My 2000 XKR has exhibited poor brake response at morning startup. For the first few minutes I have to really step hard to keep it from moving when placed in gear.
I have observed this for several months. What may be a clue is that the brakes felt great for a few hours after removing/installing the left rear caliper to replace rear shock bushings,
I had also removed and replaced the air box to replace the alternator. My pads all appear fine and there is no fluid loss. Linda reported the same issue in 2011 but not the solution. I've ordered new rear flex lines and plan to clean/grease glides any other suggestions?

 

vacuum leak? If the brake booster check valve is sticky it might bleed down overnight and then need a good warm up from engine heat in order to maintain vacuum?

how long do they feel soft? A minute or less. How long? Does pumping the brakes help?

 

Arcuda, The brakes are really soft for about 1-2 minutes. Pumping the pedal feels slow and resist-ive and does not improve pedal height. It later improves to as I drive to acceptable but they really don't ever feel really firm. If I brake hard enough for the ABS to activate a couple times the pedal is higher and more firm for a few miles. The ABS unit was removed / replaced recently and bled several times. I'm hoping its a vacuum issue, bleeding has always been tough on this car.

 

I would say start by checking the vacuum line to the brake booster. The other end is at the throttle body with a weird brass fitting and plastic collar type of setup. There is an o-ring involved too. Make sure it looks good and air tight. My car has 2 separate sections for this line as well. FWIW, the brass/plastic assembly is available separately from Jaguar even if not listed for our XK8/XKR specifically. If the brake booster is not assisted by vacuum, you normally get hard brakes requiring a lot of pressure to operate.

Separately, have you tried to bleed the system at each caliper? Sometimes air gets trapped and acts as a "cushion" of sort, a sure recipe for soft brakes. FWIW, I believe the Jaguar procedure calls for starting the bleeding at the shortest line first, then go by increasing distance, possible the opposite of what we have all heard over the years.

Also, check that if you press on the brakes a while (at rest), the pedal does not end up all the way on the floor as it might indicate a master cylinder bleeding internally.

Best of luck, keep us posted.

 

Vacuum seems likely so I've ordered the vacuum fitting for the supercharger elbow. I pulled the check valve after is sat for awhile and got a loud hiss so I expect the check valve and diaphragm are OK. I'll check again overnight. Last week when one rear caliper off it was both suspended at an unusual angle and slightly squeezed to re-install. I believe there must be a link here to why the brakes were so great just after this (albeit only to overnight). Perhaps trapped air moved or perhaps a collapsed brake line was temporally opened up.
Air is the lines is still possible since I've had the ABS computer off several times and there are many lines disconnect each time. .

 

If you got a loud hiss, it says the booster is under good vacuum so maybe its something else.

See if you can start the car, get some vacuum develop in the booster. turn off the car and pump the brakes.If its hard, and solid like you have brakes, thats good. keep pumping and try and use a;; the vacuum and feel the pedal change.

Need to feel the difference between an assisted braking and unassisted. there might be a bubble in the line. might be vaccum. might be mechanical

 

I replaced a leaking o-ring at the PCV line at the Intake tube and I found that the resonator tube's previous repair had failed leaving a large leak.
While in there, I wiggled the booster line at the supercharger elbow and connector. It appeared snug.
I probably stressed these areas last month when replacing the alternator that also when the leak started.
Right now its warm outside and the brakes are strong. I may have to wait a few months for another cold morning to see if the problem is gone.

Its an ugly repair but two intake leaks fixed.

Two Vacuum leak locations, one causing oil leakage.

 

Yup. That could definitely do it. Good luck!

 

My intake tube repairs yielded no improvement in braking. Issue is only during the high idle period of 0-90 seconds while idle is around 1100-1200 RPM.
Once the idle drops to 750, I'm usually down the block and the brakes are responding better. I'm usually in reverse within 12 seconds and keeping it still is a challenge.

 

Do the brakes come up without pumping if you let the car idle for a short time, or must they be pumped ? The latter is almost certainly air in the system.

Entertaining the thought that it might be vacuum related, using your obd reader and scan tool, monitor the manifold pressure at startup. Does it start high and slowly drop, or does it quickly stabilize at the expected value?

If you are certain the brakes are properly bled, I'd be questioning the integrity of the brake booster bellows at this point. You might have just large enough a crack that it takes awhile for the system to find a balance.

 

mhminnich, I will try your suggestion to read vacuum levels with OBD tool and let the idle drop before using the brakes. I bled it 2-3 times and I was not getting air bubbles out but I may never be certain that all air is out of the ABS system. I'm still at a loss why they felt great after unbolting the and moving the rear caliper around.

 

At idle MAP is about 50-51 KPa when first started. It drops 2-3 KPa to about 48-49 as the idle drops to 750rpm. The brakes do feel reasonable after the idle drops without need of pumping the pedal.

 

I'm starting to think it may be that the torque converter is not slipping sufficiently at startup. The brakes feel pretty good if I take it out of gear during the first couple minutes.

 

Installed 4 new rotors with new pads as well as new intake tube due to crack at PC vent hose entry. Emergency brake shoes were not previously well adjusted. Cleaned throttle bore. Some improvement in reverse but she still overpowers the brakes for first minute after cold start.
Next I may try cleaning or another MAP sensor.
Brent

 

Update: Brake performance has been restored.
My EGR tube recently broke off at the header. While installing Jag's new beefier EGR tube I located a small vacuum hose crack at the throttle body.
While in there I also cleaned the throttle body. The result: brakes feel much better, engine is less noisy and performance has improved.


Jag has TSB about this failure.

Deteriorated hoses ends.

Improved EGR tube was a bear to install. It is very hard to force/bend into position and due to its larger size there is no room for tools.

New EGR tube weighs 8X as much as original.