Not sure if your issues is grime or just needs some TLC but there’s a well documented issue with Guides having issues when in the heat / direct sunlight, it’s in the lever apparently not allowing the oil to return and sticking the pistons……. I sent some brand new Guide RS’s back this week and the shop said, “Not another set!, don’t worry we’re returning them all, have your money back”… Lots of activity on this issue on here and other forums.
Was thinking along these lines as well. Although I’m not sure if its hot enough yet for this happen?
I have just been through this process with my Guide RS’s. If you didn’t now about the lever plunger issue you would prob assume its the caliper sticking, but in actual fact its the plunger sticking inside the lever body. You can either take the plunger out and sand the ends down (just don’t try and take the seals off – I tried and ripped one of them so had to buy a rebuild kit) or by a new kit. You can use the Gen 2 plunger for the older guides, which I have done. The plunger has been redesigned and the plastic looks/feels different. My front brake feels 100% better now and my rear brake that was the “good” feeling brake, now feels like crap compared to the front. I have just received a new plunger from Taiwan (rather than buying a whole new kit) to do the back. Its a pretty simple process – explained very well in Sram Guide Manual.