Injector pressure drops after several tests

Hi,

I’m currently testing my old diesel injectors (from a Fiat Ducato 2.5TD 1991) and I’m getting some pretty weird results.

The first few runs (testing with a manual injector tester) I’m getting good pressure on the injector, around 3000 Psi and a very nice spray. After tester a few more times it does not hold the pressure anymore and drops to 0 Psi after around 5 seconds. Disassembling and reassembling the injector solves the problem again for a few pumps and then back to pressure loss. I made sure all the internal parts were lubed with diesel before putting it all together, and the injector head holding it all together tightened at 60Nm. I also checked the sealing at the injector tester side but I don’t see any leaks there.

Any idea why this is happening?