I have a real problem with my 2001 XK8 converible. I did a complete brake job; new pads and changed the brake fluid and bled the brakes several times. I have great brake pedal with the engine OFF, but after I start the car, I have a VERY soft pedal and it is difficult to stop the car when backing out of the garage. After 6+ blocks, I get good brakes again. There is NO air in the system because I bled the brakes several times and I have done many brake jobs before, but this is a new one. It must be in the ABS or power brake system?????

Does anyone have any suggestions?

 

Check for a vaccum leak in the booster system...

 

Hate to disagree, but if the pedal goes down too far, there is air in there. Rebleed watching your procedure carefully to prevent air getting back in there. Some people whren bleeding open the nipple and leave it open while they pump the brakes, this can get air in there, you need to close the nipple every time before letting the pedal come upwards. The booster has a direct link to the master cylinder, it won't give you a soft pedal even if it is bad.

 

Yeah, I usually do my own brakes and therefore am only semi-decent, having only done maybe 12-15 brake jobs. But I have done one or two where for some reason the pedal was soft afterwards. I tried to bleed them and didn't have much luck with that either. One time I had a loose caliper bracket and that let the bracket move when the caliper piston engaged and that's what gave the soft feel. Just a thought.

 

Hi Scott, I pulled the vaccum hose off the booster and have great vaccum and no leaks..... Again, I have great brake pedal before engine start and after I drive a few blocks..... There is no air in system. After I bled the brakes the first time, brake pedal and brakes worked great. This is something NEW and IT MUST BE IN THE ABS SYSTEM. I don't mind sending out my ABS for reconditioning as you did on your other thread. You also suggested that I just take it in to a good brake shop. No one on the forum seems to have had this before..... Thanks Scott for trying...... Ron

 

I have to agree with oldmots. I know you say it isn't possible, but I still agree with him. Bleeding brakes is a three man job: one person pumps the pedal, one person opens and closes the bleed screws, and the other tops up the reservoir every few strokes.

You could try getting a set of these bleed screws: Speed Bleeder Bleeding Brakes Bleeding Motorcycle Brakes Automotive Bleeder Screw Brake Bleeder



They have a check-valve that prevents air from getting back into the system.

Or, you could use one of the hand pumps to pull the air out rather than pushing it out with the brake pedal. In any case, I think it's air.

 

+1 On hand pumps. I have used the press pedal/open bleed valve/close valve/remove foot technique for many years with good results. However if the brake fluid hasn't been changed for years with a resultant build up of water, rust can occur in the master cylinder wall. Pushing the rubber seals over that rust spot can result in a failed master cylinder. I had this happen on some cars (junkyard candidates) that I was maintaining for my children while they were in high school.
Change your brake fluid with a hand pump every time you do a brake job or every two years whichever comes first.

 

If you let the reservoir empty and keep bleeding, you can get air in the ABS valves. If it is bad enough, you will need to go to the dealer and have the ABS valves cycled with their PDU to get the air out of it.

 

Thanks for correcting me on the booster/soft pedal issue. I didn't know that; I'm never too old to learn.

If there is air is the ABS system how does it affect the braking when the ABS isn't cycled? Isn't that system separate from the regular braking system? What am I not getting?

When I replaced my ABS module the other day I opened to lines at the ABS manifold and reseated them - twice with no change in the pedal action. I didn't have a chance to use the ABS yet, though...

 

Well, I finally took the Jag into a brake shop and they found a vacuum problem. When measuring the vacuum after sitting overnight there is very low vacuum pressure, but after the engine runs awhile the vacuum pressure increases to normal. They checked the vacuum line from the engine to the power boost and the power boost itself and found NO vacuum leaks. They said that the engine is losing vacuum when it sits overnight, but produces good vacuum after it runs. They cannot find the problem and suggested that I take it to the Jaguar Dealer. Has anyone ever heard such a thing? Are there any suggestions on how to fix it?

 

could be a internal brake booster problem.

Vacuum Brake Booster Problems Operation Diagnosis
Troubleshooting Brake Problems - Fixing a Brake Problem - Brake Problem Diagnosis

 

Linda,
I'd love to know, Did you ever find a fix for low brakes in the morning?

 

They haven't posted since 2011, better start your own thread.