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Changed my brake master cylinder last weekend, and the replacement part has an extra port. So I ran down to Pep Boys and got a bleeder screw which threaded perfectly into the hole and tightened it securely to close off the port.



No sooner do I pressure the system to 12 PSI with my power bleeder than I notice a puddle of brake fluid on the ground below the master. The rubber cap had popped off and fluid was leaking out, even thought the bleeder was quite tightly fastened in place.



Why wouldn't a bleeder screw close off the extra port? I don't think I could tighten it any more as the threads might strip out. It did occur to me that the port itself is made to accept a flared brake line end, so could this be incompatible with the bleeder's tapered point? I've never looked down into a bleeder hole on a caliper to see what it looks like, but perhaps it's different?

yup - the flared end makes the seal, not the threads

Figured it out ... a bleeder is the right way to close off the extra port ... I was just using the wrong kind. Took one out of the 928 caliper on my workbench and it worked just fine.

do you have a shot of that unit? i'm wondering if it is indeed a bleed port, or a fitting port - if it's a fitting port, things could get weird, but if the bleed screw has a ball seat, it will work if it reaches bottom

The replacement masters I have gotten have the extra port. I don't know why. I just put a bleeder valve on it. When bleeding I actually get quite a bit of air out of that port.

Could be master cylinder that fits multiple makes of cars?