Since I had new front pads and disks fitted I’ve had a problem with my XJ40’s front brakes. After I’ve not driven it for a few weeks, the car and steering wheel pull hard to the right when the brakes are applied. If I then stop the car and push really hard on the brake pedal, the problem goes away for the day and under braking the car stops straight. In an attempt to fix this, my mechanic has disassembled the LH front caliper and cylinder, cleaned out some fine debris from the cylinder, and replaced the seals. Unfortunately that hasn’t fixed the problem.
Has anyone else had this problem? Could it be a fault in the ABS module? Ideas please.
Yes, had exactly the same problem after fitting new pads to the front brakes.
It was caused by faint corrosion on the caliper piston surface on the left front brake. That part of the piston protrudes more and more through the piston seal as the old pads wear thinner. As a result, particularly if the outer dust shield is ill fitting/damaged, the exposed portion of the piston surface tends to corrode slightly. When fitting new and therefore thicker pads, the piston is forced back in to the cylinder and hence the corroded portion now ‘hangs up’ on the piston seal so the braking effect on that side is reduced and the car pulls to the opposite side.
I cured the problem on my car by removing the caliper piston and carefully polishing off the surface corrosion using thin strips of well (engine) oiled ‘wet & dry’ emery paper in a ‘stropping’ motion. Remember that the piston surface is ground to microns and providing there is no ‘pitting’ of the surface and you are extremely careful in removing that corrosion, the surface finish should be retained.
Of course, when refitting, degreasing and extreme cleanliness is essential and new seals should be installed in case the current seal knife edge has been damaged by the corroded piston.
Another possibility is that the caliper guide pins are corroded so that you are not getting the full braking force on the left side. On every service, it was my practice to remove each caliper guide pin in turn and polish the surface and then refit with a smear of copper grease.
Never had any brake problems after that.
Many thanks Bryan. I shall instruct my mechanic accordingly.
OH, BTW - the ‘wet & dry’ emery paper I used was 1200 grit - nothing too coarse of course! 
You mention that the problem only occurs after the car has stood for a few weeks. Is the car parked outdoors ? if so it might be something as simple as one front disc on one side collecting more surface rust than the other. When you brake for the first few times, the cleanest disc works better so the steering pulls to one side. It only takes a short drive to polish both the discs clean and maybe that coincides with you stopping and pressing the pedal hard and the problem going away for the rest of the day.
This could easily happen if one side of the car was sheltered by a wall or something similar, so you could try parking the other way around if that was the case to see if the pulling problem also swapped sides.
Another problem could be the pads are just not a good fit in the carriers. Different brands vary slightly in size and so do production runs from the same company. I’ve had pads that just slide in smoothly with the correct clearance all around and others that were so tight they would have jammed in the caliper if I’d not dressed all the burrs and edges with a file first. It’s only a minute amount that needs filing off but it makes a lot of difference to how they fit.
Hope you sort it out soon.
Cheers.
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One somewhat related service procedure I always adhere to is that when changing pads and retracting the caliper pistons I always open the bleed screw and force the displaced fluid out at the caliper. I contend that forcing the old, potentially dirty, fluid back through the system to the reservoir can contaminate the very expensive and delicate valving mechanism in the ABS control module. Many techs do not use this procedure which I rigorously encouraged as a Jaguar warranty manager back in the day. Another tip I use but have no proof of its success, is to periodically exercise the ABS system. On many cars used in dry locales without snow or ice the system never gets used and the valving MAY get sticky. This can be done on a gravel road by safely locking the brakes and making the system work.
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Thank you gentlemen for the useful repair advice, pointers for inspection and food for thought.
A week or so after my first post in this thread my brake problem seemed to fix itself so I did nothing further. However, a couple of months later it came back with a vengeance and I could no longer free the LH front brake by firm application of the brakes.
I had my mechanic again disassemble the cylinder/piston pairs. Both the pistons had slight corrosion in narrow bands about half way along their length. I followed the procedure suggested above by Bryan_N, polishing the pistons in a lathe using well lubricated 1200 grit wet & dry paper, removing all traces of the corrosion bands. Also, my mechanic renewed the flexible hoses to the cylinders and thoroughly flushed the lines with brake fluid.
This, so far, seems to have fixed the problem.