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.

Got the old check valve out, tested it and seemed OK. Installed new one. Both fuel pumps do kick on for a second when key is engaged, so LH is OK. Need to find a pressure gauge to fit and monitor header distributor pressure to see if there is a pressure drop (metric fittings?).

Where do I hook up to the system? I was mistakingly viewing the 104 engine fuel distributor system which has a location for fuel pressure checks but see no such info on the 103 engine. Removed air cleaner housing but not quite sure where too hook up (fitting sizes would be a asset as well, so I can visit my local auto parts store and pick up the right stuff).

[Cap'n Carageous]

This may get more complicated than you might want to deal with. There are a multitude of things that can cause your problem. Looking back at my own situation, I thought I could diagnose and repair my own MB. Now, four years and thousands of $$ later, I'm no closer than I was. IMHO you'd be better off taking it to a reliable tech. At least find someone with the proper equipment that can properly identify the probable cause and then decide whether you're ready to repair it yourself.

[TAYO]

HAD SAME PROBLEM ON MY 87300E, I FOUND OUT THE FUSE ON TOP OF OVP RELAY WAS BLOWN, CHANGED IT , CLEANED ROTOR AND CAP, NOW CAR STARTS UP AT FIRST ATTEMPT, OF COURSE MY ABS SRS ? LIGHT WAS ON [ COULDN'T BE SURE WHICH WAS IT ] BUT NOW ALL IS WELL

Thanks Tayo, I'll look into it. It's the little tidbits of info like this that I'm looking for. As far as performing my own diagnostics I don't have a problem with that, my line of work is directly related to problem solving in a number of different trades. The search tool can be very useful as I have spent many hours labouring through to obtain info, buts thats a question I want to bring up too. Sometimes I spend way too much time looking for an answer in the archives, Am I missing something here or is there a better way to find the exact phrases a person is looking for. Example (Where to hook up a fuel pressure monitoring gauge on a 103 motor?) Or do I just have be more creative in getting the exact wors right. Once again thanks for sharing your simular experience, I hope it's just that easy.

Checked the fuse in the OVP, it's OK. Cars gone back to the earlier symptom of almost starting within 1 second of first crank, and starting fine on second attempt. The only thing I have done besides changing the fuel pump check valve is adding some fuel injection cleaner, on my second tank after adding. Hoping for a total reversal.