Vacuum pressure questions

[1976internat1603]

Hello fellow bus people! I have a question about the vacuum pressure. My bus seems to ride normally at 15inHg (idk what that really means but whatever not that important) but when I am accelerating for a long time it will slowly go down in pressure and eventually a warning light will pop up. All I need to do to raise the pressure is to take my foot off the gas for a second. I looked it up and people say it's normal and it's supposed to do that.

What is the difference in high and low pressure? I've been periodically taking my foot off the gas to keep the pressure up. I feel like maybe the engine doesn't work as hard when the pressure is higher but the power is lower? I don't really know that's just going off of the sounds the engine makes.

Does the vacuum pressure correlate with the gas mileage somehow?

I am not mechanically inclined, so please keep the answers somewhat simple, I tried looking this up online and could speaking in Mechanic

[1976internat1603]

Here is a picture for reference, the first is normal, and the second is a couple minutes later after continuous acceleration

[Jolly Roger bus 223]

i have never had anything with a low vacuum warning?
as far as the vacuum pressure readings that is normal my old truck hold about 17 at idle and if i really get in it it will go down to around 4-5.
what else is being controlled by vacuum that you need a warning for on low vacuum?

[s2mikon]

The low vacuum warning is for the juice brakes. There is a check valve that is leaking. In Hg is inches of mercury as in the metal not the car or musician. It is the opposite of pressure as we know it. Going down hill it will go to around 23 In Hg or higher and up hill drop to 6 or lower. It is a good indicator of load and altitude. The higher in altitude you go the lower the idle vacuum will be. If you know the calibration of the power valve in your carburetor in inches of mercury you can drive it to stay above that set point.

[Jolly Roger bus 223]

Originally Posted by s2mikon

The low vacuum warning is for the juice brakes. There is a check valve that is leaking. In Hg is inches of mercury as in the metal not the car or musician. It is the opposite of pressure as we know it. Going down hill it will go to around 23 In Hg or higher and up hill drop to 6 or lower. It is a good indicator of load and altitude. The higher in altitude you go the lower the idle vacuum will be. If you know the calibration of the power valve in your carburetor in inches of mercury you can drive it to stay above that set point.

you sir just taught me something i didnt know.
i have always built and used older vehicles but everything has always been below the mason dixon for lack of better words.
NC to TX stuff.
i thought of northern stuff as a carb adjustment and never a power valve adjustment? i do power valves for test and tune or sputter backflow/flash issues never thought of one for altitude issues.
but makes since for tuning for the altitude that you are in.
in the military i have been to more places overseas than i have states in the US and i have only lived in 2 texas and north carolina some time at pendleton but i call that just visiting

[Jolly Roger bus 223]

just stuff in my head? that vacuum system has to be perfect.
i didnt see what the OP had but brake booster, vacuum advance, pcv, egr would have to be perfect.deleye
my old stuff?
everything is plugged or has block off plates.
age of the vehicle i would do an egr delete and depending on the carb a spacer

[Jolly Roger bus 223]

one more?
where are you reading vacuum from?
carb? intake?
whatever port?
brake booster?
need to know engine vacuum before we trouble shoot brake booster or dizzy problems
sounds like nto me you have a brake booster issue.
if you do it yourself pay attention to pushrod length the of the master cylinder rod?
some are adjustable and some aint.
if you get one that has to be cut then grind a ball on the end and polish it the best you can. i have had one that was adjustable shaft but that was a nightmare to make work?
took it out and went back to the 1977 idea/parts?

[Kentucky Dreaming]

Originally Posted by s2mikon

The low vacuum warning is for the juice brakes. There is a check valve that is leaking. In Hg is inches of mercury as in the metal not the car or musician. It is the opposite of pressure as we know it.

Mikon knows of what he speaks. From your other thread, I think you still have a leak at your brake booster. Make absolutely sure that new check valve fitting is not leaking.

When a gas engine idles with throttle closed, that is when vacuum is highest. The pistons are trying to pump air through the engine but the throttle plates won't let much air in. That causes negative pressure, or vacuum in the intake manifold. When you stomp on the "go" pedal, the throttle plates open and lots of air is welcomed into the intake manifold. More air means more pressure, which is less vacuum.

Your brake booster needs vacuum to work. If the check valve you put in is leaking, that means you are sucking air in from outside and the vacuum is too low.

I know how to diagnose pressure leaks with soapy water but not sure about vacuum leaks. I only know how to do it on outboard boat engine gearcases with a hand vacuum pump and pressure gauge.

Hopefully others can add more knowledge. Lots of smarter folks here than me. Good luck. I will be watching.

[Kentucky Dreaming]

Dang, I have a bunch of cute skunks hanging out here. Had 9, but I had to dispatch one that looked rabid last week

[DeMac]

Thank you for the detailed responses. I'll likely be asking some of these same questions soon, as I have a of the same 1976 equiptment.

After reading these associated threads, I expect to discover vacuum leaks, as well. With brakes, the only knowledge I have of boosters, is they go under short kids at restaurants.



I haven't traveled as many miles, with Noah yet, but I know my brakes are not right. He stops, very well. But the pedal needs a toe raise to release them. Not great.

[Jolly Roger bus 223]

i use something like a Halloween smoke/fog machine to find vacuum leaks hook it to one line and pressurize the system and follow the smoke.

[s2mikon]

It sounds like you may have a brake master cylinder issue. Is it a dual circuit or single.

[1976internat1603]

Hello everybody! Thank you all for your help! I was looking for a leak just by the sound it makes as the engine revs as that was how I found the original leak in my previous posts. I noticed that a very similar sound to the first leak I found is coming from the air filter part so soon I will look into that! A friend of mine who had multiple internationals also recommended that I look there so I'm 75% sure that's the culprit. I will update once I figure it out!

[1976internat1603]

Can you run the engine without the air filter or is that dangerous?

[s2mikon]

Yes you can run it without the air cleaner. The air cleaner keeps dirt out of the engine and is good at suppressing backfires. Safety note: If starting an engine and it backfires while cranking keep the starter cranking and open the throttle. It will almost always suck the fire back into the manifold and consume it. You should have a fire extinguisher at your side when working on these old beasts. Don't ask me how I know. I don't know of a good reason to drive it down the road without the air cleaner though.

[Kentucky Dreaming]

Originally Posted by s2mikon

It will almost always suck the fire back into the manifold and consume it. You should have a fire extinguisher at your side when working on these old beasts. Don't ask me how I know.

I know first hand about having a fire extinguisher on hand. Almost burned down my 1970 Chevelle SS396 once, due to a loose hose clamp on the fuel line at the inlet to the Holley 750 double pumper.

BE CAREFUL!!!

[Ronnie]

One other note about vacuum, where is it reading it from? the intake manifold or the brake vacuum tank? If from the vacuum tank then the check valve as mentioned by others, or a leak in the tank, or booster.


If from the manifold then those are normal readings.

[cadillackid]

Originally Posted by Kentucky Dreaming

I know first hand about having a fire extinguisher on hand. Almost burned down my 1970 Chevelle SS396 once, due to a loose hose clamp on the fuel line at the inlet to the Holley 750 double pumper.

BE CAREFUL!!!

Lost my first bus (prob my favorite of all i have ever had).. because 2 dummies poured gas in the carb of a chevelle in the shared garage trying to get it started .. they spilled a bunch and then cranked anyway (without the air cleaner).. it of course wasnt happy and belched fuel with flame.. they tried to beat the fire out with rags and their hoodies rather than grab one of the dozen fire extuingishers on the support poles of the building.. or one of the 3 out of my bus.. one of which was sitting on the floor by the front wheel.... by the time they figured out theres fire extuingishers everything was flaming and they did the right thing (for once) and ran out.. alas the chevelle was complete toast.. my bus was damaged badly enough the insurance said no to repairs.. to this day I wish I could find another little bus like that one..

[1976internat1603]

Originally Posted by Ronnie

One other note about vacuum, where is it reading it from? the intake manifold or the brake vacuum tank? If from the vacuum tank then the check valve as mentioned by others, or a leak in the tank, or booster.


If from the manifold then those are normal readings.

How do I know where it is coming from?

[Ronnie]

Follow the line from the gauge.