300E Starting getting worse, filter?check valve?

Purchased a 87 300E, (love the car) when I first got the car started fine but has progressively gotten worse. I found that in the first month it started to take two attempts to start it, going perfectly on the second attempt. It has now gotten to the point that it has to be pumped and cranked for extended periods to go. If its turned off after running and started again it's fine, indicating to me thats I'm losing fuel pressure, as the longer I leave the car the harder it is to start. I'm under the car right now changing out the check valve on the second in-line fuel pump, hoping that this has failed. Question number one, does the first fuel pump have a check valve as well. Question 2, could this be a fuel filter problem. Question 3, could it be the pressure regulator. Has anyone had symptoms that proggressed like this before and if so what was the end cause. I realize it could be a leaking injector, would this be the last thing to try and change. I dumped in some injector cleaner a tank ago with no help. Please your advice would be appreciated. Also hard to tell, because the car is very new to me, but it seems a little slower in performance.