Hi all - I rarely post but I figured this was too good to pass up given the relative lack of information available about this particular issue on the internet.
The brakes on this car use asymmetrical caliper guide pins. The lower guide pin for each caliper has a rubber sleeve (in addition to the accordion-style boot that retains the grease) that sits in a groove along the lower aspect of the pin (fully retained within the caliper bracket bore). The red arrow points to this rubber sleeve on this Subaru parts fiche diagram I edited and attached.
Despite the caliper guide pins moving well in their bores, it turns out that even the most minor swelling of this boot is catastrophic. This swelling can be impossible to see with the naked eye (as was the case for me) and can variably allow the guide pin to be reinserted into the bore without issue. Indeed, the left and right sleeves on the two lower pins on my front brakes appeared identical, but my right sleeve variably wouldn't re-insert into either bore while my left did. This was true before cleaning, after cleaning, and after greasing with a high-temp silicone grease. This sleeve, if left untouched, can cause the caliper guide pins to fail to fully re-seat after piston compression, essentially binding the guide pins and causing the brakes to drag. What is so annoying about this is that the pins will still move freely and the loss of pin travel in the bore is so minor it a) doesn't affect replacing the caliper on the bracket or aligning the rotor and pads within the assembly and b) can even still allow the wheel to spin relatively freely after reassembly. This makes it really hard to diagnose as there are many more obvious and common issues to consider, including a stuck/misaligned caliper piston, faulty replacement parts (e.g. warped rotor surface, unevenly milled rotor hat, brake pad misalignment, and even brake hose failure).
This rubber sleeve, according to a 2007 patent filing (see:
https://patents.google.com/patent/US20080093181 ), has three purported design goals. As speculated in various forums and blogs (see:
https://forums.anandtech.com/threads...his-is.259493/ and
https://blog.1aauto.com/what-does-th...t-actually-do/ ), the primary goal is to reduce caliper vibration, presumably by dampening the rate of return along the guide pin and/or compensating for upper guide pin wear. The secondary/tertiary goals as described in the patent filing are to possibly improve guide pin alignment/centering and even improve lubricant distribution, although it is unclear in my brief search if there is actually any data behind these claims.
Anyway, I wanted to provide a sample size of one (1) to state that removing these sleeves doesn't appear to degrade brake performance in any noticeable way. I did apply a little extra grease in order to compensate for the loss of volume after sleeve removal. I'll update if I notice any issues, including increased noise or wear.
Hope this helps some of you if and when you encounter overheating calipers with no readily apparent cause.