I agree with sixto, bent rods ....
Seriously it is a vacuum problem. It seems your pump does generate a vacuum but it takes a while. Next startup, just let it run for the few minutes to get sufficient vacuum .... but in the long-term you need to find the leak.
With the 82 300SD I had, it turned out to be a brittle connector off the main vacuum line from the vacuum pump to the the brake booster. I was of course, all ready to replace the pump ($), the booster ($) etc and a friend told me to check the tube as it was a common problem. He was right and I bought him a great lunch with all the money he saved me. Check your vacuum lines.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)