Ford diesel tractor with Simms pump cold start 

Thistle pump you replaced is a lift pump that brings low pressure fuel to the injection pump. The larger blue unit it mounts to is the actual high pressure injection pump. It take the fuel from 5 or 6 psi and in time with the compression stroke injects each cylinder at several thousand psi. The only place the fuel is under high pressure is the 4 steel lines from the top of that pump to each injector. Good quality rubber line is fine anywhere before the inlet of that blue injection pump. For the 4 lines from there to each injector rubber, brass, copper, or aluminum would blow out or swell instantly. And never reach the required pressure to open injectors.

Awesome info, I didn’t know about bleeders, I’ll try it now, I’ve been struggling for weeks !

The tractor comes with metal lines in the discharge side of the pump that’s where the high pressure is. The suction side comes with hoses because the pressure is way less and they are fine for that application

It's looks like a 5000 row crop book called them a 5200

So you use 15w 40 engine oil in your pump??