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).