I highly doubt that the springs have relaxed. If they did, bending them is not the solution as any bend you put in won't yield any additional tension for very long as the wire will continue to relax (assuming the wire is overstressed already).

The real answer to your problem likely lies in a few different areas. First, since this is a used bike and you likely don't know the whole history, replace the brake cables and housing as a starting point. The rear brake housing on most road bikes is prone to corrosion and many bikes have a non-ideal routing for the housing which only compounds the issue.

While you have the cable removed, you'll have an oppurtunity to check the caliper out by itself. Squeeze it by hand and see if it returns properly. If not, either your pivots needs to be cleaned/lubed or the return springs themselves need some lubricant. The best way to clean and lube the pivots is to disassemble the whole caliper but you start by just adding a few drops of chain lube at each pivot. The springs should have some sort of plastic guide on the legs where they contact the caliper arms to reduce friction. Check to make sure this piece is between the spring leg and arm as they can move over time especially if someone messed with the brake. Add some chain lube at these contact points as well. You'll know you have things working properly when the caliper moves freely and immediately springs back out when you let it go.

When you reconnect the cable, wind the barrel adjuster all the way in, pull the slack out of the cable by hand, hold the caliper closed against the rim (or just very close to it), and attach the cable. Use the barrel adjuster to fine tune your lever feel. Use the centering screws on the caliper to make sure both pads are an equal distance away from the rim. Note that you may need to true your rear rim a bit to be able to get the pads as close as possible without them rubbing somewhere else.