I noticed this last year but it came and went. I have my car in a garage but as the weather is now getting much, much colder when I leave the car sit for a long time and get back in to start the brake pedal is stiff as a board. As soon as I turn the car on it goes back to normal.

What the hell is it? Seems to me like it is a warning of something to come that I should have fixed.

I think I found the answer via Google from a 3 series forum....


SI B34 06 07
Brakes November 2007
Technical Service

SUBJECT
Additional Effort Required to Press Brake Pedal Prior to Engine Initial Start


MODEL
E90, E91, E92, E93 (3 Series) produced up to 9/07


SITUATION
A customer may report increased brake pedal effort prior to engine start. This situation can occur if the vehicle sits for extended periods of time. Although the brake pedal can be depressed, the customer may note a change in pedal feel.

CAUSE
The vacuum check valve, fitted at the brake booster, may leak due to component tolerances within the valve assembly.

CORRECTION
Replace the vacuum valve and hose assembly for the brake booster; refer to Repair Instruction RA 34 33 051.

PARTS INFORMATION
Part Number
Description
Quantity

34 33 7 577 336
Vacuum Pipe
1

WARRANTY INFORMATION
Covered under the terms of the BMW New Vehicle Limited Warranty.

Please refer to the latest KSD for all applicable labor operations and allowances.

If the appropriate labor operation is not contained in KSD, then a work time labor operation should be used.

Defect Code
34 33 00 14 00

http://www.bmwtis.com/tsb/bulletins/...p/B340607g.htm