If the customer drives about 10 km the brakes bind

This is either the calipers are locking up and not releasing or the brake lines are swelling and not allowing the fluid to return to the master when you let off the pedal.

I would suggest lifting the vehicle and operating the brakes to find out which ones are locking up and then break the bleeder on that caliper loose and see if they release. If they do then the issue is in the lines. If not the caliper is locked up.

Here is some info on this process if you have to replace the calipers:

https://www.2carpros.com/articles/brake-caliper-replacement

Below is the process from the manual if you need this as well. Let us know what questions you have and we can go from there. Thanks