Plugging a master cylinder port, am I about to do something dumb?
 

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I only need two lines for the Mk60 ABS module I’m adding, so I went ahead and plugged the extra port. I was standing here thinking about the way the master works and realized I that there might be two pistons in the master and that capping the banjo port might completely seal off one of those pistons, which would give the fluid nowhere to go, which would cause me to have a very bad day.

Did I do a bad thing?

Yes, you did a bad thing. You are liable to have issues for a few reasons:

• You're asking for too much "fluid movement" out of one piston (four calipers' worth)
• You can't bleed the sealed volume of the unused piston
• Excess movement and odd return-stroke pressure differentials will wear out the MC
• You lose the "tandem MC failsafe"

You should use the banjo port and one of the inverted flare ports. You'll probably need to cobble together a couple parts (like an AN adapter and an AN cap) to make a "plug" for the M10 IF port.

Thanks.

I hadn't even got to thinking about the "fluid movement" issue. Or the wear.

I bet the pedal would feel funny too with either the trapped air or trapped fluid.

Easy fix at least. Thanks again!

You could cap off one of the two ports on the front piston. Would not change the fluid volume of the cylinder.

Probably want to get rid of the adjustable prop valve as well.

I believe that's the port we blocked off on our Mk60 install, haven't had any issues. I can get some pics for you in 2-48 weeks.

That is the new plan.

I don't understand this logic. I still need some "static" bias front/rear, even if the Mk60 is dealing with bias at the limit. The BMW doesn't have a proportioning valve, but it does have two different sized pistons in the master (which is why I was staring at the master for a while and started thinking I did something wrong).

Sorry, not quite sure - you blocked off the banjo with no ill effects?