NO. The guy who wrote the sheet you've linked-to is WRONG. It is NOT a "proportioning valve", it's a Metering valve, or Holdoff valve. While I do agree with you, The 1968 GM Service Manual refers to it on the brake system as a Proportioning Valve. GM manual goes on to call it a Metering Valve. If you read the instruction sheet, the first line refers to the valve. The PV68 is a hold off valve.
If you look up the definition of a Proportioning Valve. https://en.wikipedia.org/wiki/Proportioning_valve.
Do not fear, although the service manual says to "press" the button when bleeding, this is a misprint in the service manual. These are not my words. Just info that someone had posted on the brake setup. Like I said in my original post, Sorry I'm not much help to you.
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68 Firebird. IA2 block, 505 cu in, SD Performance E-head, Solid roller 3600 weight. Reid TH400 3:90 gear. 28" slick.
Best so far 9.95@135 mph. 1.39 60 ft.
76 Trans am, TKX .81 o/d, 3.73 Moser rearend,
468 with KRE D-ports, Doug headers, 3" Exhaust
950 Quickfuel.
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