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Old 02-07-2019, 03:01 PM   #1
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Join Date: Sep 2010
Location: Ocala, FL
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Year: 1995
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Chassis: International 3800
Engine: Navistar DT466/Alison MT643
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Hydro-Max power steering/brakes

I finally started the engine on my bus project after letting it sit for a long time. It started right up on the very first crank. Amazing. It had not been started in over six years. (I did change the oil and filter before I cranked the engine.)

But ... now, when the engine is running, the "BRAKE PRESS" light and buzzer are on. The owner manual says this stays on when there is a fault in the Hydro-Max brake boost system and that it should NOT be driven when this system isn't functioning properly. The same pump supplies the power steering and that is working fine. I put it in gear and rocked it back and forth a bit and the brakes seem to be functioning as normal.

The bus is a 1995 International 3800 with a DT466 engine, MT643 transmission, and hydraulic disk brakes.

Anyone have any knowledge of the Hydro-Max system and how I might begin to troubleshoot it?
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Old 02-07-2019, 07:41 PM   #2
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When the ignition is "ON" do you hear the electric pump motor running? You will find this electric motor near the hydroboost on the firewall where the foot pedal attaches.
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Old 02-07-2019, 08:39 PM   #3
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Quote:
Originally Posted by Johnny Mullet View Post
When the ignition is "ON" do you hear the electric pump motor running? You will find this electric motor near the hydroboost on the firewall where the foot pedal attaches.





Hmmm....I hear my brake booster pump anytime I touch the brake pedal as long as the batteries are connected, not just when the ignition is on. In my mind this is the better way to let you know you have brake boost at all times.

What do you think of that setup Johnny? I assume it was built that way for safety, kind of glad it is rather than only when ignition is on.



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Old 02-08-2019, 06:24 AM   #4
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The hydroboost should operate the brakes when the engine is running. The electric part is there in case it's not working for safety.
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Old 02-08-2019, 08:05 AM   #5
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The hydroboost should operate the brakes when the engine is running. The electric part is there in case it's not working for safety.

Ok ty Johnny, that explanation works for me. I don't hear the electric when driving so I guess that is a good thing then.


So nice to have a real mechanic on board to bounce things off, thanks again man!


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Old 02-08-2019, 08:24 AM   #6
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Join Date: Sep 2010
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Quote:
Originally Posted by Johnny Mullet View Post
When the ignition is "ON" do you hear the electric pump motor running? You will find this electric motor near the hydroboost on the firewall where the foot pedal attaches.
No, the pump motor doesn’t run with the ignition key turned on while the engine is not running nor when it is running.

And now that you mention it, I remember I used to be able to press the brake pedal with the engine off and I could hear ... something. I never knew what it was, but I suppose now that it was the electric pump assisting the brake accusation with the engine off. Is that correct?
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Old 02-08-2019, 08:37 AM   #7
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No, the pump motor doesn’t run with the ignition key turned on while the engine is not running nor when it is running.

And now that you mention it, I remember I used to be able to press the brake pedal with the engine off and I could hear ... something. I never knew what it was, but I suppose now that it was the electric pump assisting the brake accusation with the engine off. Is that correct?
Correct. That is how it works.
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Old 02-08-2019, 08:43 AM   #8
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So I guess the reason for the alarm is because this electric pump motor is not operating. I'll start there. Hopefully it's something simple.
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Old 02-09-2019, 10:29 AM   #9
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This should help
Attached Files
File Type: pdf Hydromax booster.pdf (408.1 KB, 3 views)
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Old 02-09-2019, 11:18 AM   #10
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This should help
Thanks for that! I've got some reading to do.
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