Brake lag after starting

trstjohn · Post by **trstjohn** » 16 Feb 2011, 13:42

Not sure if it's just a cold weather issue, but i've noticed it takes about a 10 second lag for my brakes to be fully functional.

Anyone see this as a major issue?

TexasCoder · Post by **TexasCoder** » 16 Feb 2011, 14:55

Interesting, you mean from the time you press the brake pedal, to the time you feel the vehicle start to slow down?

trstjohn · Post by **trstjohn** » 16 Feb 2011, 16:30

Yes -- for example, when I put it in reverse it doesn't slow down when tapping the brake. I have to 'floor' the brake to get the car to stop.. The more I think about it, it's more noticeable when going reverse then forward.

As I said, it's only when I initially start the car. At all other times during driving the brakes are fine.

road1will · Post by **road1will** » 16 Feb 2011, 20:25

perhaps try bleeding the brakes. When was the last time you did a brake system flush?

Post by **jblackburn** » 16 Feb 2011, 20:28

Find a big hill - preferably where there's no other traffic, and stop on it for 5 minutes with your foot on the brake and the car in gear. If the pedal begins to sink to the floor after some time, it is likely blown seals in the master cylinder allowing air into the system.

Post by **jimmy57** » 16 Feb 2011, 21:19

When the engine is cold the exhaust variable cam timing is advanced and timing is retarded for emissions and quicker warm-up. Both these affect manifold vaccum. An electric vaccum pump is fitted to give adequate power brake vacuum in this situation.
Your electric vaccum pump or its switch is faulty. Try checking fuses first to see if the vacuum pump fuse is blown.
If you turn key to position II and do not start engine the vacuum pump should run if you pump brake pedal and bleed off vacuum.

TexasCoder · Post by **TexasCoder** » 17 Feb 2011, 01:25

I like jimmy57's comment and suggestion, along with road1will and jblackburn. All good comments! Certainly investigate these areas and find the issue and get it resolved. Any malfunction in your brake system is a huge potential for disaster. With the XC90 being as heavy as it is, you do not want to find yourself needing to stop...and the XC90 not willing to comply

Let us know what you find, my bet is on a vacuum leak somewhere, air in the system could be a contributing factor, but I would suspect you would also see hesitation in forward braking too. It may eat up a saturday or sunday, but take your time and go through the entire brake system "just to make sure" everything is top notch...vacuum, pads, rotors, fluid...

trstjohn · Post by **trstjohn** » 17 Feb 2011, 05:45

Great feedback everyone -- I will check it out..

trstjohn · Post by **trstjohn** » 18 Feb 2011, 06:42

Would you recommend this kit for bleeding the brakes?

http://www.harborfreight.com/brake-blee ... 92474.html

I saw it on this post.

https://www.matthewsvolvosite.com/forums ... =1&t=32021

Post by **jimmy57** » 18 Feb 2011, 08:26

I'm not discouraging bleeding brakes but air doesn't get in brake system hydraulics and leave on its own only to come back again when the engine is cold again.
The electric vacuum pump is fitted to that vehicle EXACTLY for the low vacuum and resultant hard brake pedal while the cold engine warm up/emissions routines are taken to extremes when the temp is low enough.
When the S80's came out in 99 you would have a harder brake pedal when temps were below around 40F and it always was more noticeable in reverse. Not sure the condition was actually different in reverse but if you park where you have to back up to leave you will be backing when the vacuum is lowest and cold idle speed increase is greatest. If you park heading out you likely don't have to completely stop while the lowest vacuum is present.
The vacuum pump is mounted in area next to frame rail on driver's side, next to air cleaner but a bit forward.
There is a black plastic manifold with a switch having a two wire electrical connector on driver's side more or less beside the strut tower. The pumps can fail and the switches can fail. The pump failures are usually because the switch sticks and pump runs continuously. Usually the pump will only run on cold starts or if you pump brakes several times rapidly.

Do get it resolved. Low vacuum is some risk as a number of extended functions for rollover, stability control, and emergency brake assist for panic stops relies on brake booster to be normally functioning.