My slx’s did that towards the end of their days. It’s probably dirt ingress on the lever assembly. I took the lever off the bars and cable opened the res hatch then cleaned it all out with hot water and a bit of muc off or whatever, dried it all off then lube the piston and everything else with mineral oil and reassemble :)
They’re on their way to the bin by the sounds of it but you should be able to get them through the worst of winter.