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Q: Brake Master Cylinder: Which brake line port is primary, which is secondary? on 2002 GMC Yukon XL 1500

There are two ports on the brake master cylinder where brake lines connect...referencing the picture in the link below, which one is primary and which one is secondary?

http://www.google.com/imgres?imgurl=http://ecx.images-amazon.com/images/I/31H346X9VCL.jpg&imgrefurl=http://frugalmechanic.com/auto-part/raybestos-mc390542--raybestos-mc390542-brake-master-cylinder&usg=__Fd5oTrwhDf93TdWUtUG_OInyt0E=&h=302&w=300&sz=16&hl=en&start=18&zoom=1&tbnid=R9xsZcmXb4WMqM:&tbnh=166&tbnw=154&prev=/images%3Fq%3Dimage%2Bof%2B2002%2Bgmc%2Byukon%2Bxl%2Bbrake%2Bmaster%2Bcylinder%26um%3D1%26hl%3Den%26biw%3D1007%26bih%3D494%26tbs%3Disch:10%2C759&um=1&itbs=1&iact=hc&vpx=131&vpy=107&dur=3781&hovh=225&hovw=224&tx=130&ty=132&ei=LePmTP6jLsibnAeovZCMDQ&oei=KOPmTJheg5SdB7e-rf0M&esq=3&page=3&ndsp=9&ved=1t:429,r:0,s:18&biw=1007&bih=494

Thanks!
-dstrasin
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The primary port on any two port master cylinder is the port closest to the mounting surface. Pressure to the primary (first ) port comes from the primary (first) piston in the master cylinder. The secondary port would therefore be the second port with pressure supplied by the secondary piston. Please click on the following link for more information - http://repairpal.com/brake-master-cylinder.
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