Occasionally, and generally after the car has been parked for a while, when applying brakes the brake pedal goes down farther than expected, and the car swerves to the right. That suggests that there is some form of dual circuit / diagonal brake safety setup that 'believes' that a brake line has been broken and / or brake fluid / pressure isn't detected.
However, after a release of the pedal and a re-application, the brakes operate exactly as they should. I drove 15 miles, periodically performing 'emergency stops', but the brake were perfect.
But, after the car had been parked for a 1/2 hour, the same thing happened again, and was corrected as before.
Brake fluid level is normal. No dash messages posted.
Any thoughts, anyone?

 

I would look at each caliper and ensure the calipers are floating freely on the guide pins. The pins can rust and freeze in the caliper bores , especially if the rain caps are missing from the pins.

This can prevent the calipers from properly centering themselves against the rotor, and requiring the piston on one side to over extend to reach the rotor. When sitting the piston can retract far enough away from the rotor surface that on first application you don't get full braking force on that initial pedal push. The piston will be closer on next application, so more normal braking.

 

Good input, thanks! The question then becomes: how, after having 'exercised' the pedal and then having the brakes behave normally for miles, does a caliper get out of position again following a stop?
Cheers

 

+1 to mhm - first thing to check

On an old Beamer, I had the exact same issue. It was because the brake hardware ( spring clip) on the pads had worn/warped (probably never changed by previous owner) so when parked “it relaxed” but when in use it was fine.

 

I'll check the Jag parts drawings and workshop manual to see if there is anything that can pull the hardware back. I always though that the brake pistons relied only on the rubber seals to pull back ever so slightly when the brake pedal was released, just enough to prevent squeal and wear, but if something else is capable of creating a 'gap' that would be great to know about. As I have no codes showing, I have dismissed the ABS system (after YouTubing how to repair same!), but what role can the brake master play in this, if anything?
Thanks.

 

Here is an update: I will look at the caliper pins, as suggested, but it still seems an unlikely cause: When the car has been parked for a day, the brakes pull hard to the right. If I plaster the pedal - usually a couple of times - the brakes behave as normal, and they continue to do so, until the car is parked and left for a time - one day in the most recent test. How would the caliper - I am assuming it is the left side (DS) that is faulty - get so far out of position while parked, and then require a massive emergency stop to get the caliper centered again. The brakes have now been pressure bled, and nothing untoward was noted. The brakes operate effectively once they are working and the pedal feels normal: How about a sticking piston?
Thanks for any input

 

Wouldn't I get an ABS fault light, even if my OBD reader can't see the ABS codes ?
I decided to start another thread on the continuing saga, because the problem has morphed into something even more obscure. Please see my posting "Brake Puzzle: Suggestions Please". Thanks