locking pump operation question

[locry]

... what triggers the pump to lock/unlock? and what tells it to stop?
car is an 87 300TD

i'm in the process of installing a remote, i want to eliminate the connection to the door/hatch to the pump, and rely instead on the remote system and the pump alone.

all i THINK i know is the pump either pumps/sucks depending on a signal positive/ground which is supplied to any 1 of the 3 prongs in the round connector at the pump.

as of now i can supply both positive and negative signals to the 3 prong plug via 2 single throw relays... is that all that's needed? (don't have double throw relays at the moment)

i bench tested the pump by applying a signal less than a sec(not sure if positive or ground) to one of the 3 prongs and the pump ran as it should... only thing is... it kept running, i had to disconnect the rectangular plug to stop the pump. the vac line was disconnected when i did this...

so... MAIN question really is... what tells the pump to stop running? an opposite signal? or is there an internal pressure switch? (hoping there is... makes things much simpler)

thanks guys

btw, read all about the crutchfield type installs... doesn't apply to me as i'm going to be installing direct to the pump and removing all connections to the doors/hatch