I assume it’s the same as doing a brake bleed, except that I need to completely empty the system through the bleed nipple: Correct?

There are 2 different version of SLX brakes, you need to be specific in what you are talking about, M666 or M675.

If it’s M675, get the Funnel, which includes instructions.

They are the M665s, the old style.

Plenty of how toos around, found this one the other day which is pretty comprehensive, it’s about XTR’s, but the same principles apply for M665/666 SLX brakes.

By bleeding the brakes, you are effectively changing the oil. When you push the oil through using a syringe, you will normally get an air bubble at the start, by the time that has appeared at the other end (lever reservoir), you have both bled and changed the oil at the same time.

Yeah, I found that one too: But the oil in there at the moment needs changing completely, not just bleeding. I would have thought that using that process would just contaminate the new oil with the old, dirty oil.

Just keep bleeding until you are just getting clean, fresh fluid out. As above that usually the same time that all the bubbles are gone anyway. Just remember to push the pads apart before you start so there isn’t much fluid stuck at the caliper end

You could do it by draining completely I suppose, after all that is effectively what you do when you have to replace a lever, hose or caliper (or a much smaller cheaper part in the case of most other bicycle brake manufacturers 👿 )

In practice, (if you bleed it like a car from the top down and out the bleed nipple), the amount of brake oil left in the caliper is really rather tiny once you have retracted the pistons and put the bleed block (or a 10mm allen key) in. This means that the impact of mixing of old and fresh oil really is minimal: a tiny bit at the top as you begin topping up the reservoir with new oil, and a tiny bit at the bottom as the fresh oil pushed out out the last of the old oil.

If it was me I wouldn’t bother draining and then refilling, just pushing out old oil with new will be fine.

PS why does the oil need changing? Mineral oil brakes can go far longer between fluid changes than DOT systems, as long as they are working firmly and properly. A friend has my old deore 525’s, and the rear one has never been bled or anything in eight years, and it still works very well indeed.

PS why does the oil need changing?

The oil that’s in there at the moment is black.

It is much easier to continue to flush new fluid through until you get new fluid coming out rather than draining the system.

If you drain the system first it is going to be much harder to rid the brake of air when you refill it. If you push about 50ml through each brake not only are you going to get clean fluid coming out you will probably flush out all air without too much tinkering at the end.

I’ve just done mine (M785s) for the same reason. 2 weeks in the Alps took it’s toll!

I drained all the oil & then did a bottom-up bleed (the stuff that came out in the funnel was still slightly dark), then replaced the oil in the funnel & did top-down. Probably belt-and-braces, but it’s nice a clear now & they feel rock-solid. Ready to boil it again with a week in Sierra Nevada.