The Underslung rear caliper gets a lot of road crud thrown at it, sticking pistons, slider pins etc are common. Most of the time a quick clean up won’t do therefore the caliper will have to stripped.
The Sprint I recently got was a prime example of this, it had even worn the pads clean away back to metal
The caliper itself was none to pretty either ! The rivets that had held the pad material had fallen out into the pistons !!
A lot of folk find they fall at the first hurdle of stripping these calipers because of the pad pins. A 5mm allen key is a loose fit in the cap head (it’s actually 5.1mm). So if they are at all stuck it’s waaayyy too easy to round out the head of the pin !
A common rescue for this is to hammer in a slightly over size torx bit into the head and this should grip it enough to remove the little sucker. If not you’ll probably be drilling the head out ! The pad pin torque setting is 18Nm but tbh I only nip them up (10Nm is fine) they never back out and makes disassembly easier by avoiding the above.
Smashing a Torx bit into rounded out brake pad pin !
Then you can remove the carrier and sliding pins, remove the pistons (I use a tool that grip them from the inside and is invaluable for this job) and see the culprit which is corrosion in the dust seal grooves. This pushes the dust seal hard up against the piston, effectively trapping it. Damage occurs and the problem just gets worse !!!
Use a metal pic to lift out the rubber seals and to scrape any corrosion out of the seal grooves, a quick tip is (after a wash) to pop the caliper in a low oven for a while when you then start to scrape the corrosion out it powder away quite nicely
The piston will need a clean up, I start with Autosol and then if there is any stubborn marks or minor damage you can use some 1500 grade wet and dry sand paper to sort them out
A quick rub over will normally do, any pitting that remains after this may mean new pistons have a good look and see if they are smooth and able to create the seal necessary to make the brakes work without leaking.
I give the calipers a go over with brake cleaner then into a bowl of warm soapy water and a good brush to get the crap off. Then either leave on top of the wood burner or in a low oven to get totally dry and hot. This means any aluminum oxidization powders off a lot easier,
Once totally dry I can clean out the seal grooves (I normally use a little brass wheel brush on a Dremel), care needs to be taken not to damage the finish of the seal grooves or bore of the caliper ! Reassemble replacing and seals or pistons with red rubber grease.
The caliper slider pins will need cleaning and lubing with rubber grease and the pad pins will need cleaning and lubing with copper grease or hi temp aluminum grease (preferable), the back of the pads gets a very light smear of Alu grease which acts as anti squeal.
Pay attention to the rubber boot that is situated in the hoop on the caliper. It can corrode in the hoop squeezing the boot onto the slider pin, I remove the boot clean out the hoop and ease the boot back in place with red rubber grease.
The use of copper grease to lube the rubber boots and sliding pins swells the rubber and pinches the pins avoid it !
Cleat out the seal grooves and if necessary replace with new seals lubed with red rubber grease
Lube the piston and press home