Are you talking about the part circled in red?
If so you are OK driving the car without. I have the MC/Booster assembly on a bench right next to me and the holes are just to access the retaining bolts for the rod to the pedal assembly. That plastic piece is just to keep crud out.
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Gilles & Kathy
Happiness is not having a Porsche in the garage... Happiness is having a Porsche on the road!
86 Porsche 911 Cabriolet, 2011 BMW 1200RT, 03 Saab 93 Cabriolet, 06 MB E350 Estate