Problem starting my 99 4Runner Ltd.

I have a 99 4Runner Ltd. I recently installed a new fuel filter, fuel pump and starter. My problem is that the truck will start when sitting level or when sitting with the front end pointing down, but when the front end is slightly elevated (parked in an uphill parking spot) it will not start. For example, I drove it for a couple of hours and then parked in an uphill parking spot for approximately 20 minutes. It would not start. I rolled it down to a level area and after a few minutes it started. I considered that it might be a faulty fuel regulator but I read that symptoms of a faulty fuel regulator include blackened spark plugs and black exhaust. The truck does not have either of these symptoms.
Any suggestions/ideas?

 

Soundslikeyou may have a leak in your fuel lines somewhere. Might want to recheck all your connections. Your fuel system isn't holding its pressure

 

Thanks. If the truck is not holding pressure, is it possible that the fuel regulator is the problem?