Adarn

Hey everyone, sorry if this has been answered before, but I couldn't find anything after an hour of searching.

but anyway, I just got a pair of shimano 600 brakes from a friend and installed them on my road bike.
I was having problems before with my old brakes but the "mushiness" is even worse now. The brake pads are still in pretty good condition and I put on brand new cables and housing yesterday, so I don't know what the problem could be. When I pull the rear lever (they're aero levers if that helps) it feels like the cable keeps compressing, and the cable isn't going anywhere.
the front brake works great though.

Thanks for the help,
Adam