I should have discussed this in my first reply. You should replace all the brake fluid in your brake system once a year (as MB recommends). Brake fluid needs complete replacing because brake fluid absorbs moisture from the atmosphere and then becomes contaminated (recall the warning on brake fluid containers about not using fluid from a previously opened container? water absorbtion from the atmosphere is the reason and why brake fluid goes from a near clear fluid to nearly opaque over time). Water contaminated brake fluid will cause rust/corrosion in the brake system leading to premature failure - very expensive to replace those brake components. Or, the chance that vapor lock will occur because of the moisture contamination.
I suggested the Mityvac vacuum pump so you could not only bleed your brake system now, but also every year as part of a preventive maintenance program. And, the possible future need to do other diagnostic tests on other components using the Mityvac.
By far, the cause for the vast majority of brake system failures due to rust/corrosion are vehicles where the brake fluid is not changed on a regular basis.
Try to change the fluid when the relative humidity is low (as MB recommends). When you have to replace brake fluid for repairs and the relative humidity is high, then I suggest doing a complete brake fluid replacement as soon as the relative humidity is low ( or as soon as your schedule allows when the RH is low).
My 1977 300D has never had any brake component fail (other than disc pads) probably because I replace the brake fluid every year when the humidity is low (same results for other vehicles I've owned, or responsible for as a professional mechanic/technician if I was in charge of maintenance from when the vehicle was new or nearly new).
I have not used the new synthetic brake fluids that are on the market, yet. I had a supply of Pentosin Super Dot 4 brake fluid that is now nearly gone. I plan on trying one of the new synthetic brake fluids very soon since it has been nearly a year ago that I last changed it and Texas weather soon will have much lower RH levels (Valovoline synthetic brake fluid since I use Valvoline oils in all places needing oil). The synthetic brake fluids are supposed to be much more resistant to absorbing moisture and compatible with existing non-synthetic brake fluids. However, I understand that synthetic brake fluid does need to be changed regularly - but not as often as non-synthetic brake fluid (every 2 years? perhaps longer?).
Anyone have any experience with synthetic brake fluid?