Consider the possibility of a fault in the brake master servo mechanism, and have any fault eliminated. Needless to say, the vehicle should not be operated while the fault remains.

It's a seal in the master cylinder letting by,the fluid won't leak out but the pedal will go to the floor.

Some years ago I had a Humber super snipe and I started to loose brake fluid, I checked the servo, took all the wheels and drums off and found nothing, I even parked over a white sheet overnight but no sign of fluid,I was putting about a cup full a day in by the end, and in the end I dicovered the fault when a police officer waved me to stop for some stupid mistake I made at a junction which he was directing traffic at, he was waiving his arms about and I took it as a signal that I could turn right, (it was a one way system) he more or less jumped on my bonnet/hood,He asked me what I thought I was doing,I asked the same question to which he said "pull over to the kerb sonny" at the time I was a sonny,I digress, the road being four lanes meant travelling right across all the lanes to the kerb and being a " sonny " put my foot down to indicate my annoyance,and that is when I discovered the fault with my braking system, a huge cloud of thick white smoke poured out of the exhaust blocking out all the high street so I thought I had better not stop because he is bound to think I did it on purpose,so I made good my escape,went home and changed all the seals in the servo.Job done thanks to a nice policeman, I wonder if he had asthma?

Bazzer

Love the story! So what was it, servo filling up with brake fluid?

Yes, it gradually filled the servo untill it reached the vacuume pipe,which was about two thirds up the chamber so when I put my foot down and swung to the left it was enough to slosh the fluid into the pipe and into the intake manifold.

Bazzer

.....but the OP is not losing any fluid. The failure you descibe would suck the fluid out of the brake system.....

By the way, its a fairly well known problem including the smoke when driving in some cases....

Good answer!

btw, how long do I have to keep doing this?

It dosent suck it out the brake system just the servo, It's pushed into the servo by the master cylinder,

Bazzer

...helped by the vacuum!!

The bellows are "sucked" together when braking to exert extra pressure on the master cylinder.....they are needed on all cars with modern brake pads without asbestos.....some cars had them before asbestos was got rid of, but only higher quality cars, at least in europe.

I would take it to another shop...good mechanics are hard to find....

I had this happen to me. You are correct, I believe.

I also believe the OP should take the bill for this to the Nissan Dealer and give it and the defective part to them.

I agree with ozzb and PWSlack - master cylinder.

I agree with lyn. The Nissan dealer probably charged you a pretty penny to not fix the problem. Get them to fix it ( but don't expect too much!) Perhaps you could give them some hints. They won't like you for this, but perhaps you could point out that you have the best engineers this side of Alpha Centauri behind you (try saying that in the monosyllables they might comprehend!)

I agree with SolarEagle - find a mechanic/garage you can trust (easier said than done, and always my biggest concern when I have moved house).

One of the problems these days is that cars are designed in a modular fashion. This, in itself, is not such a problem, in fact it is highly logical. But it seems that mechanics these days are trained to just keep replacing modules until the problem is solved (unfortunately your Nissan mechanics obviously got bored before the job was done). It seems they are not trained (or helped to gain experience) in problem solving or root cause analysis. It is all done by 'standard work', no thinking required! Basically they don't have a clue. And we are trusting them with thousands of dollars worth of equipment (our biggest investment beyond the house) and our lives. These are not mechanics, they are wrench monkeys, filling their time until the next Friday night out.

Do not make the mistake of assuming that your main dealer knows best for your car make/model, or that they will provide the best service, If you can find a good mechanic, you are set for life, and can then handle all the vicissitudes...etc...

btw. I have noticed that brakes are quite useful, if not vitally important!

Comparing them to monkey is giving them too much credit. Is also defamatory to monkeys which can figure some things out. To few mechanics that can actually troubleshoot. As long as we continue to pay them to throw parts at a problem until resolve they will.

Master cylinder is leaking internally! Common problem, see it everyday!

Classic bad master cylinder problem. Pumping the brakes may raise the level of the pedal. Don't drive it this way.

I had a failure where a piece of drum hardware broke on a single master cylinder car. This resulted in total brake failure. I did manage to drive it 10 miles to home using just the emergency brake to stop. These days the "emergency brake" is more like a "parking brake" which is not the same thing in my opinion.

Master Cylinder Failure, absolutely, definitely, no question. Unless you have a flexible brake paddle which softens over time like saltwater taffy... no... really. It's the master cylinder.

http://cr4.globalspec.com/thread/22457

http://forum.miata.net/vb/archive/index.php/t-354173.html

http://www.trustmymechanic.com/master_cylinder.htm

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake1.htm

http://www.tegger.com/hondafaq/mastercylinderreplace/index.html

http://www.import-car.com/Article/39076/basic_brake_hydraulics_classic_symptoms_of_a_failing_master_cylinder.aspx

I agree with Engineering Superman.

You will probably find that if you stomp hard that the brakes will apply. But if you apply a constant force, the pedal will slowly move to the floor. The internal leakage is between the pressure side of the piston and the reservoir. This could be caused by a failed piston seal or a scratch on the bore.

Have them replace the master cylinder with a new or factory rebuilt. I would not trust this group to rebuild the unit in their shop. I would tell them that our contract was to repair the problem and to randomly replace parts. In the end you should only have to pay for the master cylinder, and the brake bleeding. If they do not want to refund charges for booster pump, ask them to reinstall the good booster pump that they removed. As part of your negotiation tell them that you will not charge them for the use of your vehicle in training their technician. If this is an authorized dealership they should have factory trained technicians. Ask to see the technicians certificate in brake repair. You can also contact Nissan's district service manager about your situation.

I worked for several Nissan dealers back in the days when they sold vehicles under the Datsun brand. Back in the 1970's, Nissan, although not necessarily all the dealerships, took custom satisfaction very seriously. They had a VOR (vehicle off road) program to get cars quickly repaired. If a was not available to fix the vehicle that was under warranty, the district service manager was authorized to scavenge the part from a new vehicle.

I don't know where the master cylinder is in your car but in my 1976 Ford Van it's on the right side under the hood(bonnet) and attached to the firewall. When I would have this problem I would stop at the auto parts store, purchase a fresh one, and install it. A few bolts and two brakes lines were all I would have to disconnect. It would take about fifteen minutes and I could do it right there in the parking lot. The new master cylinder would come with instructions on how to prime it so you wouldn't have spongy brakes.

Master cylinder seals are possibly damaged because the brake fluid is not being "moved". Or the master cylinder bore is damaged in some manner. Rust being possible when the hydraulic fluid is too old and contains too much water for example.....test fluid using a testtube sample and a small gas flame to heat it up, if it "crackles" when hot, replace the fluid and of course the master cylinder at the same time....

As no fluid is being lost, no need to look further or replace anything else, what has already been replaced was a waste of time and money....nothing to do with the actual problem, it simply demonstrates a lack of understanding how brakes work OR a way to make extra money from customers!!!!

To the OP, it is a pedal not a paddle. You use your foot not your leg which is why it can also be called a foot pedal!!!

See the new title, use that instead

definitely sounds like the master cylinder, though another possibility is soft hoses from the hard line to the brakes. This could be expanding as you depress the pedal giving a spongy feel. but I would still bet on the master cylinder (cost wise you might take a peek at the hoses and see if they are deformed.

Ok, I'm from the points, plugs, condensors, and brake shoe school, but if I had those symptoms, I'd either rebuild or replace the master cylinder... Good luck with your project!