fuel pressure not holding
 

our 94 S500 fuel pressure is draining after a while it take lots of cranking to start what causes the fuel pressure to leak down

Two things.
Faulty injectors or a fuel pressure regulator.

I am assuming that you had the car hooked up to a fuel pressure gauge and that you are 100% sure that there is a fuel pressure problem. There are many possibilities. If the pressure drops quickly you could have bad check valves. If it's a slow leak it could be a bad injector or regulator. Try to find out how fast it's dropping. Does it drop down to 0?

Vinamg

yes it drops fast i will time it in the morning i do have a fuel ps gauge what time frame do i look for and were are the check valves?

Do this simple test first. Start the car let it run for a while and shut off. Disconnect vacuum line to fuel regulator and see if there is fuel coming out of the regulator. Also, check if the regulator holds vacuum. You'll need a vacuum tester.

If the pressure drops quickly (within 10 minutes) to 0, then inspect check valves. The check valves are on the fuel pumps.

Vinamg