Jaguar Repair: 84 XJ6 ball joints, caliper pistons, caliper piston

Question
QUESTION: Howard,
Ok, not exactly the ball joints, but while changing my lower front ball joints, I managed to break the hard brake line that runs from the caliper to the flex hose on the driver side. No problem, I cut and re-flared the tubing and bled the caliper, but now on two short, 20 mile drives, I have lost braking effort while never having any problem during a two hour drive between the two. Each time I lost pressure (pedal going to the floor, but still stopping with greater effort), I have stopped and verified that there are no leaks and that the reservoir has not lost fluid. Each time, as I cut my cruise short, and headed back home, pressure returned to normal in terms of pedal feel. Would you say that maybe air was pulled back in to the system, and perhaps I should flush and bleed the entire system at this point?
Thanks,
Michael

ANSWER: You could have had some air still in the system and as you used the car the air migrated back into the reservoir. When you foot is off the brake pedal there is an open and clear path from all the calipers back to the reservoir but since the lines are so small it takes a long time and many pedal movements to get an air bubble to migrate back up.

Also there is the possibility that since you were working with that caliper, if you moved the pistons, their seals may have gotten flexed in the wrong direction causing loss of pedal due to an excess travel of the caliper pistons. The caliper piston seals have two functions, One is to seal the fluid in and the other is to retract the pistons away from the rotors when you release the pedal and if the seal gets moved too much it will be flexed too far and require too much fluid to make it's move to the rotors.

If they are working normal now and there is no leaks, I would not worry about other then to be sure to use DOT 4 fluid.

Howard

---------- FOLLOW-UP ----------

QUESTION: Howard,
Everything you said not only makes sense, but was in line with what I suspected other than the fact that the condition has come and gone. On a 30 min trip around town last Friday night, it occurred, but pedal feel returned as I returned home, the next day, I drove all over for nearly two hours with no problems, then today to the exhaust shop and back to have the pipe repaired, then this evening, on a similar route as last Friday, lost "pedal" and regained it at about the same interval as the first instance. Not sure what grade of fluid is currently in there, but it is very dark. Maybe a flush, refill and bleed with DOT 4 is in order.
Thanks again,
Michael

Answer
Yes, but look closely for any leaks and watch fluid level. One more thing to check. Jack the front up and check wheel bearing free play as that will cause the caliper pistons to be moved in too far and cause the loss of pedal on one pump but will regain pedal if you pump the pedal several times. Other than a master cylinder going out I can't think of anything else.

Howard