You got a loose vacuum hose... or zero vacuum to inside the car. Look in the
hood and find those yellow checkvalves by your brake master cylinder...
trace those vacuum hoses... make sure they are tight. Trace it all the way
back to engine.

You can pull the vacuum hose out and see if it has vacuum.

I have an '82 300 TDT, so my system is not exactly the same as yours,
but the general principle is that your vacuum system is not working
right if you have automatic climate control.  The other option for
your car is a system with manual controls that are operated with
cables to levers.  I think all were automatic in 1990 however.

If you have the automatic system you need to find the vacuum diagram
and go through the system step by step to find out what component is
leaking.  There are several possiblities.  It could be your check
valve, but there are many other possiblities too.  This would be an
expensive job to take to Mercedes, and I have not had good experiences
with them trying to fix the vacuum system on my car at the dealer.
Let's face it - they like to sell and work on NEW cars.

I would recommend either doing it yourself or trying to find a shop
that specializes on Mercedes and is used to working on the older
models.