The valve you cite is called the switchover valve and is indeed a safety
valve that's triggered by a 16 psi sensor on the intake manifold. It
should NOT be by-passed.
The line to this valve comes from the intake manifold where it does
sometimes get plugged up with oil and soot, but at that end, not at the
by-pass valve. The purpose of this line and valve etc. is to transmit
the turbo's boost pressure to the injection pump so the IP "knows" to
add additional fuel to the engine as the boost rises. The safety valve
ensures that the engine won't run away if the turbo's wastegate (11 psi
pressure relief valve) were ever to stick shut. In that rare event too
much boost would be sent to the injection pump which would add even more
fuel and the engine would be uncontrolled. So don't by-pass it.
Finally, the transmission's shift smoothness is controlled by a VACUUM
powered modulator whose vacuum is controlled by a white plastic vacuum
valve attached to the rear of the injection pump.
Find someone who knows the difference between pressure and vacuum, some
one who knows what he's talking about, not someone who's illogically
stringing together a bunch of handy phrases.