Noticed lever wasn’t returning properly. Took the lever apart and it was all I could do to get the piston out – I had to push (hard) with an allen key from the hose end. Couldn’t see anything visibly wrong with it though

I have ordered a replacement – hopefully this will sort it

anyone else had this?

https://www.bikeinn.com/bike/sram-spare-parts-kit-piston-maneta-guide-rsc-ultimate/136336069/p?

Yes it is a common problem on all versions of Guides. I sanded a tenth of a mm off the diameter of the piston and reassembled with sram dot grease. Has been ok since. For some reason the plastic seems to swell slightly with age/heat and get stuck.