Can a Shimano BL-M425 brake lever be used with a BR-M615 caliper? As the title says, can a Shimano BL-M425 brake lever be used with a BR-M615 caliper? I have an old BR-M447 caliper that I want to replace, and I was wondering if it would be possible to exchange it for a BR-M615 while still keeping the old levers.
Because the hose attachments are the same, you can do it, but the performance of the brake is going to suffer for the following two reasons:


As pointed out by Nathan, the hose inner diameter is different for the recommended hose for each component
The desing of the levers of both types of brakes are a bit different as well, the newer master pistons at the levers push a bit more oil.


So you can assemble the brake and it will work, but the feeling at the levers is going to be a bit weird, and you migth not get the full stopping power out of them.

Finally, you migth also want to update the rotor, the newer ones are designed for the narrow type rotor, you can check the "Disc brake caliper and Rotor compatibility" on the following link:
Shimano docs