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I coulldnt find this situation anywhere else so I'm looking for answers and asking for help. I have a 2012 6.7 with no brake assist. Several months ago my vacuum pump went out and so I replaced it and have had no problems since until the other day. I was stopped at a red light when i heard a funny noise and the brake pedal came up and got hard as a rock. From that moment on I had no power assist brakes. I checked all vacuum lines and hoses so I decided to check the brake booster. Well long story short oil from the vacuum pump has worked its way into the booster and over time the rubber boot inside had a tear in it. I removed and replaced the booster sure that it would fix my hard pedal but surprisingly it didnt. I eventually replaced every line, hose and even the check valve but still have a hard pedal. I checked the vacuum and even with 25 all the way to the booster it still has no power assist brakes. Anyone have any ideas???
 

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Any time this happens and oil is found in the vacuum system, REPLACE EVERYTHING: the brake booster, all vacuum lines, check valve, vacuum reservoir and the vacuum pump at the same time. Before we assume something else is wrong here, all of the contamination and the source of the contamination must be eliminated. It is likely that the reservoir has filled with oil effectively reducing the volume available for the vacuum booster. If you lightly depress the brake pedal with the engine running will it slowly pull down?
 

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Any time this happens and oil is found in the vacuum system, REPLACE EVERYTHING: the brake booster, all vacuum lines, check valve, vacuum reservoir and the vacuum pump at the same time. Before we assume something else is wrong here, all of the contamination and the source of the contamination must be eliminated. It is likely that the reservoir has filled with oil effectively reducing the volume available for the vacuum booster. If you lightly depress the brake pedal with the engine running will it slowly pull down?
I did try that and pedal does go down a very little. Also I did have some oil in the reservoir but it wasnt very much. I tried to clean it out with air and that didnt help at all. Do you think it could have possibly done anything to the master cylinder?
 

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I did try that and pedal does go down a very little. Also I did have some oil in the reservoir but it wasnt very much. I tried to clean it out with air and that didnt help at all. Do you think it could have possibly done anything to the master cylinder?
The master cylinder is separate so it should not have been affected by the oil in the vacuum system.
 

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I ran out of things to try so i ended up replacing the master cylinder anyway and couldnt believe that was the problem along with everything else. I cant explain why other than maybe while trying to stop the truck when the booster went out. Oh well, I dont really care since that took care of it.
 

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It's kind of odd that a master would do that. I am skeptical but keeping my fingers crossed for ya! :)
 

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I ran out of things to try so i ended up replacing the master cylinder anyway and couldnt believe that was the problem along with everything else. I cant explain why other than maybe while trying to stop the truck when the booster went out. Oh well, I dont really care since that took care of it.
Could very well be the case. I have had a MC do some crazy crap after slamming on the brakes into a full lock up. I felt a pop, and ever since then it took more effort to stop. Turned out a cup had gone bad in the middle of the cylinder between the front and rear lines, and was pinching in the bore, I forget which is first, rear or front, but in my case only one pair of brakes were working. I tore it out, and said Resistance is Futile, and replaced it with a new one.

Before returning it as a core I tore it down, and one cup had shifted a bit sideways, you could see the original spring marks on it and where the spring was sitting after it shifted. So basically the cup bound in the bore causing the hard pedal.
 
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