[924er]

So I have been chasing this stiff brake pedal during cold starts problem on my 335i for about 2 years now. Used to come and go then it gradually got worse.

I researched it plenty times apparently it's a common issue.

Some say it's normal because the brake booster isn't on vacuum but this isn't true. For me it was inconsistent

Others say there is a vacuum leak in the booster system and there is a BMW SIB for it.

One sticky thread claimed it was a defective seal in the brake booster system.

I tried it all and none of it worked. Finally took it to Bmw 2x and they couldn't replicate it but this time I got lucky.

The tech found that my master cylinder was leaking into the servo unit which eventually damaged it. So I got both master cylinder and servo unit replaced. This also required removal of the DCS unit. Parts alone were around $1k but I paid nothing since it was CPO. Problem is fixed. Brakes feel less sensitive now but functional.

So for anyone that has a stiff brake pedal in the morning, it's not normal and it will get worse affecting your braking system.
Read the sticky first and try that fix if not you might need to replace the entire brake booster system.

Good luck