Brake caliper piston seal leaking

[ROZZER]

Refurbed my Z1b brake caliper as there was evidence of fluid leaking that took off paint on lowest part of caliper. Have rebuilt with new seal (piston looks new and is stainless steel). Not used bike but I found brake fluid drip on floor. Have cleaned up and left, again fluid drips found so caliper piston seal is leaking. On stripping and examining the piston there are tiny longitudinal scratches one of which you can just feel with your nail. Is it possible to use fine emery paper to remove the longitudinal scratches using say 600/1000 grade (by rubbing in same plane as the machining marks) together with new seal, or will I need a new piston? Looks like the piston has been made rather than bought as NOS/pattern part (previous owners mate owned an engineering company) and the finish is very fine, can it be too fine?
Have tried to photograph but the images don't show the scratches as they are so small.
Whats the best way to insert the piston when seal is in place, I have always used a tiny amount of brake fluid to lube the seal, is this OK cos getting it in dry is almost impossible. I have heard of using grease, would a high melting point grease be OK?

[london calling]

Special red rubber grease on rubber parts

[Rich]

I always use a slight wipe over the piston with brake fluid if starting from scratch or WD40 if freeing off the pistons when they're on the bike. Try the 1000 W&D then metal polish if it is stainless and new seal you have nothing to lose.

[DogsbolloxofZ1B]

John,
If you can feel it with your nail then I'd replace it. The tolerances in this area are pretty tight so I'm amazed something got in there in the first instance. Was anything embedded in the flat seal? I'd replace that too.

Mike

[DogsbolloxofZ1B]

Castrol Red rubber grease can be bought from:
http://www.mgocaccessories.co.uk/acatalog/MG_xtras.html

Mike

[DogsbolloxofZ1B]

Be careful not to get it on yer private bits...

[chrisu]

Be careful not to get it on yer private bits...

I don't want to know how you know that bit of useful information !!

[ROZZER]

I have used 1000 grade paper to remove the very tiny sratch, then used solvol and reassembled using new seal. Bled system and pumped brake periodically over last couple of days - no sign of leak as yet so I think it's sorted.
No sign of any debris on old seal and the scratch really was very slight on what appears to be a new piston made of stainless, as Rich said, I don't have anything to lose by trying a quick rub with fine W&D paper other than cost of another seal if it doesn't work.
Will monitor over next few weeks and if any leak found will replace seal and piston.

Dogsbollox - How do you know what the effect of the red grease on yer family jewels is

[Big Fluff]

John - Before going any further try this.!!!!

It is very common for the piston to stick to the main seal.... when the brake is applied and as the piston moves out it tries to take the seal with it, buckling the seal and causing a leak. I've seen it loads of times and it's woth a go before splashing out more cash.

With the pads removed and the caliper off the fork leg, remove the dust seal. Next pump out the piston to around ? of it's travel, wipe with a lint free cloth with brake fluid on, then push the piston home back into the caliper. Wipe around where the piston sits, do NOT yet fit the dust seal.
Mount the caliper & pads back on, pump up and see if any leakage occurs.

Good luck, Will

[DogsbolloxofZ1B]

Ohhhh I can't go there i'll get myself in all sorts of trouble.