difficult braking in my 380sl
 

the brakes are very hard to apply almost feel like the power assist is not there at all. Do the booster lines fail? What are the common causes of this?

if you look under the hood you will see a big black vacuum line running from back of intake[approx in centre]to the brake booster.if this line is not cracked you probably have a broken diaphram in the booster.

2nd what david said, check your vac line as it is imperative to the function of the brake booster, if not, your problem lies in the booster its self. If it were a master cylinder your breaks would feel spongy and the petal would sink to the floor.