a Dwyer vac gage I use for tuning large gas train burners so i can see what its pulling during heavy engine decel. Figured I could just run the hoses into the front seat and watch it while driving?

A bad master cylinder could cause the brake issue you are describing, no pressure in the pedal, and no return...also a flooring sensation...when you step on the brakes, they go to the floor, or so it feels.

Replace the master cyclinder and maybe the brake booster. I may try that. But the sensation is not that the brakes do not work. The pedal is firm and does not go the floor. Brakes and ABS will work. Just tested in the rain. What happens is when you touch the brake pedal and the pedal almost moves by itself. A very lite feel. Again it seems like too much vacuum on the diaphram. And then it stays down and the brakes stay engaged and drag. You have to stick your throttle foot side of your shoe under the brake pedal and pull it up to release the brakes. What parts on the car modulate the vacuum for a given RPM?

A bad AOS wouldn't give you more vacuum, don't know why the dealer told you that, that's BS. Still a good idea to have replaced it on a '99 before it makes a mess. What happens when it fails is it applies the intake vacuum directly over the crankcase oil sump through the intake runners, so you end up with high vacuum on the OIL filler side, which sucks all the oily vapor fumes in to the intake, causing smoke and an oily mess there. Which, of course has nothing to do with the brakes, so the Dwyer gauge idea is a waste of time. Vacuum is just generated by the suction of the intake valves being open when the piston is dropping, because of the expelling of the expanded gases through the exhaust valves. There is no vacuum regulator that I ever heard of, which would be nothing more than a bleed valve to let air into the system to reduce vacuum. Vacuum is vacuum everywhere. Does the brake behave the same or similar way with the engine off? If so, it sure ain't a vacuum issue. I think leak somewhere in fluid side, maybe master cylinder like suggested. Haven't seen this before on brake, just on clutch, and it is usually fluid leak.

Thanks for all this. I did see oil in the intake when I replaced the AOS. I will chalk this up to "preventative maintenance" as I daily drive this car. Ok, so no regulator on the vacuum. Hmmmm.. I will replace the master cylinder first. Lower cost. And then replace the vacuum booster. I will report back and I very much appreciate the advice. I would add that wrenching is not a bad thing and to me its a passion. Time is the shop is time in the temple. Update will be in January.

Same here. As long as I can be left alone to work at my pace, I love it. Problem is, wife only seems to want me around when I want to work on my car. Annoying as hell. There will always be SOME minor oily residue in the intakes, that's normal. The AOS is not 100% efficient, it's just knocking down as much airborne droplets & vapor as possible, while still sucking in the cumbustibles. The actual intake throttle plenum has a volumetric low point collection area, and mine had maybe 1/32 of a teasoon of oil sitting in it, and my intake valves were sparkling clean after 48k miles. The runners had some very minor dirty carbon build up, but all that wiped off easily with a paper towel.

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Four stages of brake boost. 1) Brake pedal released. Vacuum is present on both sides of the diaphragm plate. 2) Brake pedal depressed. This closes off the vacuum to one side of the diaphragm plate and opens this side to atmospheric pressure. This helps apply the brakes. 3) Holding. The driver is applying constant pressure to the brake pedal. 4) Releasing. The driver is releasing brake pedal pressure. This closes the atmospheric port and opens the vacuum port, again exposing both sides of the diaphragm plate to vacuum. As the atmospheric pressure is removed from the diaphragm plate the diaphragm return spring will push the plate and release all pressure on the master cylinder. Based on what you reported -- in another post -- it would appear the valve that keeps vacuum on both sides of the diaphragm plate is bad and allows atmospheric pressure to enter which starts boost action. Furthermore because of this when you release the pedal and this valve doesn't block atmospheric pressure and allow vacuum on this side of the diaphragm plate the diaphragm plate return spring is not able to overcome this and you have to manually bring the brake pedal up. But do not replace the brake booster on my say so. Best get trusted mechanic's opinion/diagnosis before you go ahead and replace the brake booster.

That is a very good explanation of what is going on. Brake Booster..... Darn things are worth their weight in gold..... When I buy one I will be taking the old one completely apart. Pics to follow of course.

Also Porsche has redesigned a few things there to fix the water in the booster problem. So make sure you get the latest parts.

There are good suggestions here but some not so good descriptions. The vacuum in the intake is actually regulated by the throttle plate. So the throttle plate is a regulator but NO ONE calles it that. Also, the valves are not really creating the vacuum either. At wide open throttle or on diesel engines (they don't have a throttle plate) there is no vacuum.

Almost right. The throttle doesnt create vacuum any more than the valves do. (A small portion of my post was deleted because I was typing on a stupid PITA Nook, and I corrected bad spelling and erased it, if you ever use one, you will know what I mean. It's been corrected.) If there was no vacuum at WOT, no air would be sucked in as the pistons went down to the fill stage. An engine is just an air pump with the expanding gases driving the piston down. Air flow is highest during WOT, obviously. The throttle is throttling the mass of air by impeding flow, which regulates the amount of fuel via the MAF for that level. Vacuum only changes significantly during throttle changes, then levels out at equilibrium. (Simplified). Though the vacuum is reduced in the plenum as the throttle is more open. For instance, with true variable valve timing, no throttle is needed in the Otto cycle either, as you can time the intake valve opening to be open the right length of time to fill the chamber with air. If you want more air mass with more fuel for more power, per unit time, either make bigger pistons rotating faster, or pressurize the air to cram it in the same space. Either way, there is a vacuum somewhere, or the air doesnt move.

At WOT the vacuum is inside the cylinder, not the intake plenum. You put a gauge on the plenum and it will read 0.

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The zero is really atmospheric pressure. The gage is providing gage pressure not absolute pressure. The low pressure exists from the intake valve on down into the cylinder as the piston move down the cylinder during the intake stroke.

I'm an Instrumentation & Controls engineer for the last 33 years. I don't want to get in to a pissing contest. Vacuum is just a way of saying less than gage for the atmospheric pressure at that location, just as Macster said. Air (any liguid or gas) moves because of differential pressure between the 2 points. The vacuum was always created by the draw of the piston, so yes the "vacuum" is inside the cylinder is a way to word it, but there are degrees of vacuum, and believe me, regardless of what a gage reads, (which depends entirely where it is hooke dup) at WOT, the air in the plenum ina running car is at less pressure than atmospheric pressure. There are pressure drops starting at the intake to the air filter, and the drops increase as you get closer to the cylinder. All this is moot, as it has nothing to do with what the OP was asking in the first place, which was "Is there a vacuum regulator that regulates the vacuum to the brake booster?" The answer is no. Whatever vacuum exists in the intake plenum, which charges the vacuum cannister, is what is at the brake booster (minus losses). If there was no vacuum in the plenum at WOT, there would be no vacuum for the brakes or anything else at WOT. And there clearly is. Let's just drop this and see wha tthe OP finds. I hope Macster is right and it is just the booster. Excellent description, BTW, Mac.

Cliff notes Removed ABS and repalced the brake booster and also the master cylinder. The brake booster inside was dry but rusted. So it was sticking. Could not get the brakes bled. Free AAA to the dealer. Back on the road for $200. Now, I talked with the senior tech there. He said that what I did was correct and exactly what they would have done. Except that the AOS might not have needed replacing now. But certainly at some point.... Tech told me that for these early cars that the brake booster is most always the culprit. Check your drain plugs fo shizzle. But if the brake pedal starts to act weird, repalce the booster. Back on the road.....

Thank you for the follow up.

Glad you got it fixed. Maybe we can agree on low pressure area instead of vacuum. Air pressure causes the cylinders to fill in a NA engine, not vacuum. This thread has regressed to the basic Otto cycle. (Old auto mech students will remember this)

So it was the booster, as Mac said, and the only issue that you needed to have it towed for was to have it bled? Just want to make sure. If that was the case, did you try with a Motive Pressure bleeder or just the ol' pump it method? Just curious.

he replaced the MC and booster... it was rusted therefore sticky... Bleed the brake lines often!

The master cyclinder was fine. But since I had to pull it, for $80 a new one was intalled. I have pumped the brakes for fresh fluid a couple of time over the years. Just using the old pump the brake methods. The culprit was for sure the booster. Even though I pulled the vacuum hose and put a long stick into the unit. It showed no water. But once I got the MC off you could see some rust inside. The guide bushings were dry and had rust. So at some point water had gottne inside. I did the old pump the brakes method. And I would have liked to have tried the Motive Pressure Setup. But I made the choice to just use my free AAA towing and be done with it. EDIT ; search is a good thing https://rennlist.com/forums/996-foru...iy-w-pics.html [B]Now I need to fix the horn bezel, blown fuses, and dead horn. Anyone know how to disconnect the airbag wires from inside the steering wheel? There is an electrical plug that for the life of me I could not get to disconnect./B]