The lever has started returning slowly and the first cm of grab on the lever is loose. Brakes work fine apart from the loose lever feel and slow return.

Sticky piston.?

It probably just needs bleeding, it’s a dead easy job on M4s in my experience.

How do you mean “loose”? Is there any friction/resistance at all? What does the bite point feel like? Do the pads bind when you’ve released/returned the lever? When was the brake last bled? What did you have for breakfast?

I bleed them every year normally when I swap frames. The brakes aren’t binding, there is a the slightest of contact but thats nothing unusual. When I pull the lever the first cm or so of lever travel does nothing then there is a normal lever feel after that.

Oh and two pieces of wholemeal toast and fresh ground coffee.

I’d say you need to replace the lever piston seals. Cheap and relatively easy to do! While you’ve got it apart do the caliper seals as well, all you need to do then is add fresh pads and the brake will feel like new.

Teh piston return spring in the master cylinder would be my guess as the culprit part 18 in this

My initial feeling is that one of the lever piston seals has failed. If you’re happy that the system contains no air, then there’s not much else it can be. Here’s a guide I did in case you need to overhaul.

Cheers all.

Three fish nice guide.

Best place for parts.? CRC.?

You might save a couple of quid by shopping around, but here’s the basic rebuild kit you need. You might consider just getting the lever piston seals. CRC don’t have the Mini lever spring, but a search for HBSP110 will get you one. The Tech springs that CRC have are a different part number, so probably not suitable.

I’d suggest opening the lever up and having a look at the spring. If it had failed – they sometimes rot away in poorly maintained brakes – I’d expect you to have more issues with the lever also not retracting and the pads binding.

I’d be interested to know what the cause actually is.