T700-strange thing going on..

[1G1YY]

86 TPI T700 standard wiring.
I have just transplanted an engine and trans out of a car that was running sweet and doing everything as it should, including OD working correctly.
I have just finished the eng/trans swap into my 62 stepper and had the motor running, but not driven it.
All wires seem to be in place, incl at the ECM, as per FSM.
I have all of the wiring exposed and am doing final checks before cleaning up the wiring and final assembly.
While checking the TCC cutout switch my test light went out as it should with brake pedal depressed when connected to the purple trans wire going into the trans plug on the driver's side.
Terminal F on ALDL also showed no test light on when brake pedal depressed...BUT(!), when I tested this circuit with pedal depressed my multimeter showed voltage, both at the trans terminal and at term F on the ALDL.
The voltage from the ignition side of brake switch is 12.7volts. Voltage on trans term is .5 volts lower, as it is at the ALDL connector.
My Snapon test light won't light up as I said earlier with pedal down, but I checked with a small LED and it lights up.
The checks I have done are without the engine running and stone cold.
I am a little nervous about finally assembling everything when I am not sure that the TCC will dis-engage when I finally get to road testing.
I don't understand how a 12v test light won't light up but my LED one will and there is over 12v showing on my multimeter.
Can anyone shed some "light" on this please, as it don't make much sense to me...?
TIA