Don't know much about MB, but I'm helping a sister-in-law maintain hers. I have 2 possibly-related problems.

The brakes don't work as good as I think they should, takes
a lot of peddle pressure to stop it. I've replaced the pads on the
front passenger side, and the caliper on the front drivers side (the pistons were stuck).
Bled it OK. Plenty of fluid in reservoir. Haven't looked at the
rear disk brakes yet, other than to confirm that there are no
leaks back there.
The other problem, is the trunk won't open, even though the key
and push-button seem to work properly.

I did a search and got the impression that on some models of MB, the trunk uses a vacuum line to operate it. The brake booster also uses vacuum.
Other posts mention something about a 'vacuum motor' rather
than engine vacuum being the source. What is that ?

Could these two problems be related ?
Any advice/suggestions would be greatly appreciated.