I am planning on installing front disc brake kit on my 66 Chevelle Malibu. I have a highly built 355 that pulls about 14" of vacuum at idle. Currently, the brakes aren't power assisted at idle unless I rev it a bit. I have also changed from a 850 Demon to a 750 Demon, without checking the vacuum yet. Will I pull more vacuum with a smaller carb? Will I have enough vacuum to activate the power disc brakes I will be installing? Or should I get the deep pocket master cylinder and use a manual disc setup?

14" should be more than enough. Is that the vacuum at idle in gear? You can add a check valve in the vacuum hose to the booster too, it'll help trap some vacuum in the booster. I used the check valve from an 80 camaro that I took apart. I only have 6-7" of vac at idle, and my brakes work ok. I have a stick though, so maybe different with an automatic.

Also, carb size won't really affect idle vacuum just WOT vacuum (too much vacuum at WOT means too small a carb) The cam size and general condition of the engine will have much more affect on the idle vacuum. Idle speed is a big one too... if you idle too slow, you won't have much vacuum.

I;d say that 14" is plenty. If you have that much vacuum ,and you have troubles with the brakes, I'd check everything over good. Make sure everything is up to snuff, and that you've got the correct m/c, booster, prop valve, and that everything is bled. Make sure the rear drums are adjusted properly too, could make a big difference in stopping power.

Good luck! smile.gif

chris

I'm pulling 20" in park/neutral. Have no idea wht the minimum is. But at one point this same car idled with about 15-16" and the brakes worked just fine then too.

The 14" of vacuum that I measured was with the 850 carb idling in Park, with the original power drum brake setup. I have already purchased the power disc brake kit, but have not yet installed it. I was just wanting to know if I had enough vacuum to run the power front disc brakes that I plan on installing. Last year I installed a new M/C and used all new brake fluid and bled brakes accordingly. So I'm pretty sure the brakes are working properly. Thanks!! I will purchase a check valve and go ahead and install the power disc brake kit. How much improvement will I see with the disc versus the old drum brakes?

You might want to check the idle vacuum in gear too. It will be lower than in park. Hopefully some of the automatic guys can give you an idea of how much vacuum they have with power brakes in gear.

That said, you should notice a huge improvement in braking with disc's up front. I did.... but I had manual drums before I switched. Not only do the discs have more stopping power, they aren't affected as much by heat. And I alos noticed that my old front drums were pretty twitchy, and sensitive to adjustment and contamination. The discs don't require any adjustment, and they stop straight and smooth everytime. smile.gif

chris

Master Power Brake Co. says there should be 18" of vacuum. I think that is a little high.

on other vehicals I've built/worked on the power brakes seem to start loosing power around 12#hg vacuum, under 10 is getting pretty bad.