the isolated white wire is Not connected to the pump, just one leg of the feed off the backside of the pressure switch.

Well, there you have it. The white wire is connected to a switch. Therefore it's a hot wire. You said it was one leg of the feed off the pressure switch. The switch switches power. Unless it's switching the neutral. But then it wouldn't be controlling the 240 VAC. Best it could control would be 120 VAC.

Answer this: Is one of the two wires going to the pressure switch a black wire? The other is obviously the white. Where do those two wires go? Do they go to a control box or the pump? Where exactly do they go? Can you give us a drawing of the circuitry? As best you can should help us get a handle on what's going on. I'm thinking they (whomever wired it) used the interrupt method. If you need, I can provide a drawing.

 

Here you see an image of an interrupt wired light fixture. You can imagine anything you like for the fixture, a pump, a fan, a light bulb - whatever. Notice that the hot and neutral both run to the fixture. From there two wires (black and white [silver in my drawing]) run down to a switch. They did that for many years before the updated version of the NEC came out. Interrupt wiring was cheaper and easier, but it meant you could get a shock at the light switch too easily.

Those two wires running from the fixture to the switch are black and white. Hot and neutral. You can see in the drawing that the hot goes to the device. Your case a pump motor winding or two. From there back to the switch I've run a black wire. But notice that the black wire from the switch is tied into the house neutral. It's possible someone reversed that feeding hot directly to the switch via your white wire. If you ground your white wire you (apparently) get a short and blow the breaker.

Keep in mind this is all theory. None of us can see your house from where we sit. So at best we can only guess at what might have been done.

While the Interrupt method was being used, toward the end some electricians took to putting a black piece of electrical tape around the white wire to indicate it was part of an interrupt circuit. If that came off then you wound't know for sure. And it's ALWAYS easy to reverse wires. I've gotten plenty of shocks because of backwards wiring. Even the pro's can do it.

So draw us a picture or take a picture or two. Be thorough so we don't have to keep asking for complete or better pictures.

 

While the Interrupt method was being used, toward the end some electricians took to putting a black piece of electrical tape around the white wire

When ever I do 240V wiring using standard romex, I color the white wire with a black magic marker.

 

Back in the dark ages, machine tools (mills, lathes, etc) use a control transformer to reduce the 440VAC to 110Vac and control things like relays, solenoids and other low power devices. In order to detect one of the wires being pinched or worn through, the design engineers let that 110VAC "float", that is, not being grounded. To detect a fault, they tied two light bulbs in series with the common point being tied to ground. The other sides of the bulbs were tied to each one each of the floating 110VAC. Each bulb would then light to a low intensity. If one of the floating 110VAC wires found itself grounded via a short, the light tied to that wire would go out and the other would light to full intensity. Codes have been revised since those old days and now, one side of the control power is grounded.

The situation you started out with, to me, sounds like a way to detect a motor winding having a short to the case by lighting the bulb.

 

Codes have been revised since those old days and now, one side of the control power is grounded.

.

Although NFPA79 states it is optional, and control circuitry shall not exceed 120vac, if non grounded then both lines are fused.
Max.