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cat_engines_only, Other
Category: Medium and Heavy Trucks
Satisfied Customers: 2346
Experience:  Repair CAT machines only
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I have a 79 international truck, about a 3 ton truck. The

Customer Question

I have a 79 international truck, about a 3 ton truck. The brake system has, what looks like to me 2 brake cylinders. One on the firewall and one down under the driver on the frame. When the system has been bled the brake pedal gets real hard and there is little braking action. On the first pedal operation the pedal kicks back up at you and then remains hard. Any idea of what is going on with tis?
JA: What is the complete model and serial number of your machine?
Customer: not available at this time
JA: Anything else you want the equipment mechanic to know before I connect you?
Customer: no
Submitted: 7 months ago.
Category: Medium and Heavy Trucks
Expert:  Pete replied 7 months ago.

My name is ***** ***** I am a professional here at Just Answer. I noticed that your question had not received a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.

If there is an issue with the brake master cylinder this can sometimes cause a fault like this here.I have seen incorrect fluid or old fluid over time damage the master cylinder internally.If this has not been checked into this should be done to see if any issues are found with it needing replaced.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service,your 5 star rating is appreciated.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

Customer: replied 7 months ago.
Brand new master cylinder at firewall and the booster under the driver had been replaced as well
Customer: replied 7 months ago.
I have been charged 46.00 a. Already. Is this charge of 25.00 in addition to that?
Expert:  Pete replied 7 months ago.

O.k.,looking through my diagrams and info on this model here unfortunately I do not have all of the info needed to diagnose this fully myself but others here may be able to help further.

I will open your question up for others here to see if someone else can assist further on this as there may be others here that may have more info on what to check or do with this issue.I hope you can get this resolved with ease.No need to reply to my last post here unless it is a last resort as it will keep your question locked to me and others will not be able to view it.If you feel you want to continue to work with me on this feel free to reply at any time but keep in mind in some cases it can be difficult to fully diagnose or help repair this without seeing your vehicle.

Thanks Pete

Customer: replied 7 months ago.
You did not Answer the question from the last reply
Expert:  cat_engines_only replied 7 months ago.


You will not be charged per reply. This is an open chat site were experts should be helping you through a series of questions and answers about your problem.

I am not an experienced technician on these old brake systems but, I have done some work with them. (I jut want you to be familiar with me to understand I want to help.) (Two years with an IHC truck shop, some 25 years ago.)

You brake system is call (by most) a Hydro-Vac brake system. If you want to know more about this brake system use that phrase to point you to relevant information.

The first thing I was always told about these brakes is to "bench bleed" the master cylinder (firewall mounted unit) before installation.

You must also be certain to bleed the air from the line at the booster too. Any air in this section will cause all kinds of strange operation.

I am assuming this is a gasoline engine or if you have a diesel, you must have a vacuum pump. Either case you need to have a strong vacuum to the booster to have "power" brakes. If not, they will be hard to push like when your car is off and you press the brake pedal.

So, to start this troubleshooting, verify no air is in the line from the master cylinder to the booster cylinder. Next verify you have good vacuum to the booster chamber.

Let me know what you have found and I will be glad to help as much as I can.

You can ignore the rating emails until you are satisfied with the answers. The site only holds he charges and will not pay us (experts) until you are happy with the responses.

If at anytime you are not happy, please contact customer service and they can process you a full refund.

Expert:  cat_engines_only replied 7 months ago.

Here is a link to a simple diagram to show a normal vacuum setup.

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