I have a feeling something else isn’t right. 12” of vacuum at idle should be plenty to run power brakes. Heck I run power brakes without issue with about 10” of vacuum. I’ve got to get down to about 9” before I start feeling an issue.
One thing to note about the vacuum reservoirs is that you are absolutely pulling more vacuum than 12” during low load driving operation. Probably over 15” of vacuum, if not more. That’s what you would actually have in reserve. That’s also the vacuum level your booster is going to be seeing as you decelerate and break.
That’s what specifically leads me to believe you have something else going on.
Are you running an actual vacuum hose to your booster and not fuel hose? Vacuum hose is specifically designed not to collapse. While fuel hose is not. If that hose collapses, there goes your vacuum assist.
As mentioned already I’d also be looking at line size, verifying the combination valve is setup for disc/disc operation. Disc brakes need more fluid volume than drums. I’ve seen a lot of people try to reuse the original drum/drum combination valves and that’ll be a recipe for poor braking performance.
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-Jason
1969 Pontiac Firebird
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