Brake pedal goes straight to the floor after bleeding four times?

XAVIERMARTINDALE

MEMBER
2008 NISSAN TITAN
5.6L
V8
4WD
AUTOMATIC
164,000 MILES

Before doing anything, the brakes acted as they should on a 2008 Nissan Titan King Cab LE V8 5.6.

TLDR
TLDR: Bled brakes 4 times, twice using the pumping brake method, once by a place that only does brakes (Brakes Plus), then did a half assed job at with the Mityvac MV8000. I would read just the bolded and then watch the videos.

Recently we replaced the front driver caliper. There was an attempt to bleed the brakes but didn't get far and decided to wait till the next day. My dad said he did not go below the minimum line on the reservoir. The next day while bleeding the driver rear, the nipple was rusted so badly that it actually broke off. We then replaced the driver rear.

Bleed #1) Next, we started the first full bleeding. We broke the nipple on the driver rear so we replaced the caliper and started the bleeding over. Starting on the passenger rear, I pumped the brakes a few times (3 to 6+times, attempting to get it firm but not working), not pushing down slowly. I held down on the brake pedal, most of the time it was already at on the floor or as far down because of how bad they were. As my dad opened the bleeding valve, I believe some air bubbles came out. We did it until we didn't see anymore bubbles from that wheel. The valve was open when the brake pedal was on the floor pushed down to the max, if that changes anything. Then we repeated all of this going from driver rear, then to passenger front, and then to driver front. At some points it felt like I would pump it and it would get half firm, but then after pumping it again, all that pressure was gone. We went out of order after doing all of that to see if there was any air left. We also did some of it while the car was on and off. Gave it a test drive and brakes went to the floor immediately, similar to what happened in the videos.

Bleed #2) I did some research and 2 hours later we tried again. This time I would pump the brakes and it still didn't get completely stiff. However, the car was off the entire time this bleed and I would not let the brake pedal touch the floor and be maxed out while the valve was open, I don't know if this makes any difference. My dad said he saw some small bubbles at first, about pin heads, and eventually went to very very tiny bubbles that were barely noticeable. We bled each brake until we saw clear fluid with no air bubbles. Also, this bleed, my dad opened the valve a small amount compared to the other times, if that has any effects. The car was jacked up maybe less than 6 inches off the ground to be able to get to the nipple without taking off the tires, it was not even. Even after bleeding it this time, very little changed, said that there was a very small improvement, might have been placebo. It still took us about 3 seconds to stop going on a relatively flat paved road going about 25mph.

After these 2 bleeds, I think that the SLIP light came on sometimes.

Bleed #3) Took it to Brakes Plus to have them bleed it because maybe we didn't do it correctly. They couldn't get the brakes to get better. My dad mentioned something about it possibly being in the ABS motor thing even though we didn't let the reservoir get below the minimum, unless we did just a little bit and didn't notice. The guy said something like how it could be in there but didn't have the tools for it.

Next day we decided to take it to a gravel road and get the ABS to activate, it worked. We did it probably 15+ times, activated for an average of 3+ seconds each time. Did not make any difference.

Bleed #4) I then by myself I used a Mityvac MV8000 to bleed the brakes since I was alone. At this point I think I found the service manual for the car, picture of how it says to bleed the brakes. I did it while the car was off and all evenly jacked off the ground about a foot. I went in order that it said (pic below). I disconnected the negative terminal of the battery because I didn't want to unplug the ABS. I attached it to the nipple, which was not easy, and then to the MV8000. Even with the valve closed, it would loose about 1 PSI every 3 seconds. I opened the valve, fluid started to come out but then a bunch of huge air bubbles came out, to the point where more bubbles than fluid came out. I did this one caliper probably like 20+ times and went through about 5+ oz of brake fluid. I thought that maybe I had the valve opened too much so I opened less and instead of huge bubbles, medium bubbles that were about the size of a pin head with that pearl thing on top came out. Opened it less, and smaller ones came out. Opened it a bunch and a bunch came out. I thought it was maybe that caliper so I switched to the driver front, like it says in the service manual, and did it there. The same thing happened. I have a video of me using the MV8000 and then me pumping the brakes, it is till connected to the MV8000 but there is no pressure built up. You can also see that fluid leaks from the threads when I pump the brakes. I don't know if air slipped between the threads when I was using the MV8000. I only did those 2 since the bubbles were coming out nonstop and I felt like there wasn't a point.

If you have questions maybe look at regular text, I tried to be somewhat specific even if it could have a very small impact on what happened.

What is the problem? Can you tell from the last 2 videos?

We are going to get another thing to bleed with that I believe attaches to the reservoir or something and then connects to an air compressor. When we do this one last bleed, is there anything specific we can do to make it better?

Is it a bad master cylinder? no visible leaks on top or near it, can send photos

A line broke at some point? I don't see any lines leaking and brake fluid level doesn't go down from what I can see. Nothing is dripping on the ground.

Still air in the ABS?

Is air getting between threads, if so, what can we use to make it air tight?

Below are 1 picture of the service manual that I found online and 4 videos.

Video 1) Me using the MV8000 and you can see all the bubbles.
https://youtu.be/XeaUUl8Ms-0

Video 2) Pumping the brakes with the MV8000 attached but no pressure
https://youtu.be/1O4yHqW3Ld0

Video 3) Current state of the brakes while off. When I say it isn't getting stiff, I meant that it is resisting but if I wanted to, I could stand up or put more pressure and it would go down further.
https://www.youtube.com/watch?v=CzOEGlGAlzAVideo

Video 4) Current state of brakes while off and on and lights that show up. When I say "It is hard to pump it" once the truck is on, I meant that it is really easy to push down but hard to pump in order to get the brake pedal stiff.
https://youtu.be/mvVve4YyvFo

1 Reply

CARADIODOC

MECHANIC
33,871 POSTS

You brought up some important points without realizing it. First of all, it is a pretty good bet the master cylinder has been damaged by pushing the brake pedal to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal over halfway runs the rubber lip seals over that crud and can rip them. This can happen when pedal-bleeding with a helper or when the driver is surprised by a popped line. Rebuild kits are available, but they often cost more than a professionally rebuilt unit with a warranty.

My first question was going to be whether the vehicle has anti-lock brakes, but you mentioned that. You also said the reservoir never ran empty, so there shouldn't be any air in that area, however, you need a scanner to perform the bleeding procedure on most ABS hydraulic controllers. That is the only way to expel the air that can pool in two chambers. Performing skids like you did to activate the hydraulic controller won't work. That does pulse the valves open, but the air has no place to go when the bleeder screws aren't open. With the scanner, it commands the ABS Computer to open the valves while you have the bleeder screws open, as directed by the procedure. (The scanner's screen tells you what to do).

There is no correct order you must bleed at the wheels. Even when you read that in a service manual, it doesn't stand up to logic. Some people start with the right rear, thinking getting one line air free will get the most air out, making the other wheels go faster. Other people do the left front first because it takes the least time and fluid to get one circuit air free. I always start with the wheel I'm standing closest to.

Something to consider when bleeding rear wheels is pickup trucks, minivans, and any other model that can have a wide variety of loading, front to rear, will usually have a rear height-sensing proportioning valve. Car models have a simple proportioning valve in the brass combination valve under the master cylinder. Its job is to limit the amount of brake fluid pressure going to the rear wheels. That's to prevent rear-wheel lockup under moderate to hard braking when much of the vehicle's weight shifts to the front wheels. If you add rear passengers or a heavy load, you need to recalibrate the rear brakes to maintain the carefully designed-in front to rear balance. That is done with the special height-sensing proportioning valve. That will be in the rear with some type of linkage connected to the rear axle.

When you raise the vehicle off the ground to gain access to the bleeder screws, the vehicle must be let down and supported on jack stands under the axle, not under the frame. If it is supported under the frame, the axle will hang down similar to how it extends during hard braking. That will close off the proportioning valve, then no or very little fluid will flow to the rear brakes, making it almost impossible to get any air out.

One of the clues the master cylinder has been damaged is the brake pedal will act differently during a repair session. You might see it go all the way to the floor one application, then get a low but firm pedal the next time. When you do get somewhat of a good pedal, hold light pressure on it, then observe if it suddenly, slowly sinks to the floor. If the pedal holds steady, modulate, or vary the amount of foot pressure on the pedal without fully removing all pressure. In other words, maintain pedal pressure, but vary how hard you're pushing. That can induce the pedal to slowly sink to the floor, exposing the defect. For the pedal to sink like that, brake fluid has to be leaking somewhere. I'm sure you've checked many times for an external leak and found nothing, or you'd have a puddle on the floor. Instead, you have an internal leak when the lip seals in the master cylinder are torn. The fluid bypasses the seals rather than being pushed down to the wheels, under pressure. This slowly sinking pedal is very different from a simple low pedal due to air in the system.
If you need to replace the master cylinder, let me know before you start. I have a trick that eliminates the need to bleed at the wheels.

You are correct about air sneaking past the threads of the bleeder screws. My response to that is to ignore it. Air in the lines comes out in huge, long bubbles, never a steady stream of tiny pin-size bubbles. When you see that steady stream of really small bubbles, they're being pulled in past the bleeder screw's threads by vacuum-operated bleeding equipment. Ignore them. Even if they did somehow come through the system, they're much too small to cause a low brake pedal.

The only bleeding procedure I've used since forever is gravity-bleeding. I use that too to replace old, moisture-laden brake fluid on my old cars. When doing brake work that requires replacing a part that will have air in it, I do those repairs first, then let each circuit gravity bleed while I work on the other side. Once fluid starts dripping from the bleeder screw, close that bleeder, then wait for the other wheels to do the same thing. Once all the bleeder screws are closed, "irritate" the brake pedal a little by hand to wash any remaining stuck bubbles into the calipers and wheel cylinders. Open each bleeder once more to expel those last few little bubbles. The reservoir cap has to be loosened to do this, otherwise a vacuum will build up in the reservoir as the fluid runs out. That vacuum will stop the fluid from flowing under gravity.

Here's a list of a number of related articles to check out:

https://www.2carpros.com/articles

There's one for replacing the master cylinder. Let me know how far you get with this, then we'll continue on.