Jaguar Repair: 1984 XJ6 heat, friction material, would like your opinion

Question
QUESTION: Howard,
So it seems I have my heat system working reasonably well, and I sorted out the electrical problems that I no doubt caused while removing and replacing the blower motors. When replacing my rear brake pads this evening I found the driver side pads not quite half worn, the inboard passenger side pad worn slightly more, and the outboard passenger side pad was clear down to the backing plate. I am asking for nothing more than your instinctive opinion here, based on having a few decades more experience than me... So, the outboard piston was not completely stuck, but did require much more effort to compress, and there was some visible leakage of fluid. There was negligible scoring of the rotor, so little that I felt comfortable with replacing the pads and not servicing the rotors. I did install the new pads, and while the rear of the vehicle was still on stands, had a helper process several brake applications as I spun the rear wheels and noted positive grip, and release with no leakage, even after two road tests. So, here is what I would like your opinion on: would it be reasonable to think that perhaps the friction material broke loose from one of the pads, which caused my previous episodes of long pedal travel, the over-extended piston was the cause of leakage, and that all may be ok now, even though I plan to monitor closely? I remind you, this vehicle has sat, and sat, for years. I imagine these may not be factory pads, but possibly the first replacements. Just your thoughts...
Thanks,
Michael

ANSWER: That is possible but it is very rare to see any pad come off. I would have been more suspicious of the piston or the pad sticking and wearing out due to continuous contact with the disk.
Howard

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

QUESTION: That was definitely my first, and the most likely assumption. I checked the pads visually when I bought the car in July and was happy that they were quite thick, and have never sensed drag, smelled burning pad, and have driven the car less than 300 miles in this time. Just a thought, I'll have to keep an eye on it.
Thanks!

Answer
It is true that 300 mi is a short time to ware a pad down. So it either broke (least likely) or either one of the pistons is sticking on that caliper. The piston that had the worn pad can fail to retract each time it is applied

(the retraction device is the piston seal itself retracts the piston away from the pad)

Or the opposing piston is stuck and each time the brakes are applied the worn side forces the disk over to meet the stuck piston pad. Either one is common and will give you a soft pedal feel and ware out one pad long before the other. On a few rare occasions the worn pad itself can be stuck in the caliper and thus hold pressure on the disk even though the caliper piston has retracted. This can happen when the caliper is rusted due to a long period of sitting or a poor fitting aftermarket pad that is a tight fit.

In the dealerships we would require that both calipers be rebuilt or replaced each time a car came in with this problem. Brakes are something you do not want to short-cut on.

This happens on front calipers too and we often get a car in that this had happened on and the customer just replaced the pads once and it repeated so they brought it to us then. Most customers will not attempt to replace the rear pads after they see where they are.

Howard