vacuum help

[jfp3205]

Hello everyone
I am trying to troubleshoot a problem I am having. My brake pedal goes from being good to rock hard all the time. When I am moving at slow speed backing out of a parking space, the pedal is rock hard and hard to stop but normal driving it seems ok .
I thought i had a bad vacuum pump but was told to check it first. The hose from the back of the pump showes a nice solid needle around 25 but the hose going into the brake booster shows a needle bouncing all over the place around 15.

I pugged up the vaccum ports coming from the manifold that the pump feeds and i have no change until i unplug the hose that has the tee coming out of it. when I plug off the prt that has the hose going to the front of the truck, the needle settles down. The hose looks like it goes to some canister under the battery tray behind the drivers side headlight (cannot get to it)

Can someone tell me if I am troubleshooting this correct and what is the canister under the battery try and what does it do?
Thanks

 

[OLDBULL8]

The canister under the battery is a reservouir of vacuum for one time braking if the engine stalls. Just a tomato can, it may have a rusted hole in it. The plug going into the booster is a check valve, a hard brake peddle is usually caused by a bad booster, you have no braking help from low vacuum in the booster. Plug off the line going to the canister, if vacuum settles down, canister is bad, if not then booster is more than likely bad.

 

[jfp3205]

I plugged off the tee side going to the can it the needle smoothed out but it seems like the problem is better but still there. What else can I check

 

[smolkin]

Are your rear brakes in good shape? I know the ones on my truck were pretty finicky until I replaced EVERYTHING. Check your bearings too.

 

[MIDNIGHT RIDER]

My 1985 has a soft-ball-sized black plastic ball-shaped vacuum reservoir instead of the paper-thin rust-prone pork-and-beans can.

Like already said, that can is a vacuum reservoir that ?might? allow one more assisted brake application.

Your process of elimination that you described will pinpoint the problem, so long as you isolate each branch of the system as you go along.

When my vacuum booster went bad, the pedal was like I was trying to shove it through concrete, and it even would "kick" back at me.

I have TWO vacuum gauges in the cab, monitoring truck and trailer vacuum, and the gauge would show good vacuum until I applied the pedal; considering this, you might need someone press and HOLD the pedal while you monitor the gauge.

With a healthy booster, I would expect the vacuum to drop on initial application of the pedal, but rapidly regain vacuum while the pedal is being held down.

 

[icanfixall]

My thoughts are one of the vacuum hoses has a hole rubbed in it. Check the cruise controll too. Thats operated by vacuum too and its driven off the tomato can resivour.

 

[jfp3205]

I know that my cruise does not work. Can I use a vacuum pump to see if the lines hold vacuum? Can I also make some other kind of canister since I cannot get to the can below the battery tray without taking everything else off? Any suggestions

 

[MIDNIGHT RIDER]

I know that my cruise does not work. Can I use a vacuum pump to see if the lines hold vacuum? Can I also make some other kind of canister since I cannot get to the can below the battery tray without taking everything else off? Any suggestions

Cut and plug the line that goes ONLY to the can; do this in such a place that you can splice/sleeve it back together if the problem is not the can.

Once the can's line is plugged, see if the problem goes away.

Myself, I would not fear temporarily driving the truck minus the can; it serves very little purpose other than making some safety freak feel better.

You can scavenge a much better plastic or steel vacuum chamber off of something else.

One can even be made by capping both ends of a section of steel pipe, tubing (round or square), or the like, and fitting a hose-nipple into it.

A short section of Schedule-40 PVC would be stouter and much longer-lasting than those tin-cans.

The vacuum tanks on my goose-neck trailers are plain old steel air-tanks just like found on semi-tractors and trailers; it takes several brakings to run one of those out of vacuum.


As for the cruise-control, that tin-can plays no part that I can imagine.

You are far safer when the cruise-control is non-functioning as it is just as dangerous as blabbing on a cell-phone.

 

[MIDNIGHT RIDER]

I forgot to add :


It matters not where that can/chamber is located, so long as it is connected to the system via a vacuum line.

It could be mounted inside the cab, taped over a head-light, or zip-tied onto a trailer-ball and it's function would remain the same.

A capped bumper air-tank fashion would make a far more sensible reservoir than those tiny cans.

 

[dgr]

Jfp,
I'm not sure how you would detect WHERE a leak is with the vacuum pump?
Vacuum lines are meant to hold a pressure differential. You can use them to hold a small amount of positive pressure, too. Say lung pressure and a nice cigar. Worked for me in my cherokee.

If you do have a vacuum leak, i would be real interested in how long it takes your blend door actuator to release after you turn the key off. I thought i had a leak but minutes after shutting down, that actuator was still pulled tight.

 

[MIDNIGHT RIDER]

One other thing :

It is just good business to install a permanent vacuum-gauge inside the cab.

Equipped thus, one can pick up on situations much quicker/easier that trying to trouble-shoot with a hand-held gauge.

Of course, the hand-held gauge is still necessary for isolating the pump from the rest of the system.

With a dash-mounted gauge, one can observe what happens when the various vacuum systems are operated, without the need of a second person.

 

[jfp3205]

What is the best way to install this vacuum gauge? If you had my truck, how would you do it? Thanks