What should the vacuum be on the hose that goes to the brake booster?

What model/year/version? (Where are the Profile police when you need them)

Have to say that I don't quite understand your question (i.e., how a "missing" cylinder would contribute to an inefficient intake vacuum process). Are you saying that you've measured the vacuum level in the line to the booster (between the checkvalve and the booster), and the checkvalve is working OKish but the overall vacuum level is lowish?

I think worst case (for something in a single cylinder to influence the brake vacuum process) would be on a carbed 308 where the brake vacuum is drawn off a single cylinder, but if that cylinder's intake process was so leaky that it didn't produce a high vacuum under engine rundown conditions (that the checkvalve should then hold in the booster for braking use) you'd need a serious rebuild/repair anyway.

Please give more information/problem description/etc....

 

(Since you say "plenum" I'll assume it's an injected year/model F) If you measure directly at the fitting on the intake plenum you should measure something like ~16 inches Hg vacuum at idle and it would vary as you the gently blip the throttle (maybe dropping to 10 inches Hg and rising to 20 inches HG vacuum on the rundown). In the line between the checkvalve and the booster you should measure a vacuum level corresponding to the highest level seen at the plenum fitting -- so if you blip the throttle a few times it should "pump down" towards the 20 inches Hg level and hold fairly constant after shutoff.

 

Steve, I don't want to hijack this thread, but would a replacement hose that's rated at 28 in/Hg be sufficient? What should a temperature rating be? Aeroquip makes a similar black cloth braided hose ~12x20mm with the quoted vacuum rating and a temp rating of 212 degF.

Thanks
Carmine

 

I've never instrumented up the booster side to see the maximum vacuum value obtained during actual vehicle running (where due to engine braking you can have higher vacuum levels than by just statically blipping the throttle) so can't say for sure. If the manufacturer refers to it as being used in brake booster applications I'm sure it's OK.

The temp value is probably OK -- but engine bays do get up in the 200 deg F ballpark. If you had an application where the hose was more exposed to the radiant heat transfer from the headers I'd be concerned.

One point I'd make is that the labor is the dominate cost/hassle to replace the long hose so I'd err on the side of an over-designed hose. It won't match the original, but IME high-quality brake booster hoses usually have an extra inner liner and much bigger OD/ID ratios (like 7/16" ID and 1.38" OD) than the "stock" F brake booster hose. If you want to keep the stock-look, you might consider adding another connection somewhere above the rear wheel well so the (easily-replaceable) underbonnet/underfender hose sections are stock while the long buried hose section is better quality/life -- just a thought...

 

On my carb'd 308 (which has good leakdown and compression stats), I seem to recall 15 ins or so of vacuum when we measured it -- consistent with Steve's report.

I usually get the Wurth hose from Continental. Randy stocks it in bulk and will cut a piece to the length required.

 

The vacuum measured in the line at the booster end (between the checkvalve and the booster) should match the vacuum measured in the line at the engine end (between the checkvalve and the booster) -- no "losses" to account for.