hi, ocassionally when driving, my brake pedal will go rock hard,then when i release it and press again its fine, buy then the abs light comes on ?? any ideas anyone. thanks ben.

 

Did you manage to solve the issue of your brake pedal going hard? I am having a similar issue with my 96 xj12.

 

2 of my X-type Estates do this. Usually on start-up after sitting for a couple of days. Brake pedal is hard for a bit, I will drive it aound for a couple of mnutes, stop the car, turn off the engine and restart, then the brake pedal feels normal. I have looked at the brake booster vacuum lines and can't find a leak, but have not hooked up a vacuum gauge to see if there is alway a vacuum.

Hope to see if others have solved this problem.

 

Yes, I would first check for vacuum leaks. Spray the lines with WD40 or starting fluid as the motor is running and listen for the motor's RPM to change. If it does that is the spot it is leaking.

At the booster vacuum line, you should have a check valve (one-way valve) that could be going bad.

This is not a problem you want to go unrepaired, you need someone who knows what they are doing as it can kill you and/or other people.

 

Been there, did all of that. On one of my 2 X-types that have the same symptoms, I did a thorough vacuum leak test, and found no leaks. Then for good measure I replaced the vacuum line/check valve for good measure. No Difference.

I haven't done any more investigation, but the feeling I get is that something is happening inside the booster. My problem happens repeatably after the car has sat for several days or longer.
I start the car, notice that the brake pedal is almost rock hard, then I push the brake pedal for 5 to 10 seconds, turn off the engine, restart the engine and all is normal. In my case, (OP wrote that his happens while driving, so perhaps my problem is different) the only time this happens is the first time a car has been started after sitting for a while. This week, I have driven the car every day and have not seen the "hard pedal" issue.

 

You might want to look at possibly replacing the master cylinder. I had a similar problem and it turned out that the pushrod in the master cylinder was not returning all the way back when the pedal was released which didn't allow the master cylinder to relieve pressure by the fluid draining back into the reservoir. Also there's a pushrod adjustment on the brake booster which may be holding pressure in the master cylinder. When the pedal is hard, trying bleeding air out of the vacuum line running to the booster which would confirm the pushrod problem.

Hope this helps.