I did a brake service, and noticed there is a very amount of brake fluid like sludge on the piston surface and brake pad. There is any overtly leaking fluid, just a semi wet sludge (made up of fluid and rust particles).

Do I need to replace my caliper? It seem based on the limited spread that it is probably leaking at like a rate of 1ml per month or something.


When I was in the Army working on trucks, they had a rating system for leaks in vehicles.

  • Class 1 - Film of fluid
  • Class 2 - Drip is formed, but you don't see it drop
  • Class 3 - You see a physical drip or can definitively state a puddle came from a given spot

The Class 1 & 2 would not deadline a vehicle (deadline meaning it could not be used). A Class 3 is a deadline and must go to the shop to get fixed.

It was this way for all but two different fluids. The first being fuel. The second being brakes. Either of these were automatic deadline items, both because they are huge safety issues.

Most likely, you're not in the US Army, however, I think the same way now (and yes, this is my opinion). If you see any type of leakage coming from the brake caliper, you need to either rebuild it or replace it. The cost of your life, those who may be riding with you, and those around you are at stake. Do as you will, but I'd be fixing it. It's only a matter of time before it fails.


Are you sure it is brake fluid and not brake grease? Brake grease is sometimes applied to the piston face and/or back of the brake pad to make the two slide efficiently when the brake pedal is pressed. Both materials are made of metal so the grease makes the two slide well when they make contact with one another. I often use this technique when doing brake service.

  • No because I did the pads last and I did not put grease anywhere near the back of pads or piston.
    – Scorb
    Jun 26 '21 at 23:02
  • @MikeH - It's a good point, though. +1 Jun 26 '21 at 23:22
  • @Scorb You should have used some brake grease around the caliper bushings and pins, on the "ears" at the top and bottom of the pads, and on the backs of the pad where the springs / clips make contact. It will be OK without it, but it might wear faster an start making squealing noises.
    – Z4-tier
    Jun 27 '21 at 22:01

Leaks usually only grow stronger, at least while the fluid is available.

In regard to brakes, the only exception I am aware of is brake cylinders leaking when left unused for a season at extremely low temperatures (-20C or lower). They can lose all their brake fluid and then work acceptably when the weather improves and brakes are filled with fluid and bled.

Either way, the brake pads should be absolutely dry (rainwater doesn't count).

If your brake pads are wet, even only at their backside (facing to the piston), fix the leak.

  • Point of note. I live in one of the coldest cities in Canada that regularly reaches -30c over night in January, and because of remote work, my car was sitting 9 days out of 10.
    – Scorb
    Jun 27 '21 at 22:45

You could have corrosion or a scratch on your piston. When this part of the piston is against the seal it may leak. It may have been when the piston was fully pressed in to the calliper for the new pads that the bad part of the piston was against the seal and hence the leak.

It would still be best to either refurbish the calliper or replace it, since if it is corrosion it will only get worse. It may not leak again, but that would be taking a risk.

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