Question for those of you who have installed an aftermarket Bendix master cylinder on their Bendix brake booster, I have a question.

The aftermarket unit has a groove around the periphery of the puck that inserts into the booster. Should an O-Ring be placed in the groove? If not, will the booster leak vacuum? What is your experience? If an O-Ring is required, then what ID/OD? My OEM MC didn't have a groove nor O-Ring. Thanks.

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If you think— it needs an O ring bring it to your local auto store to find  an O ring that will fit  the back of your MC 

 You might ask the guys in the auto store what they think about the difference in your stock part and the back of the new MC and if an O ring might be needed

i have no direct experience with the Bendix  brake parts 


I road tested it without the o-ring and it held vacuum fine.

Do not put an o ring there.  
That's the Delco master cylinder design, not the Bendix, and that groove was originally intended for a felt ring 'air filter,' which usually dries up and disintegrates or is discarded.   The booster breathes through the front and rear, and if you seal it off, you will find your brakes locking up after about an hour of driving after the vacuum slowly builds each time you push the pedal.  

Thanks good to know. I didn't see any o-ring or seal between the mc and booster in the service manual Bendix cross section drawing so I didn't put anything there. The booster seems to operating fine.



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