I am not sure what you are seeing when you say "fade". If you can post pics it would be very helpful. It could be clogged nozzles that are freeing themselves as you print a little. But if it does this every time, it more likely could be an electrical issue or loose wire in the head. Does the carriage move freely on the rails? Have you cleaned and re-greased the rails since you got the machine? Have you checked the encoder strip, does it show signs of ink splatters? If so, you need to clean that really well. Honestly, there are all sorts of things that could be wrong, but again, without a picture, I'm just throwing darts and trying to hit something.
FWIW - There are no valves on the head itself. The nozzles are either open or closed - ink either comes out or it doesn't. What does your nozzle test look like? Any missing nozzles, if so how many? If your head isn't getting a tight seal on the cap on the maintenance station it allows air to reach the head, drying out nozzles and causing clogs. Also, a similar thing can happen when the drain lines become clogged/plugged. I can't stress enough the importance of having a maintenance station in good, clean condition. If your maintenance station is failing it will cause all sorts of problems that you will not resolve until it is replaced.
