Rear Brake Caliper 'issue'

[bentbeemer]

OK, I pulled my rear wheel to get a new tire put on it and of course I had to pull the rear caliper. I examined the pads and they are very thin... need new ones. I know the new pads will mean I have to push back the pistons and fluid will rise (overflow) in the reservoir, so I poured out about half of the volume of fluid. I am thinking OK good so far. I put a wedge (wood) in between the pistons and pushed hard (very hard).
Here is the problem when I removed the wooden wedge I noticed that one of the edges of one of the pistons (where it presses on the brake pad has chipped off. Is this a disaster or can I ignore it?? If it is a mess can this single part be replaced.... I can't find a part number for just the piston.
Thanks, John

[MIXR]

Two things. The chunk will have depth into the piston and will reduce surface area. Depth will affect seals. Area will affect seating against the pads. Neither should be a problem if it's just a few mm in depth or double that in area.

Even at full retraction with new pads, the chipped edge should be clear of the caliper body. Even if it extends into the body, it will be clear of the seals by a good way. However, a chipped edge that is 5 or 6 mm 'deep' indicates something hard has either hit the piston face or been lodged in ithe caliper. It's still no big deal. A massively large chunk missing is unlikely to exceed a few % of overall piston area, so the piston will still sit flat on the pads.

So .............. If the chip is less than a few mm deep (into the piston), or several mm in area on the face of the piston, I would completely and utterly ignore it. Different if it has a radiating crack though.

I have quite a large chunk missing from one edge of mine. It is not uncommon on high mileage bikes that have been used on dirt roads or extensive back-roads. The caliper is prone to picking up crud, including small stones and other debris. Maybe post a photo if you are really concerned. Or PM me.

A lot of such chips are also caused because people use inappropriate tools like screwdrivers to prise the pads apart or push-back the caliper pistons.

[bentbeemer]

Mixr, Thanks for the post. I think I will be all right it is just a small amount of missing material. I did notice the level in the reservoir didn't increase when I pushed the pistons back.... I have ABS on this machine and I am now thinking the fluid must go back into the ABS pressure modulator and reservoir, guess I will have to check that. Do you know if this is correct? Thanks

[richierex]

Yup, thats where it goes, might be worth checking the level, but if the self-test and warning lights are all OK, you should be alright.

there are some excellent walkthroughs on here, search for " bleed ABS " or wait for someone else to point you in the right direction. Good luck.

[MIXR]

Just fitting new pads should never be a problem with the fluid level unless someone has 'filled' the reservoir when really old and worn pads were still in the bike. Obviously, the new thicker pads have to displace some fluid and that cannot happen if the reservoir is already full. You will hopefully find that out because the excess brake pressure will lock the brakes on before it blows past the reservoir seals!

My service guys told me I had really worn pads. I had a set at home and said I'd fit them. They still 'filled' the reservoirs with new fluid. I had some bleeding to do to get rid of the excess!