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I aM glad it might help you. However, I want to apologize that after reading my suggestions I might not have been as clear as I could have been, as others have been with other's answers. And I want to be more on target:
The problem IS THE booster VACUUM CHECK VALVE loosing it's sealing capability in part, intermittently, or entirely by getting oil contamination caused by a "going bad" or bad PCV valve.
I would recommend replacing (or thoroughly cleaning both parts. Engine decreasing cleaning additive solvent, break cleaner (short length of time soaking) water clean out/drying in my case)) REPLACEMENT of both parts better/safer). In my case I cleaned & so far so good.
Again booster check valve and PCV valve NOT just one is the problem in this case of intermittently loosing break/pedal going/spongy/sinking break pedal!!!!
I decided to clean the brake check valve & hose but thanks for clarifying.

I feel certain that this is not a hydraulic brake issue. There are no brake fluid leaks on this car. A hydraulic leak will not recover an hour later on its own. A brake booster will not repair on its own either, That leads us to vacuum supply, or lack of, therefore cleaning the vacuum valves makes sense as a repair attempt. So far so good.
 

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You are correct, verification of the check valve/hose could be done (is what I did & how sure I was and after a year w/o any problems after cleaning). However, I am a DIY back yard mechanic. Do NOT expect a dealer or any shop to suggest to clean bc of liability issues (the reason I suggest replacing the part). Also, dealers, shops, etc charge by labor hours...
Weather it takes them 10 min or 3 hours they rightfully will charge what the book hour values.
On THIS specific issue I successfully resolved it by cleaning the booster hoses (I think if I recall the check valve is INSIDE the booster hose (TIP: You will see a LUMP on the hose that'sthe check valve) and not in two pieces where you can replace the check valve. Maybe that's why some dealers aren't aware of it...

Trust me it is NOT complicated, easy to do, and if you have engine degreaser/break cleaner/ soap and water on had already--costs nothing but time. This was the case for me.
Don't over complicate it bc it's that easy. Just bc the new cars have very complicated things on it, on this Murano it is a BASIC old fashioned item AND CHEAP if you do this FIRST.
GOOD LUCK
Before I forget. When I say clean out the vacuum hose I refer to the CHECK VALVE (lump inside the vacuum hose). What I did was used engine degreaser SERVERAL times inclusive I let it SOAK for about a total of about 1+ hours. IT took years of accumulation to build up, YOU need several soaking/cleaning to remove all dirty build up. Also, IF you use break cleaner (a strong solvent) I used soap & water afterwards, right away,, to remove al cleaner solvent out of the "rubber hose" which could potentially detereate the hose in long term. Point being make sure you have a good sealing of the check valve AFTER cleaning. (Replacement is best but it worked for me successfully & still going after +/- a year after cleaning both of the check valve and PCV valve again).
Please, please let us know how you make out!
 

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You are correct, verification of the check valve/hose could be done (is what I did & how sure I was and after a year w/o any problems after cleaning). However, I am a DIY back yard mechanic. Do NOT expect a dealer or any shop to suggest to clean bc of liability issues (the reason I suggest replacing the part). Also, dealers, shops, etc charge by labor hours...
Weather it takes them 10 min or 3 hours they rightfully will charge what the book hour values.
On THIS specific issue I successfully resolved it by cleaning the booster hoses (I think if I recall the check valve is INSIDE the booster hose (TIP: You will see a LUMP on the hose that'sthe check valve) and not in two pieces where you can replace the check valve. Maybe that's why some dealers aren't aware of it...

Trust me it is NOT complicated, easy to do, and if you have engine degreaser/break cleaner/ soap and water on had already--costs nothing but time. This was the case for me.
Don't over complicate it bc it's that easy. Just bc the new cars have very complicated things on it, on this Murano it is a BASIC old fashioned item AND CHEAP if you do this FIRST.
GOOD LUCK
Additional information: I did used the following and as I said, after cleaning the check valve, it successfully resolved my problem, however, before that I had ordered a Nissan check valve assembly, off another vehicle (part number 47478-03B00) just in case cleaning it did not resolve the problem, cost me $35.00--but it is still in it's original bag unused.
Just in case it happens to help anyone.
However, cleaning both the PCV valve and the CHECK VALVE resolved the problem. Cleaning the pcv valve has the advantage to additionally prevent oil leaks to the valve cover gasket & other problems down the road.
Sorry for the repeat
 

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Additional information: I did used the following and as I said, after cleaning the check valve, it successfully resolved my problem, however, before that I had ordered a Nissan check valve assembly, off another vehicle (part number 47478-03B00) just in case cleaning it did not resolve the problem, cost me $35.00--but it is still in it's original bag unused.
Just in case it happens to help anyone.
However, cleaning both the PCV valve and the CHECK VALVE resolved the problem. Cleaning the pcv valve has the advantage to additionally prevent oil leaks to the valve cover gasket & other problems down the road.
Sorry for the repeat
I appreciate the time you have taken to be specific. I will order the brake line/valve to be safe. For now i feel good about the cleaning of the brake line. Its only happened once at least far as I can tell. I do not think this is a chronic problem and can be resolved with the steps you have mentioned.
 

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Additional information: I did used the following and as I said, after cleaning the check valve, it successfully resolved my problem, however, before that I had ordered a Nissan check valve assembly, off another vehicle (part number 47478-03B00) just in case cleaning it did not resolve the problem, cost me $35.00--but it is still in it's original bag unused.
Just in case it happens to help anyone.
However, cleaning both the PCV valve and the CHECK VALVE resolved the problem. Cleaning the pcv valve has the advantage to additionally prevent oil leaks to the valve cover gasket & other problems down the road.
Sorry for the repeat
Correction: ... cleaning DID resolve the problem, cost me $35.00...
I appreciate the time you have taken to be specific. I will order the brake line/valve to be safe. For now i feel good about the cleaning of the brake line. Its only happened once at least far as I can tell. I do not think this is a chronic problem and can be resolved with the steps you have mentioned.
Additional information: I did used the following and as I said, after cleaning the check valve, it successfully resolved my problem, however, before that I had ordered a Nissan check valve assembly, off another vehicle (part number 47478-03B00) just in case cleaning it did not resolve the problem, cost me $35.00--but it is still in it's original bag unused.
Just in case it happens to help anyone.
However, cleaning both the PCV valve and the CHECK VALVE resolved the problem. Cleaning the pcv valve has the advantage to additionally prevent oil leaks to the valve cover gasket & other problems down the road.
Sorry for the repeat
Break problems are of covery scary and insane to drive knowing that it could fail when you know that IF/ when you need them during an emergency it COULD not engage 100%. However, sometimes taking the car elsewhere could be as scary because you might be "rapped" with so many OTHER costly parts replacement parts. As you can already see by the so many DIfferent answers posted--cuases so many DOUBTS in one's mind. Suggestion: before you read or continue any other thing, suggestion GOOGLE "symptoms of bad break booster check valve". Does it give you the symptoms of your problem? If it matches this as a potential culprit, it should give you a bit of piece of mind that you are attacking the correct part! Don't get misled by other potential problems when we should be attacking the obvious FIRST!!!

Best of luck
P.s. please help the other person & post your experience if this helps you successfully!
 
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