2011 XJL Supersport, 45k miles, serviced regularly.

This has occurred intermittently in the past but is happening more often recently. When pressing the brake to start the engine the pedal is very firm and won’t readily move down enough to let the pedal switch engage allowing the starter to spin the engine. It requires holding strong pressure so the pedal sinks enough to allow engagement. The second the engine lights up the pedal softens noticeably and acts and feels normal and drives as expected. When the car starts as usual the pedal is more compliant, starting as usual.
Thanks for reading this, would like to hear opinions and is there any failure likely?

best regards, Don M

 

Donm,

I'm thinking.....

It sounds like your brake vacuum isn't dumping on shut down,

That's my best input, I'm sure someone else will chime in.

wombat

 

This happens to mine too, it's just the vacuum assist bleeding down overnight. It only happens overnight- if I leave the car for just a few hours there is still plenty of boost to press down the pedal easily. Perhaps there is a tiny leak somewhere in the system.

If the leak becomes too large it will eventually throw a check engine light, but for now I won't worry about it.

 

Not sure if and where located in gas engine XJ, but most cars the vacuum pipe to the brake booster have one way valve. An simple plastic "ufo" looking device on pipe. If and when this start to leak it release the accumulated vacuum of from resevoir / brake booster causing pedal hard to press.

 

question, this is a supercharged engine, does the intake manifold create any vacuum? Maybe there is a vacuum pump?

 

Well technically it (the intake manifold) doesn't create a vacuum, but it wouldn't work without it.
And there is a vacuum pump, on the front of the engine just to the left (as you're looking at it) and slightly below the crank shaft pulley.

 

Common on most any car but does vary depending on how long the vacuum holds out. The brake booster has a check valve built in and that could be leaking or failed.
After you shut the engine down with everything shut off hit the brakes a few times and see how the pedal feels. You usually can get 2 or 3 applications before the vacuum is exhausted and the pedal gets hard.
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Follow up……a preowned brake booster and master cylinder cured the issue. New booster was not available stateside but a booster/ master cylinder was so we are back on the road.

thanks all for the replies!