if you have any thing higher than 2.5 volts then i would use a 1 watt 3.3 volt zenner diode in series with the led.
as far as the glow goes some pinball manufactures may like to keep the bulb filaments warm so the standard incandecent lamps illuminate quicker (just like williams kept their flash lamp filaments warm)
also make sure the filter cap for the insert lighting is up to scratch, you will find if this capacitor is stuffed or weak then you will be getting some A.C. ripple on the column and row transistors causing them to pulse at 50 hertz which will make your leds glow, if this is the case then the more incandecent lamps that are lit then the brighter your leds will glow. it's all food for thought. it might just be that as i said at the start or this reply, the designers may have just wanted to keep the lamp filaments warm.
so if you have a small amount of voltage then follow Marty Machines advise, but if this doesn't work then think about what i have suggested.
when trying to work out what voltage you have do it with a analog meter with your pin in test mode, try the individual lamp test then try row and colum test.
