Guys I have to admit I dont' know the answer to this one for sure, so let me bounce it off you:

Setup: I'm on an empty road going 60 or 70mph and hit the brakes as hard as I can. ABS works, Car stops perfectly.

Result: For the next few minutes the brake pedal is perfect, better than normal. It is high and hard, what I mean is if I need to use the brakes it feels like they'll engage immediately when I hit them. I feel like taunting all the deer - Ha ha go ahead and jump boys, I'm ready for you now. (Lord knows I shouldn't tempt fate with a joke)

Normally there is a bit more travel before the brakes engage, quite a bit more travel to engage fully, and certainly a lot more pressure required to acheive the same result.

Question: What causes this? Is it just plain old physics? Heat causes expansion; so all the parts in the brake system need move less to transmit the pressure to the rotors/pads? Seems too simple to be the only answer, but let me ask.

Maybe my first question should have been - is this the same as your XK8?
I've read countless posts about the intricacies of bleeding our brakes. Could there be just the littlest air in there as demonstrated in this comparison?

This topic reminds me of an article I read about the latest F-Type. They advertise that the brakes are pre-pressureized to reduce pedal travel and stopping distance. Wonder if heating our brakes up pressurizes something.

Let me know what you think. Thanks,

John

 

Most probably the friction coefficient of your pads/rotor combo changing with temperature causing them to feel more powerful.

The higher pedal is probably due to the pads being in closer contact to the rotor than normal. Pads get 'knocked back' by a few thou after application due to rotor runout, wheel bearing play, etc.

 

Johnken,
I believe what Jaguar is refering to is the master cylinder residual check valve. This valve is installed in the outlet of the master cylinder, and is designed to retain some brake line pressure, (5-10 psi) within the system that will ensure a rapid brake response after the system is 'charged'.
It sounds Like your check valve is leaking down, therefore I suggest you have the master cylinder rebuilt with a new residual check valve. Kits are available and are not terribly expensive. Rebuilding a master cylinder is not a difficult job, just don't get any brake fluid on your paint!
Regards,

 

Thanks WhiteBear, Wow can I assume that your car doesn't do what I described? The brakes show no difference in performance after such a hard stop?

If so, it looks like I'm about to learn how to rebuild a master cylinder, Yikes! Please let me know (and I invite others to chime in) if your car does not do what I described.

Thanks, I appreciate the input.

 

I would do a little research before attacking the master cylinder. The check valves are usually aftermarket devices used on hot rods, particularly the older ones with drum brakes.

Doing a hard stop will definitely heat the pads and rotors, causing them to grab better. Of course, repeated hard stops will result in brake fade, especially if the fluid hasn't been flushed in years.