Item 1 - I think that 'extra valve' you refer to was a feature on the K-jet as I recall. I vaguely recall that it was a large black plastic knob that you could adjust. I think the last year of the 140 series got the Kjet on the B20 engine.

Item 2 - As noted by others, the discharge on the pump has a check valve. If the check valve is leaking you will lose pressure causing slow starts. If you still have the original Bosch pump, the check valve screws into the discharge port of the pump. The Bosch check valves are no longer in production. However, the pump used on the 1979 - 1980 Nissan 280 ZX is a dead ringer for the Bosch 2 port pump. The Nissan pump is still in production and the Nissan check valve might fit the Bosch pump and might be available as a separate item from the Nissan dealers (that is two 'mights' in a row).

As a suggestion, if you unscrew the check valve, you can try rapping it and see if there is any foreign object stuck in the valve which might be holding the check ball open allowing drain back of the fuel. However, the reality is that they do wear out.

Finally, the other source of pressure loss is leaking injectors. Seems to me I recall you were fiddling with your injectors already; but, if you haven't done this, pressurize the system with the injectors pulled out of the holders and watch for leaking fuel at the tips. Should be zero.