PowerMaster pressure switch leak

[Jeff]

My PowerMaster has a slight leak where the pressure switch screws into the Master Cylinder. The result is that a little fluid/pressure is lost when the pedal is first pressed, resulting in a no-power brake situation for the first few seconds until the pressure builds up, despite the leak.
I'd like to fix this, it's a dangerous and annoying condition.
My question is what size/type o-ring to use to seal this fitting?
The car came with a spare used MC and pressure switch. I have already swapped pressure switches with new o-rings to no avail. I guess my next option is to swap the master cylinder, but I would have to get the fittings to bench bleed the thing. More work than I really feel like doing right now.
Comments?
Thanks,
Jeff
P.S. I think I'm going to keep this beast, and this is the first thing I need to fix.

 

[Chris Taylor]

Try another O ring. May be a little thicker one. Put a little oil on the O ring so it want tear.

 

[ChrisCairns]

Make sure the rubber gasket under the cover isn't leaking. It'll make the pressure switch appear to be leaking because that's where the fluid will end up. I've just gone through this....duh.

 

[Jeff]

At first I thought it might be the rubber gasket, but it is definitely the switch.
The brake warning light comes on when you first press the brake pedal, then once the pressure builds past the leak the light goes out and the powermaster kicks in like it should. Kind of bad during panic stops where the pressure spikes quickly and the powermaster doesn't kick on for a second or two.
I have installed it with another o-ring and lubricated it as well. I have a pretty good hydraulics background, used to design subsea oil well hydraulic drilling control systems. That's why I was wondering if anyone knew exactly what size o-ring and material. I have experienced first hand how important proper o-ring size and material is.
Thanks for the responses,
Jeff