2005 Brake pressure problems in the Morning

mbsjshad · Post by **mbsjshad** » 16 Nov 2010, 13:25

I have a 2005 S60 AWD. When I start it in the morning there is no pressure in the brakes until I pump it up and the car gets warmer. After this there is no problems until the car sits overnight or a long period of time. This really sounds like a check valve of some sort but I can't find anything on the net regarding this. I have seen that others have this problem but they get no replies. CAN ANYONE HELP ??? What is happening is that it losses its build up of pressure that's supposed to stay there to give you brakes when the car starts after it is cold. After it warms up (the car)it builds back the pressure. I have gotten to this point (realizing its pressure of some sort)but cannot find the solution. Thanks in advance to anyone that can help!

Post by **igel513** » 16 Nov 2010, 19:35

Same problem w/ my 03. I checked the wye check valve beside the intake man. and its working fine but still have that problem. I have checked other sites and have found no threads re: this prob.. Maybe some of the gurus here could help out.

otis44 · Post by **otis44** » 16 Nov 2010, 20:55

The check valve is the little round thing that plugs into the brake booster. One way to check it is to pull it off with the engine running and be sure you have vacuum. Check the hose that runs from the check valve to the intake manifold for leaks as well. To check the check valve (connected) per Vadis; With the car off pump the brakes a few times until the pedal has no travel and hold the pedal down. Start the car, the pedal should travel and feel softer. Release the brake and shut off the engine and wait one minute, the pedal should feel soft, but become stiffer as it is depressed several times.

To check the booster itself, the procedure is the same as above. It is noted that if the pedal feels hard and unresponsive with the engine idling the booster is in need of replacement.

I couldn't find anything on the master cylinder itself, hopefully this will give you something to go on.

RaymanSean · Post by **RaymanSean** » 17 Nov 2010, 07:08

You should not have to pump up the brakes even if the booster was failed the pedal would be firmer than normal (require more effort to get the same braking power). I had a similar problem as mb described. I bled the brakes until the fluid ran clear again. (I bought the car used and the fluid was the color of dark roasted coffee and had water droplets in it. ) Brake fluid is hygroscopic thus the system should be bled until the fluid is clear at least every two years. Otherwise you run the risk of reduced performance in the short term and in the long term the fluid will begin to boil under otherwise normal conditions forcing some of the brake fluid into the reservoir making room for the vapor which will then condense and pull the brake caliper pistons in which requires more fluid than necessary to engage the brakes and thus the need to pump the pedal to get a firm pedal.

jda2000 · Post by **jda2000** » 17 Nov 2010, 08:01

with the car on, stick your head under the dash by the brake pedal, and listen for a hiss when you press the brake pedal. If you hear a hiss, then you have a vacuum leak at the booster.

Retired MVS Contributor · Post by **Retired MVS Contributor** » 17 Nov 2010, 08:30

There is no "pressure" built up when the car is not running...You supply the pressure when you press the pedal...The vacuum booster does not supply it, it provides a power assist so that you do not need as much foot pressure for stopping...When you get in the car and before you start it, the brake system is a conventional unassisted hydraulic system and should give you a pedal if you step on it before starting, albeit a hard pedal...If you have your foot on the pedal as you start it, the pedal should sink a bit as vacuum takes over and assists you...There is a vacuum reservoir, but it's purpose is to provide a reserve of power assist if you stall on the road and attempt to stop...That is why you do not want to pump the brakes to stop if it stalls, you will use up the stored vacuum and the car will become hard to stop...

The first thing I would check is the vent in the reservoir cap...More about that later...Then the system should be bled...All indications are that there is air in the system...I cannot begin to guess how it could get in there, but it is my suspicion...

Now to the cap vent...In 2006 my son-in-law bought a new V70R...It was a stick shift, and stick shift cars use the same fluid reservoir for brakes and clutch...Depending on weather conditions, mostly temperature, the clutch pedal would be at differnt levels each day...One time he could not get it into gear until he pumped the pedal...He took the car back to the dealer for this problem 5 times and they could not find the problem...After 5 trips back to the dealer he demanded and got the car replaced under the lemon law...Eventually one of his mechanic friends told him the factory took the car back and had determined there was a master cylinder venting problem, but he did not know exatly what...To this day we still don't know why that clutch behaved the way it did...

RaymanSean · Post by **RaymanSean** » 17 Nov 2010, 10:01

Jerry, I have been there with a BMW albeit the problem started when it had 200K on the clock. Back to MB's problem I doubt that the vent cap is the problem, but the cap could cause the problem MB described and it it stupid simple to check by loosening the cap in the morning and see if the symptoms return the next morning. I am still hedging my bets on old brake fluid.

Retired MVS Contributor · Post by **Retired MVS Contributor** » 17 Nov 2010, 13:10

RaymanSean wrote:Jerry, I have been there with a BMW albeit the problem started when it had 200K on the clock. Back to MB's problem I doubt that the vent cap is the problem, but the cap could cause the problem MB described and it it stupid simple to check by loosening the cap in the morning and see if the symptoms return the next morning. I am still hedging my bets on old brake fluid.

I agree...Perhaps the wrong fluid was used at some time in the past and is boiling, forming bubbles...There is no other way the pedal could be pumped up, exept for air (or vapor) in the system...It's a dangerous situation though, the symptom could return at any time, God forbid at 70 MPH...

Jerry