I have a Cube Nature Pro (2017) bike and the brake lever for the rear brake has too much travel and braking is less effective than usual.
The pads don't look too worn as there's over 1.5mm on each, so probably not time to replace them yet.
So how do I adjust the brakes so there is less travel?
I have looked it up, but can't find anything to adjust.
Here are some photos in case it helps:
This last photo shows a rubber cover I have removed, but not sure what this was underneath it:
There may be air in the reservoir.
Remove the wheel and put in the pad spacer, or reset the pistons and use a bleed block.
Screw the funnel from your bleed kit into the lever bleed port (set level), put some fluid in it (1/3rd full) and pump the lever until the bubbles stop coming out. Pump a few times, hold, wait pump, hold, wait.... eventually all the bubbles will be gone. The lever will feel much firmer. Unless you reset the pistons and use a bleed block, the system probably now has slightly too much fluid in it and some may need to be released when you change pads.
It's a quick and dirty bleed technique that works, for when you need a result right now and can't risk making a mistake or having to do the whole thing properly. (time constraints perhaps, or very little air in the reservoir).
I presume from your statement "...less effective than usual." the brakes are not working as well as the used to. This means they probably need a bleed.
First thing to check is reach adjust, that the M315 have, It could be the lever is hitting end of travel before getting to full braking power. In this case the bite point will be close to the start of lever travel, just not much lever travel.
If reach adjust is set all the way out, or has not been changed, then it is likely a brake bleed is needed. If the brakes are spongy its a definite sign of air in the system, and need for a bleed. (The rear will be a bit softer than front on M315s as the hose is not the highest quality and expands a tiny bit). If the brakes are not spongy, there may not be enough fluid in the reservoir, which is now empty. This is causes the brakes to have a late to bite (M315 do not have bite adjust).
M315's, like most bicycle brakes, are open systems (fluid moves in and out of a reservoir), but unlike brakes in cars, the reservoir is sealed and changes volume, usually via a flexible bladder. If the problem is not enough fluid, it’s possible to top up though the funnel without doing a full bleed. If there is air in the system, you can sometimes avoid a full bleed with just a funnel, but Shimano brakes are so easy to bleed, and all you need extra is a syringe and length of hose over doing a partial bleed, I would go for a full bleed.
While I say bleeding is easy, it requires basic bike mechanic skills and the correct equipment. First time, take your time and watch YouTube videos. Have plenty of fluid and clean-up cloths. Another option is to hand the job to your local bike shop (LBS).
Dont know too much about bike hydraulic systems but done plenty of cars. I suspect that this problem is because the M315 may be a closed system ie hydraulics dont automatically compensate for pad wear as a reservoir system would. I think only option would be to top up fluid level which would create extra fluid which would need to be removed when new pads are installed down the track. I had same problem and ended up changing pads which were cheaper than buying bleed gear :-}
