Have you checked the signal coming from the push button control unit? There's a 3 prong connector up behind the brake booster (yellow, black and red I think) where you can test.

The PBC's are known to go bad due to bad solder joints. Do a search and you'll find a wealth of information.

Good Luck

__________________
-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi