Vacumue problems ????

[ihfarmer1700]

Hi I have a 1974 loadstar 1700 single axle grain truck that has been having brake problems. You have to push the pedal all the way to the floor for brakes and then when your driving along the vacumu light comes on and then when you leave off the gas the vacume gauge goes back up I checked the brake resvior and it is fine is the vacumu booster going out?

 

[MarkO]

Somewhere in your system you have a leak.

I would trace the vacuum lines and make sure you don't have one that is cracked and leaking. I would check the booster last since it will be the most expensive part to fix.

When you trace out your vacuum system you should find a vacuum chamber that "stores" vacuum so you can have at least one good brake application with the engine off. Over time those can become contaminated and end up with reduced "storage" capacity.

Under load you have lower manifold vacuum. As a consequence when you let off the gas your vacuum will go up and the light go off. Which is why in the newer trucks the electric over hydroboost systems were developed. That and ihc went to all diesel engines that don't have manifold vacuum.

 

[ihfarmer1700]

To get a general idea where do the lines run also shouldn't I hear it dumping air some where. So the booster helps the motor?
My brakes act the same way all the time wouldn't it get better and worse if I had and line leak.

 

[MarkO]

It has been a really long time since I have looked under the hood of a gas powered loadstar so I will have to rely on a fuzzy memory.

Iirc, there is a very large vacuum line coming off the back of the intake manifold. That line will go towards the hydra-vac and the vacuum resevoir. You will also have the more normal vacuum lines that operate the governor (if so equipped) and the vacuum advance on the distributor.

If any of those lines had a leak it would show up in poor engine performance. The engine would not want to idle well at all unless the idle speed was really kicked up, in excess of 1000 rpm's .

If however the engine doesn't change in performance except for when you apply the brakes then you might be able to deduct the problem is in the vacuum diaphram in the hydra-vac unit.

But if the engine doesn't ever really change then I would point the finger at the vacuum reservoir.

Normally you should be able to hear a vacuum leak. But in my experience a vacuum leak, unless it was really large, would not be heard over the engine and road noise--loadstars are no where close to the "can even hear the clock tick" ambiance you will find in other makes.

I would not be too worried about a bad hyrda-vac unit. Most of them are pretty troublefree, even after fifty years.

From your description it sounds as if your hyrda-vac unit isn't getting any or enough vacuum to operate correctly.

Again, if your engine runs smoothly all the time I would suggest you most likely do not have a vacuum leak but a problem with your vacuum system going to the hydra-vac unit. If the reservoir is all gummed up it won't hold enough vacuum to assist the hydra-vac.

The reservoir can get all gummed up for a lot of reasons, most of them age related.

One way in which to eliminate the hydra-vac as the problem is to disconnect the vacuum line(s) to it. If nothing changes then you know the problem is upstream from the hydra-vac. If the engine won't idle you know you have a clear shot up to the intake manifold and the problem is in the hydrac-vac.

If you want, contact me directly and I will see if I can help.

 

[MarkO]

By the way, irishmike who spends a lot of time over at oldihc.org and justih.org has a lot of information he has made available online.

You might want to check with him to see if he can help you with where your vacuum lines should be.