There's not much pressure out of the pumps when the lines are disconnected. The pressure is a combination of the pump and the injection nozzle that has a spring seat that the pressure must rise to overcome. This is the case with these old Cat pumps but not necessarily so with other types, so the guidance to avoid doing that is sound. Can't remember if there's a check valve in the top of the pump which could contribute to some pressure but nothing like injection pressure. That said, you finding that there is not much pressure is too subjective to be much help, unless you can get some gauge readings. Even with a severe blockage, you will probably get some fuel so the procedure I use is to start at the beginning of the fuel cycle (the tank) and methodically work forward eliminating potential sources of problems. If you really want to work backwards, remove an injection valve, turn it upside down and reconnect it to the line and run the pony. If working correctly, it would spray a fine mist (stay out of the way of that if it does and don't breathe it). I suspect you won't see that which means you're not building sufficient pressure to crack the injection valve. Work methodically backwards to find why it's not getting the supply. It could be a regular blockage - my tractor's lines were 90% choked and I doubt it would have run if I tried before refurbishing it. It could be choked filters, it could be a broken transfer pump shaft. It could also be that your timing is out, could be blocked intake, decompression lever not working as expected, many things