Vince,
you do need to measure system pressure. If the system does not hold pressure, the fuel vaporizes in the lines due to pressure drop and heat soak. You then have a vapour lock. As you do not have hydraulic pressure in the lines, the injectors will not lift off their seats, and you have a non-starter. This does not happen on a cold engine, or one that has cooled enough for the vapour to condense. It is the classic hot start problem that affected Bosch K and KE systems where the injectors required 4 bar plus to lift them off their seats. On the age of your vehicle I expect there is one high pressure pump (0 580 254 938?) which has a non-return valve screwed into the outlet of the pump where the banjo fitting goes. If you are lucky, they are available separately, or I always removed them from knackered pumps, as they appear to be a fairly standard fitting,
HTH,
Peter