My front m4 is ok but after about 5 mins into a ride it starts a high pitched squeal. It sounds like the pads are pressing gently on the rotor. Pressing the brakes stops this temporarily and then it starts again. Any ideas how to fix this

Try spinning the front wheel – if it drags then you’ve probably got a sticky piston.

how do you fix a sticky piston??

I had exactly the same problem on mine when they were almost new. It seemed like after using them a few times they ‘bedded in’ and the alignment to the disc was differnt to when they were set up in the shop.
In my case it was easily remedied by adding another shim between the fork and the caliper. Mine was IS mount, easier if it post mount.
Never happened since. Might be worth a try.

Go and look on the hope tech web site. There is a video of…………..
how to free off a sticky piston.

From the guys that know!