Hard brakes - Diesel Forum - TheDieselStop.com
7.3L IDI Diesels (Not Power Strokes) Technical discussion of topics related to vehicles powered by the 7.3 Liter In-Direct Injection Navistar engines.

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post #1 of 19 (permalink) Old 06-30-2018, 06:36 PM Thread Starter
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Hard brakes

I have replaced the vacuum pump, the master cylinder, the brake booster and my brake light is still on and hard brakes, any idea?
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post #2 of 19 (permalink) Old 06-30-2018, 06:41 PM
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Have you tested the vacuum at the vacuum distribution manifold?

Not by just putting your finger on it but a actual gauge.

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post #3 of 19 (permalink) Old 07-01-2018, 11:05 AM
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What should that be? I have always just put my finer over the hole, and guessed. "Micrometer eye...budweiser elbow"

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post #4 of 19 (permalink) Old 07-01-2018, 11:52 AM
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I believe that it should be around 18Hg on a vacuum gauge.

While the finger and elbow test works but if you are having problems that could be related to a low vacuum issue I would think that you would want the actual reading.

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post #5 of 19 (permalink) Old 07-01-2018, 02:32 PM
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I believe that it should be around 18Hg on a vacuum gauge.

While the finger and elbow test works but if you are having problems that could be related to a low vacuum issue I would think that you would want the actual reading.
I agree, that's why I asked. I'm having a vacuum problem, pretty sure it isn't the pump, 'casue it comes and goes. A pump usually just fails. I haven't tested anything yet, just started the end of the week. I want to catch it when it isn't working.

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post #6 of 19 (permalink) Old 07-01-2018, 03:11 PM
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I agree, that's why I asked. I'm having a vacuum problem, pretty sure it isn't the pump, 'casue it comes and goes. A pump usually just fails. I haven't tested anything yet, just started the end of the week. I want to catch it when it isn't working.
When I was working I was a trouble shooter for the company and I always told people that if it was broke I could fix it. It is wasn't or if it was a come and go type of trouble that it would usually be a lot harder to fix. That is when you need to drag out the manuals and start making sure that things are within spec's . But at times you have to go with your best guess and go for it.

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post #7 of 19 (permalink) Old 07-01-2018, 04:00 PM
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When I was working I was a trouble shooter for the company and I always told people that if it was broke I could fix it. It is wasn't or if it was a come and go type of trouble that it would usually be a lot harder to fix. That is when you need to drag out the manuals and start making sure that things are within spec's . But at times you have to go with your best guess and go for it.
I've told many supervisors, that if I knew what was wrong, I'd have it fixed, then asked them to please not help, because it would take much longer to figure it out.

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post #8 of 19 (permalink) Old 07-01-2018, 07:13 PM
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Looked at my photobucket pics since I cant post pics. When someone asked a few years ago I hooked up my vac gauge to the vacuum manifold. Idling I had 25.5 inches of vac. Pressed brake pedal, dropped to 21 and took 5 seconds to recover.

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post #9 of 19 (permalink) Old 07-01-2018, 08:15 PM
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Looked at my photobucket pics since I cant post pics. When someone asked a few years ago I hooked up my vac gauge to the vacuum manifold. Idling I had 25.5 inches of vac. Pressed brake pedal, dropped to 21 and took 5 seconds to recover.
Will not get that at higher elevations, Got to really work to get close to 20 here, think it is more around 15 to 20.
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post #10 of 19 (permalink) Old 07-02-2018, 07:35 AM
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I found a chart once and I think at 5000 feet it showed something like 19 or 20. I don't know if this applies to all older model years, but most people don't know that the plastic inlet fitting at the vacuum booster is a one way check valve to keep max vacuum in the booster.

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post #11 of 19 (permalink) Old 07-02-2018, 07:40 AM
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Iirc, the check valve was the "zero loss booster."
There was a tsb about it.
Or are they all a check valve?

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post #12 of 19 (permalink) Old 07-02-2018, 08:57 AM Thread Starter
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I have not tested the vacuum manifold with a gauge, just the finger test, does it need to be a special guage to test the vacuum, sorry I am new to desiels. Is the vacuum manifold the little square box that the vacuum hose runs To? Thank you guys for all your help.
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post #13 of 19 (permalink) Old 07-02-2018, 04:08 PM
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I have not tested the vacuum manifold with a gauge, just the finger test, does it need to be a special guage to test the vacuum, sorry I am new to desiels. Is the vacuum manifold the little square box that the vacuum hose runs To? Thank you guys for all your help.
Yes that's it. Driver's side fender well, right where you can "easily" reach it. Didn't know that about the check vave, I think I'll check mine there, might be what is wrong, by the way it is acting.

Any good vacuum gauge will work

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post #14 of 19 (permalink) Old 07-02-2018, 09:54 PM
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If you have the Tomato Can on the drivers side fender, that is a reserve for one time braking and also for cruise, they do rust thru small holes, vac pump has a hard time try to keep up.

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post #15 of 19 (permalink) Old 07-03-2018, 05:38 PM
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Thanks Trucker, now I have two directions to lean

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