Leaking fluid from hydraulic brake caliper Shimano BR-M355 rear caliper.
I've very quickly managed to contaminate three sets of brake pads through fluid leaking (I think) from the bleed nipple. I've twice taken advice from local bike shop who inspected the caliper and assured me nothing was wrong, advice was to buy more premium end sintered pads. Fair enough I thought, give it a try. Few weeks later and back to square one - no braking power and very noisy.
The dust cap is missing from the bleed nipple, but I assumed this was one way and wouldn't cause it to leak?
Essentially, the question is can I fix it by replacing the dust cap, top up fluid and replace pads, or is it a case of replacing the caliper and starting again?
Any advice gratefully received. Thanks
Are you 100% sure that it is leaking from the nipple?
There are other ways to leak oil:

From a badly crushed olive in the hose connection
Cracked piston
Damaged piston seal

And other issue could be &quot;pad glazing&quot;. This has similar symtoms as getting oil on the pads.
If you are 100% sure that the bleed nipple is the issue. You can just replace and bleed the brakes.
You can't just get a new dust cap though. You would have to get a new bleed nipple. The dust cap has nothing to do with making sure the system doesn't leak. It's purpose is to keep dust out so when you bleed the system, the dust doesn't contaminate by mixing dust in with the oil. Adding a dust cap, won't stop a leak. The pressure from braking will still push the oil out.