OK first with a correction. with it wire in post 27 and brown wire removed it will crank with no start.If the brown wire is removed from the "I" terminal and the purple is still connected to the "I" terminal, as soon as the key is released from "start", the starter solenoid should open, removing power from the purple wire, which turns off the Pertronix relay and power to the Ignitor is cut. So the engine should stop as soon as the key comes off start.
What you describe makes no sense unless it is not wired as the drawing shows. With the brown wire disconnected, you should not have power on the purple wire unless the key is in start.
I considered that the Pertronix relay may be a latching type, but even that doesn't make sense, since latchers require a reverse pulse to change state, or more wires.
I would take these steps:
1. Make sure no extraneous wires are connected- only those shown in the diagram
2. Take the wire from the distributor to the coil off. Set your meter to DC volts and connect one side to starter solenoid "I" and the other lead to chassis. With the key off there must to zero voltage. With the key on there must be 12v.
3. With everything off, disconnect Pertronix relay wires red and orange. Take an ohm reading, on the orange and red, which should be infinite- no continuity. Hook up the purple wire to battery + and black to chassis. Take an ohm reading on the orange and red and it should be close to zero.
I checked all the continuity of the relay connector and they are good.
2. wires removed from distrib. 0 volts off, 11.9v on. good
3. all wires removed. Orange to red no continuity. purple to + battery , black to neg bat. . continuity check between red Relay wire and Orange relay wire . Jumps from 34 ohms and quickly drops to .2 ohms. I can gear the relay click when I energize purple and black with 12 volts.