Bubble trouble!

Just fitted a new SLX four pot rear caliper (Shimano replacement for the leaky one). Lost a bit of mineral oil out the system fitting, but flushed the system very well. Started off with a reverse bleed (drawing oil from the brake reservoir downwards to the caliper) with the bike diagonal in a stand (front wheel high). Flushed the system about 8 times by reverse bleed and pushing back through, removing a fair bit of air. The end result is reasonable, but I’m am trying to get them really bitey, like a brand new bike. I suspect there a few micro bubble trapped in the system, bite point on the lever is about 40% in towards the handlebar. I’m considering totally draining the system and starting from new, like they do in the factory. Anyone had much success with this ?

Remove wheel, pull lever once, release, rebleed.

Job done.

? .. That would push the piston too far forward surely and need them pushing back ? or do you mean with the bleed block inserted in the caliper ?

4 pot rear Shimano are a mystery. I’m going with a bubble trapped in behind one of the pistons that a “regular bleed” just won’t get.

Bleeding with the caliper higher than the master cylinder ( or at least not as the lowest point) may help as long as you can activate your vibrating and free rotating wrist.

I doubt a drain down would get you at any better point.

The 4 pot slx I got a while back was a ball ache for me to get a good bite. In the end I did similar to what weeksy said, advanced the pistons a bit, sealed everything up then pushed them back in a bit. Probably not good for them to do this but it worked for me.

Thanks, good tips there. Did you pull the level all the way in to the bar when the wheel is off ?

I always do that. Got Shimano on 4 bikes.

I don’t have the wheel in but pump the pistons out a little, bleed them and then ease them in just enough to take the disc.

Rock solid/near instant grab every time.

The reverse bleed thing is great for keeping crap out of the master cylinder. It’s always the caliper end where the oil gets contaminated. Once the whole system is clean though (and it should be if you’ve bled it 8 times), I tend to get better results from using a syringe at one end. Doesn’t matter which, a bit if a push/ pull on the plunger seems to displace air far better.

I the only way I could bleed the XTs on my hardtail was removing the whole system from the bike, screwing the caliper to a bit of wood on the floor and fixing the lever to an old handlebar clamped in a vice on the bench.

I miss external routing sometimes.

@coconut it varies, sometimes it’s half a pull, sometimes a full pull, depends really. Give it half, try it and see where you are then.

Will do, thanks for the advice.

Haven’t done a shimano bleed in a while but don’t you need to do something with the free stroke on the lever to get them perfectly bled?

Haven’t done a shimano bleed in a while but don’t you need to do something with the free stroke on the lever to get them perfectly bled?

yes. all the way out.

Update: Just rebled the system and have prefect near new bitey brakes again. All of you were correct, thanks for the help.

1. Backed off the bite adjustment point all the way on the brake lever (lever furthest point from the handlebar)

2. Put the bike in the stand with the rear caliper at the high point and front wheel touching the ground.

3. Removed rear wheel and brake pads, then pulled the brake level in once all the way to the bar.

4.  Reverse bleed whilst tapping the caliper lightly with a small spanner, this dislodged lots of tiny micro bubbles (I think there was some air trapped behind the pistons and this is likely worse on 4 pot systems).

5. Close system and wind bite adjustment point on the lever in. Near instant grab on the brakes, very happy with the end result.