... which is what I wrote about 2 days ago!

Agreed.  However, it is possible that a previous owner connected the wires form the lamp fixtures to the wrong wires in the harness.

Bulbs fitted the wrong way round in the holders - shouldn't be possible as they ought to have offset lugs, but it is doable.

This mini lives in a covered carport.  I'll check the ground.

dido.. I am thinking bad ground also.. has mini meet sitting for long time outside?/ later bc

thanks for the advice.  I'll got a real volt meter. I'll check with that too.  also I'll make sure the short fat filament goes to the brake.

If you are using your tester with the bulb in you should get 12V at the parking light terminal, as you apparently do. If you put the tester on the other brake terminal with the brakes OFF, you are seeing the voltage measured across both filaments - out through the first and back through the second one. It might read around 3-4 volts. When you keep the test lead on the brake terminal and apply the brakes, the brake light circuit will provide 12V.
If your tester can do it, you should also read the resistance between the shell of the socket and the ground battery cable where it connect to the car body. If you can't read resistance you might use a volt meter to test for voltage between the same 2 points with the lights on. Since you are testing a "ground" there should be zero volts. Even a positive flicker would suggest a poor ground.

Could be a bad ground somewhere also

Standard 2-filamen bulbs are always made so that the tail light filament is not as bright as the brake filament. The same thing applies to front parking/turn signal lights, which usually use the same bulb.
First suspect is that you have the double filament bulbs in backwards. They are bayonet mount but the two tangs projecting out the side of the brass bulb base are a different distances from the tip. You can sometimes get them in the wrong way round and get the results you describe.  Try removing them and re-inserting them 180 degrees around. The other possibility is that the sockets are incorrectly wired and are feeding the wrong filaments.

Hi fellas,
Noticed this when I was checking my 1965 mini over before inspection.
when the main lights are off the brake lights are plenty bright.
But when the lights are on you can't tell the brake lights come on at all.
I took the lens off and checked with a 12v tester.  
With the lights ON the tester glows bright at the parking lights lead but also slightly dim at the brake lead with no brake applied.  Then the brake light brightens up when the brake is applied. But is still overpowered by the other filament.  
Anyone else have this issue?  I might need a resistor for the parking light.  Or maybe someone knows of a bulb that has the brake filament more pronounced.  
I'll also see how they look at night.  But if i have my headlights on for some reason during the day I'm concerned about someone not seeing the brake lights.

thoughts?