Not enough vacuum for power breaks?

Does anybody know what it would feel like if you werent producing enough vacuum for your power brakes?
Im running a jegs power brake conversion kit and my pedal will travel pretty much to the floor after I first start the car up and back up out of a parking spot. After I drive for just a little bit the brakes work great, but at real low rpms after I havent driven it in a while the pedal travel is extreme. I called the jegs tech line and they explained to me that this is a symptom of not having enough vacuum, and that I should just add a vacuum canister. They said that since there is no direct mechanical linkage from the brake pedal to the master, that when not enough vacuum exists in the booster, you could push the pedal a long ways with very little pressure on the master cylinder. I have a pretty lopey cam, its a comp cams extreme energy cam but I cannot remember the exact specs. I appreciate any advice!

Air! Bleed the MC then the corners. I'm assuming the rear drums are spooned up. EDIT forgot about the pushrod round nub clearance- check that also, want .060" or a bit less

Master cylinder has been bled multiple times, as well as at each wheel and the rear shoes have been adjusted to be just slightly grabbing, the rear wheel cylinders are new, and the master cylinder has been replaced twice with new ones both times. And after the car seems to warm up the pedal is good. I have been chasing this for awhile, and I hadnt thought of the lack of vacuum being a problem.
Thanks for the quick response!

Put a vacuum gauge on the engine and check it, You need an absolute minimum of 13 1/2 to 14 inches at idle. I have installed a few of these resevoirs and they usually work good, but if the cam is a quite a bit more than a stocker you may need to put a vacuum pump on it.

not sure if it is not enough vacuum, or what.
I have a fairly stock engine.

when I first hop in, stopping at the end of my street and down the block seems a bit hard to do. I have to use extra effort to get the car to stop.

once I have driven up the road about 1/2 a mile, the brakes feel normal and I don't have to stand on the pedal to stop.

doesn't seem to matter how long I idle in the driveway either.

kind of just been living with it, since once I have driven a while, there are no issues.

my brakes are fully adjusted and bled, along with the push rod between the master and power booster.

sounds about the same as yours.

Yea thats about what mine has been doing, except that the pedal travel is my problem and not just a very stiff pedal. Ive driven before with manual brakes, and Ive disconnected my vacuum hose and tried that and the pedal is rock hard. Does anyone think that a lack of vacuum could cause the pedal travel to be severe? Would adding a vacuum resevoir keep the pedal firm at when I back out of the parking spot? Once Im driving Ive never had a problem stopping the car but I worry that itll be that one time when Ive used up the vacuum boost and I wont have brakes

I have to ask with all the mc replacing, did you put the residual valve back in?

that 10 pounds hold would be the difference I think on your first run of the day vs, later on.

Ive been having a shop do it that is very experienced with muscle cars. Im not sure where the residual valve is honesly. I do know that after driving I can come back 2 hours later and when I pull the vacuum line off of the booster I can feel the vacuum and hear the whoosh noise of the air escaping as I pull the hose out

Quote:

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Put a vacuum gauge on the engine and check it, You need an absolute minimum of 13 1/2 to 14 inches at idle. I have installed a few of these resevoirs and they usually work good, but if the cam is a quite a bit more than a stocker you may need to put a vacuum pump on it.

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THIS is one opinion....
I have a 440 based 493 in my Charger. My idle vacuum is under 10 at 800 rpms and drops to 9 in gear at idle. The brakes feel fine, but will get depleted in short order. I added a vacuum can from Comp Cams. Problem solved.
The low idle vacuum in my case allowed one or two good stops before the pedal firmed up and was non-responsive. The vacuum can allows so much more reserve, I have yet to find its limits.