growing up playing of cast steel calipers on cars, I hadn't really thought about what happens on cheap alloy bike ones that are left to fend for themselves for too long...
I have 25 year old Suzuki I acquired with brakes that are sticking a lot... A mate was saying it corrosion behind the seals but I dismissed it, thinking he was being a little odd in his thinking. So being rather lazy and not actually really planning on using it I thought I could rather clean up the sliding pin and just work the pistons back and forth and all would be good with the world. The rear got over itself but the front just wouldn't (and what made it feel worse was the sliding bits on the front were corrosion free and lubed up before I started !!!)
Another year of sitting doing nothing and it didn't want to push round the garage easily (even after multiple attempts pushing the caliper back and forth and pumping it back out), so I decided right pal... I'm going to unbolt the front caliper, get those pistons almost popped out and clean everything up with a Dremel and light weight wire brush. Loads of fun with furry aluminum round the pistons, whilst the actual pistons were in a state with rubber / corrosion / filth not wanting to come off. Then to clean off all the crap I needed to get the pistons to spin a bit so I could get round the back... My shortcut took forever. especially with one so stuck it took a lot of effort to get it to move in and out let alone spun round to get the gunk off.
Anyway from the outside all cleaned up with the pistons sparking and the visible fur all gone... and they were still sticking ! Still no plans for it to go anywhere, and we roll on another year in a dry garage. However this time I need to get it MOT'd and registered before Boris makes life hard. But this time I can't even move the bike. So going back to what my mate said I thought - he may have a point !!! With the possibility of actually riding it, I thought a fluid change would be a good idea by now, so not much to lose popping the pistons out and taking a good look at the corrosion on the inside of the caliper (where the fluid sits) except there wasn't any !!
With the pistons out the actual seals looked a right state.... and so it turns out the real reason they were stuck and why moving the pistons back and forth, cleaning the sliding caliper pins, having all the external parts clean, together with shiny pistons, had NO desirable affects at all is entirely due to the build up of corrosion in the caliper body behind the piston seals.... They push the seals out jamming the pistons solid... hard to believe, until you strip it properly and find the pistons surfaces and the caliper bores are like new... But after removing all the fur in the seal groves the pistons almost fall in and out on their own !!!
