C230 Komp. (W203) does not start well after replacing Fuel Level Sending unit
 

Hi,
I have C230 Kompressor, 2002 (W203) with 2.3L engine.
Last month I replaced the Fuel Level Sending unit (it is located under the rear seat, on the driver side). To do this, the fuel pump should be pulled out of the tank as well (it has hoses and wires coming to the Sending unit that need to be detached). Since this is not very complicated job, I did this work by myself.
I did not disconnect the battery nor remove the pump fuses to un-power it.

Almost immediately after this replacement I noticed that my engine does not start as quickly and reliably as it was before the replacement. Especially after long stay, e.g. overnight.
Now, I have to rotate the started for 5-7 sec. before engine starts (previously there was 1-2 sec. time).

There is a valve on the tip of the fuel rail, close to injectors (it looks like a small pin). Previously, when I had pressed this valve, even in the morning, the fuel under slight pressure started leaking. Now, when I press it, there is nothing at all. So, it looks like the rail is completely dry. That's probably why it take some time for engine to start: it pumps fuel into the rail.

I wanted to ask what would be the reason that gas is not staying in the fuel rail anymore? Could I damage the fuel pump or its return valves when reconnecting the hoses? Any recommendations?
Thank you for your help!