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Diagnosing Brake Booster and Master Cylinder Problems

Pelican Technical Article:

Diagnosing Brake Booster and Master Cylinder Problems

Jared Fenton


1 hour1 hr





Applicable Models:

Porsche 955 Cayenne (2004-08)
Porsche 955 Cayenne GTS (2008)
Porsche 955 Cayenne S (2003-08)
Porsche 955 Cayenne Turbo (2003-08)
Porsche 955 Cayenne Turbo S (2006)

Parts Required:

Master Cylinder, brake booster

Hot Tip:

Replace your brake fluid every two years

Performance Gain:

Better braking

Complementary Modification:

Replace brake fluid

The brake booster is a device that uses vacuum pressure from the engine to decrease the amount of effort needed to push down the brake pedal. It does this by using a large vacuum diaphragm controlled by a small electric air pump. In most cars, the vacuum source is the engine vacuum itself, usually connected to the intake manifold with a check valve to keep the engine from stalling or surging.

A failing brake booster usually will result in a brake pedal that takes more effort to push and also to slow the vehicle. Typically, if there is a problem with the brake booster, the vehicle will trigger a fault code indicating a problem. This does not always mean that the brake booster has failed. If there is a problem with the brake booster, you'll likely notice that the brake pedal slowly sinks to the floor when the engine is running and you are stopped. You'll want to check if the brake pedal goes to the floor with the engine off. If it doesn't, you have a problem with the system. Usually, this is accompanied with a hissing noise as there will be a leak in the system, either at the brake booster diaphragm or the air line to the vacuum pump. If there is no hissing and the pedal still goes down, you should use either the factory computer or Durametric to check for fault codes and also test the vacuum pump.

If the pedal does go down with the engine off, it's an indication that there are problems with the hydraulic portion of the braking system. Most automotive braking systems work by using a hydraulic master cylinder to pump brake fluid to each brake caliper on the vehicle. The entire hydraulic circuit is essentially a force amplifier that along with the brake booster takes little relative effort to press the pedal and stop the car.

The first thing to always check is the brake fluid level. If the fluid level is low, check for leaking brake fluid under the car and also at each brake caliper. If the fluid level is fine, try bleeding the system. A soft pedal can be caused by air in the brake lines. See our article on Bleeding Brakes for more information. If the brake lines have been bled and there is no obvious change in the brake fluid level, it is probably a good bet that the check valve in the master cylinder has failed and it is time to replace it.

A failing master cylinder can cause a few different symptoms. The most obvious sign is a pedal that goes right down to the floor with the engine either on or off.

The brake booster and master cylinder are located under the access cover shown here.
Figure 1

The brake booster and master cylinder are located under the access cover shown here. Turn the two locking pins (green arrows) to release the cover.

Shown here is both the master cylinder reservoir (green arrow) and brake booster (yellow arrow) on the Porsche Cayenne.
Figure 2

Shown here is both the master cylinder reservoir (green arrow) and brake booster (yellow arrow) on the Porsche Cayenne.

Removing the plastic cover at the rear of the engine bay helps gaining access to the vacuum line from the pump to the brake booster.
Figure 3

Removing the plastic cover at the rear of the engine bay helps gaining access to the vacuum line from the pump to the brake booster. See our article on Windshield Wiper Motor Replacement for more information.

Comments and Suggestions:
SakuraGarden Comments: Finally found the cause of my Break problem after replace most of parts.
There is Position sensor on Booster which sensor detect not only a booster Diaphragm position but also the moving speed. The signal is detected for PSM to control Atmospheric flow into Booster opposite side of Diaphragm from Vacuum side.
After replace the sensor, Brake feeling become much better. Diagnosis for Intelligent Brake system is very difficult.
September 21, 2017
  Followup from the Pelican Staff: Thanks for the follow up. We appreciate it.
- Nick at Pelican Parts
rsuarez Comments: Hello, I replace the front pads on my 08 cayenne turbe with red calipers. I was able to remove the calipers and push the pistons back without having to bleed the system. Got everything back together and now the brake pedal is hard. I'm able to stop as long as I don't release the brake pedal and press again, at this point the brake pedal gets hard and I haveto press harder. There is no brake fluid leaks and the level is full. I have ordered the brake power bleeder but I'm not sure if its a differnt issuse.
June 25, 2017
  Followup from the Pelican Staff: Does the brake booster vacuum hose have any leaks? if not, the master cylinder may have failed. - Nick at Pelican Parts  
Marc Comments: On my 2010 cayenne the right front and left brakes are sticking on after u let off with ignition on when off its fine
May 9, 2017
  Followup from the Pelican Staff: Check for a restriction in the hydraulic system. With the vehicle lifted off the ground, apply brakes so they stick. Then loosen hydraulic fitting working from the master toward wheel until it unlocks. - Nick at Pelican Parts  
Asim Comments: Hi,super helpful write up. On my 2008 Cayenne S, my pedal is hitting the floor. When car is on, I pump the pedal, it gets hard. Then when I release it, and push, the pedal sinks. Fluid level has not dropped which leads me to believe there is not leak. I have had it bled from my local mechanic twice. No change. A few question please.
1 Could it be anything other than the Master cylinder? If so, what?
2Do you have any DIY replacement documentation or can guide me to it?
3 If I replace MC myself, does the new part need to be registered into the vehicle's computer system using a tool like Durametric. Essentially, must I take it to the dealer?
March 3, 2017
  Followup from the Pelican Staff: Sounds like a master cylinder to me. If no external leaks, the internal seals are likely at fault. Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
Mike Comments: Nick, in answer to your question on crank or no crank, it's "no crank" the first couple of tries. Also, I've found that if I pump the brake pedal 3 times before turning the key, it will start, usually the first time.
February 6, 2017
  Followup from the Pelican Staff: If your Cayenne is one of the ones that requires a brake pedal to be pressed to start, you probably have an intermittent fault with one of the switches. Check out this article for more information: - Casey at Pelican Parts  
Mike Comments: I have a 2009 Cayenne GTS. My problem is when I try to start the car, if I don't pump the brake pedal three times, it won't start, and when it finally does, I get the Brake Proportioner and ABS warning light. It won't shift into gear at first, but then will "unlock" after 15 seconds or so. Once it's going, there is no problem other than the Check Engine light staying on for the next few starts. I've checked the wiring harness under the driver's side carpet, and it seems dry and I don't see any corrosion. Cany anyone think of something else to check?
February 3, 2017
  Followup from the Pelican Staff: I would start by checking for fault codes. When trying to start engine, is it cranking and not starting, or is it no crank? - Nick at Pelican Parts  
Rod in Forfar Comments: Nick: Don't overlook the obvious with a newbie. The problem was that I had mistaken the unlabeled power steering reservoir for the brake reservoir. After gradually realizing the mistake, I soon had things sorted: the brakes bled, the power steering system flushed and bled, and new front diff fluid and engine oil while I had things apart.
December 16, 2016
  Followup from the Pelican Staff: I don't have a crystal ball, so I have to assume you are in the right place. Especially with the provided photos. Glad you got it sorted, glad to be of help if there is a next time. - Nick at Pelican Parts  
Rod in Forfar Comments: While removing the right front axle I disconnected the brake line and left it dripping into a pan for several days until it stopped. Now the brakes won't bleed, even with positive pressure on the reservoir. I'm new to Porsches. My 2004 Cayenne S had excellent brakes before I went after a leaking boot. Our shop has a full hoist and most equipment. Suggestions?
December 5, 2016
  Followup from the Pelican Staff: Allow the system to gravity bleed, then try to manually bleed it again. if it won't bleed, use an advanced Porsche scan tool to cycle the bleeding process. - Nick at Pelican Parts  
Shankar Sharan Singh Comments: I have a Porsche Cayenne S 2014 model. The brakes make screeching noises when breaking hard. Is it something to do with the brake pads wearing off?
October 15, 2016
  Followup from the Pelican Staff: Yes, I would inspect the brakes. - Nick at Pelican Parts  
Stefan Comments: I have a Porsche Cayenne 2004 and I just got on the board the light brake booster failed the brakes works good I don't know what's the problem my mechanic doesn't have Porsche computer what's you suggestion can I still drive the car or no thank you !!!
August 4, 2016
  Followup from the Pelican Staff: I wouldn't drive it until you can figure out what the fault is for. Could be a major issue with brakes. - Nick at Pelican Parts  
snapon Comments: have a 2008 cayenne has a noise under dash and brake booster has hissing noise when depressed with engine on or off has super brakes but when you pump brake peddle you can hear maybe air door under dash flopping and at idle there is a noise like a motor running quits at about 1000 rpm thought maybe air door was problem if i pinch off the vac line to booster will it set a code
May 16, 2016
  Followup from the Pelican Staff: It's tough for me to help with sounds like this, without hearing them myself.

i would try to pinpoint the source of the hiss using a mechanic's stethoscope or a similar tool. maybe a smoke machine if a vacuum leak is suspected. - Nick at Pelican Parts
Trieuman Comments: I have a brake booster faulty indication on and off about three weeks ago. However the brakes seem to be fine. what should I do to make sure that my 2004 porsche cayenne turbo prevented from having bigger problems? Another issue I have is that every 2-3 weeks I have to add about 2 litters of 0w40 to the engine and there is no leaking problem. Please tell me what I must do to avoid adding oil unless its time to change the oil. Thanks.
May 2, 2016
  Followup from the Pelican Staff: Your engine may be burning the oil. Check the spark plugs for signs of oil.

Can you share a photo of the warning light that was ON?
- Nick at Pelican Parts
habib Comments: Hi! I have a corolla 98 . Its brake system is not working priperly. Pedal becomes hard on applying brake some times. The check brake light never comes on however check engine light comes on for some times. Is there any connection of brake with engine check light ?
March 8, 2016
  Followup from the Pelican Staff: Could be a major vacuum leak, causing low vacuum to the booster. - Nick at Pelican Parts  
Josh Comments: I have a 2009 Porsche Cayenne Turbo S My brake lights are on steady while the vehicle is off. I researched and see that it does not have a brake switch for that year. Any idea of what might cause this?
February 20, 2016
  Followup from the Pelican Staff: Your vehicle should have a switch. It may be called a pedal or brake travel switch.
Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
Tommy Comments: I've got a 2013 Forte and on several occasions when coming to a stop the brakes have failed and the car keeps rolling. The only way to stop the car is to throw it into Park. Also when the car is rolling the engine revs quite high without making the car move any faster. The dealer claims there's a moisture freeze up in the one way valve in the brake booster this always happens in the winter. They are going to replace the valve. Does this make sense? Thanks
January 27, 2016
  Followup from the Pelican Staff: Sounds like it could be your issue. Icing could cause that issue. - Nick at Pelican Parts  
Grea Comments: Hi i have a 2006 turbo s cayenne and so far complaint to 3 different places including dealer about brakes imho feel worst than "normal car" in panic stops as the car doesnt seem to stop, and never locks wheels, so far i have never felt the abs vibrate, and when hard braking pedal goes all the way to the floor.normal light stops seem ok. But If i am on a red light and pusf the pedal hard, it will eventuaIy go to the bottom. I have scared myself a couple of times now trying to stop after hard acceleration. Dealer tried to convince me and said i might have been going to fast. Dont think so. Ive had cheaper cars that stoped way faster. This thing has huge calipers. Dealer checked rotors and pads so did other garages and all said within specs. Also had dealer replace fluid and bleed system but same problem still. Please advise. I just know that a porsche shouldnt brake lime this. They tell me its because it is a heavy suv, but my landcruiser stops quicker and can lock wheels on emergencies. Please no dash lights and no computer codes.
November 6, 2015
  Followup from the Pelican Staff: Normally you won't feel the ABS come on, unless you lose traction. Without feeling the braking, I can;t comment on whether it is normal or not. It is too important of a feature to guess at. I would suggest seeking another opinion. If you keep getting the answer it is normal, they are likely correct. - Nick at Pelican Parts  
yas Comments: Hi. I have porsche cayenne base 2006. Computer checked car it said brake booster problem.i have been having problems while applying brakes.sometime it works when brake paddle is pressed twice. first press on brake has not good effect.I dont know if the brake paddle sinks or doesnt sinks.but there is a hissing noise when brake paddle is pressed when driving or when car is already stopped. And while pressing brake paddle the paddle has very loose/soft effect . Kindly tell if i gotta change the brake booster or brake booster can be repaired or its a problem of brake oil being low ? . And yes there is no fault code seen .
September 8, 2015
  Followup from the Pelican Staff: Does sound like a problem with the booster. - Nick at Pelican Parts  
Thrill Comments: Hi Casey - I have a 2008 base and have an intermittent "Check Brake Lights" warning coming up more and more often. All lights are operational, and I've taken the rear lamp housings off, cleaned contacts and replaced the lamps proper with no change. Started to take off the spoiler, but it was...not I replaced the Torx and left it alone. Rotors and pads are only about 5,000 miles and look good. The issue was there prior to that maintenance, but has become a near-constant warning tone both at system startup and when applying the brakes. Any insight would be appreciated.
August 15, 2015
  Followup from the Pelican Staff: I would check the voltage out of each bulb to the rear comfort system module. There may be one bulb with a faulty circuit where it is incorrect, setting the light. - Nick at Pelican Parts  
Medo Comments: Hi, i need your help. I have porsche cayeen GTS 2008 and i have problem in my break system. The break performance is not working properly and when the engine on and i press the pedal it go toward the floor. I have changed the two black vacuum hoses long and short. Bleeding has been done for all wheels and still same issue. I am confused if the issue comes from the break booster or the master cylinder. I need your advice pls.
July 4, 2015
  Followup from the Pelican Staff: Sounds to me like a fault brake master cylinder. - Nick at Pelican Parts  
SakuraGarden Comments: Vehicle is Cayenne Turbo 2004. Under the Dashbord, this car has very complicated structure and design as not so simple then 4 nuts which is for Booster is not easily see from under the Dashbord even removed the panel. please show snapshot what need to be removed besides the panel under the Dash. I would like to see how those 4 nuts can be accessed.
June 4, 2015
  Followup from the Pelican Staff: I don't have any snapshot to share. You'll have to grab a repair manual from Porsche if you need further help.

- Nick at Pelican Parts
SakuraGarden Comments: Since still problem solved, I ordered Power Bleading system from Pelican Parts and going to replace Master cylinder and Blake pipes as well as Blake Flude for just in case. I am also ready to replace Master back system in a same time. I know how to replace Master cylinder and Brake Pipes however I could not see four nuts to remove Master back Vacuum Booster at under thefront panel.
Could you put the instraction how to replace Vacuum Booster especially for how to access Bolt and nuts at inside the car. Snapshot is very appreciated.
May 28, 2015
  Followup from the Pelican Staff: What vehicle are you working on? You will likely have to remove the lower trim panel on the dashboard. Next detach the brake booster rod from the brake pedal. Then remove the fasteners near the brake pedal, at the firewall. - Nick at Pelican Parts  
SakuraGarden Comments: Thank you for your kind help. I will need maintenance house to check it. If I need new sensor then will order to you.
Thanks again.
May 8, 2015
  Followup from the Pelican Staff: You're welcome. Let us know if you have any more questions! - Casey at Pelican Parts  
SAkuraGarden Comments: Again no faults no warning. level control looks fine since it is normal to measure height at 4 wheel side and no error. Where is brake pedal position? I am suspecting the sensor zero position may be shifted. Please let me know how to check the sensor position. Thanks.
May 7, 2015
  Followup from the Pelican Staff: Sensor position is checked via the actual values page in the PSM control module using a scan tool. - Casey at Pelican Parts  
SakuraGarden Comments: All 4 tires are same bland and same age. The tire condition is actually good. I would like to know what is a mechanism that PSM setting does change emmergency brake performance.
It may describe the lower brake performace Thanks.
May 6, 2015
  Followup from the Pelican Staff: I think that is all based upon the map programmed in to the PSM unit. It uses steering angle, brake pedal position, lateral acceleration and various other inputs to determine a panic stop. Maybe you have something wrong in the level control unit? I would again make sure there are no faults in any control unit. - Casey at Pelican Parts  
SakuraGarden Comments: Thank you for prompt reply. I never got PSM warning. My Brake stops car but when emergency Braking, it just work as usual braking. I have many Car experience including BMW, Audi, Toyota etc. but this Car's brake is something wrong. I would like to recover the car Brake as usual car. One hint seems PSM behavea that something electrically out of performace like sensor offset etc. Thanks,
May 5, 2015
  Followup from the Pelican Staff: Tire size and brand can affect the PSM in strange ways like the ways you describe. Are all 4 tires the same brand and age? I don't believe there is a sensor problem with no PSM fault. - Casey at Pelican Parts  
SakuraGarden Comments: Blake Fluid was changed by Porche center engineer with carefully bleed. There was nothing change before and after change. I am sure no Air issue and no Vacuume line issue, and not related with Pads and Roters. As I mentioned PSM off and or Air Suspensiton Sports setting helps for breaking. I canged Vacuume sensor and Vacuum pump as well. I am now thinking Porche pre pressurelize Pad system and related Blake pedal position sensor issue. But I am not sure how Porshe PSM and Pre pressurize program works. Please advice if any possible mechanism and sensor or parts which need to be replaced. Thanks,
May 5, 2015
  Followup from the Pelican Staff: If the pedal travel sensor was faulty, there would be a PSM warning on the dash. Are you receiving any PSM Failure warnings on the dash? I just want to make sure I understand you correctly, your problem is that the brakes don't stop like they used to?- Casey at Pelican Parts  
SakuraGarden Comments: No. The car is second hand and I replaced brake pads two times with different supplier's. the problem is always exist and no change.
May 4, 2015
  Followup from the Pelican Staff: Then it could be air in the brake hydraulic system. Make sure to bleed the brakes from all 8 bleeder screws. How did you change the brake fluid? - Casey at Pelican Parts  
SakuraGarden Comments: This is not a comment but question. My 2004 Cayenne Turbo Break feeling is not so good as usual cars that jam the brakes on so hard but I never get ABS start to work except at rough road. I have replaced all barke pads Roters, Vacuum hoses with check valve and sensor, Vacuum Pump etc. I did replace Brake Flude. I also confirmed that inside Master Back was kept as vacuum when I replaced Vacuum hose. It was also confimed that Brake Pedal can move to flower little easier at 1st time then get harder 2nd time and 3rd time but when start engine then pedal move toward flower. It means booster works okay. It is also noticed that when I put off PSM switch then brake feeling get better. Also put Air Suspension to Sports mode then get better. Is there any other sensor need to be replaced or I totally need to replace Master Back or Master Cillinder? I think PSM is refering the sensor signal of Pedal position which makes Brake Pad distance adjust for emergency braking. Do you have the sensor and the information?
May 2, 2015
  Followup from the Pelican Staff: Did your problem begin after replacing the brake pads? - Casey at Pelican Parts  

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Page last updated: Thu 12/14/2017 02:43:16 AM