Mushy brake pedal only with engine running

[arknotts]

So I finished installing my front big brake kit from Arizona Performance, and bled the system. Everything went well, but the brake pedal was very mushy when the engine was running. It would go almost to the floor (felt like it bottomed out in the master cylinder first?) with enough pressure. So I bled them multiple times until I realized that my master cylinder was weeping some fluid between it and the brake booster. I waited a few days for a new OEM master cylinder from Toyota, installed it yesterday after bench bleeding it I pressure bled the entire system today. Start the car up, and the pedal feels the SAME! Argh!

I can feel it bottoming out if I apply enough pressure, though I don't think it's hitting the floor. It feels like the master cylinder bottoms out first. Is this correct, or should the pedal be able to hit the floor? I'm at a loss to what the problem could be. There are no external leaks of fluid now, and the pedal doesn't sink to the floor with constant pressure (I have to apply more and more pressure to get it to go to the floor). I assume I still have air in the lines, but I've bled it over and over without even the tiniest air bubble. Actually one bleeder spits out consistent bubbles but I think it's coming in through the threads because it will stop/start if I wiggle the bleeder back and forth.

Any other suggestions? My car has been down for a month now for several reasons (this being the biggest) and I really want to drive it! I would appreciate any help.

Edit: I don't know if this matters--or if it is even normal--but when I press the brake and release, the engine idle changes slightly and my wideband reads lean (about 15-16 AFR). If I pump the brake fast I can get it to go to like 17 or 18 AFR! Is this normal fluctuations from the load on the engine changing, or do I also have a leak in the brake booster? I doubt this would cause a mushy pedal (if anything it would cause a harder pedal), but correct me if I am wrong.

 

[Poodles]

Booster is probably shot.

 

[whitemike]

Booster!

 

[arknotts]

Booster is probably shot.

Booster!

I may be wrong about this, but wouldn't a bad booster cause a hard pedal? I can't see how a bad booster could cause any softer of a pedal than a properly working one, but then again I am not a mechanic and have no direct experience with this stuff so I only have my knowledge of how the system works.

Then again, I might be dealing with two separate issues...air in the lines and a bad booster causing a vacuum leak. Thanks!

 

[Poodles]

Bad booster can cause all sorts of issues. Also, the main load on the engine from hitting the brakes is electrical from the brake lights coming on. This is why it's a bit hard to diagnose, but look at a volt meter to see what's going on, it will help narrow it down.

 

[mkiiichip]

Why not test it, to be sure.
Using a hand vacuum pump, pump up the booster and see if it holds vacuum. If it does hold, then hit the brakes once and see it it still holds.
It sounds, to me, like a leaky booster and some air lingering in the lines. Was the master bench bled?

 

[arknotts]

Why not test it, to be sure.
Using a hand vacuum pump, pump up the booster and see if it holds vacuum. If it does hold, then hit the brakes once and see it it still holds.
It sounds, to me, like a leaky booster and some air lingering in the lines.

I don't have a vacuum pump handy right now, but it seems to hold vacuum fine after the car is shut off. It takes 3-4 pumps to get the pedal hard again. There is even some residual vacuum after it sits overnight.

Was the master bench bled?

Yes.

 

[arknotts]

Here is a video:

[video=youtube;VUWUD4fg8Qo]http://www.youtube.com/watch?v=VUWUD4fg8Qo[/video]

It seems like a vacuum leak in the booster diaphragm to me. I'm still not sure that is the cause of my spongy pedal though. I'm going to try bleeding the lines again today.

 

[Nick M]

If it was the booster, he wouldn't have those symptoms. When the engine is running he has vacuum assist. His improperly bled lines or leaking line is mushy when he has assist. When the engine is not running, you only have assist for one stop. One pump and the pedal will not be mushy.

My guess is conjecture because it is only based on his information he put out, and it could be a bad description.

Use a vacuum gauge, not an oxygen sensor in the exhaust.

 

[arknotts]

If it was the booster, he wouldn't have those symptoms. When the engine is running he has vacuum assist. His improperly bled lines or leaking line is mushy when he has assist. When the engine is not running, you only have assist for one stop. One pump and the pedal will not be mushy.

I'm not sure why you think I bled the lines improperly? I've tried both two-man bleeding and pressure bleeding and both have yielded the same results. That said, obviously something is wrong and I likely still have air in the lines, but do you have any suggestions to get all of the air out? I know it can be a pain getting the air out of the ABS unit even with proper bleeding since our cars have no way to purge or cycle the ABS unit.

My guess is conjecture because it is only based on his information he put out, and it could be a bad description.

I think you can tell from my description that I try and be as thorough as possible.

Use a vacuum gauge, not an oxygen sensor in the exhaust.

I am using a vacuum gauge (see video above); I'm just also curious why the wideband is going lean since I don't think it's normal for it to go lean like that when pressing the brake. Couldn't running lean for a split second indicate a vacuum leak at the booster? I'm not just going to ignore that unless someone can tell me it's normal. Like mkiiichip said I wonder if I'm dealing with two separate issues...

 

[Nick M]

I think it wasn't done completely because you said the lines are mushy. That is what happens when air is in the line. Or it leaks, which is the same thing only different. ABS is bled the exact same was as conventional brakes. In fact, unless the unit is running, you are on conventional brakes. The solenoids are both in the open position for fluid flow in default. This is so if there ever was a problem, you would still have brakes.

Customers say a lot of things that seem to be way off base. It wasn't personal.

If you step on a normal functioning brake pedal, a valve opens allowing engine vacuum on top of the hydraulic assist to open to atmospheric air creating a pressure differential. That is normal. If the valve was sticking, they usually hiss when you hold the pedal.

 

[Poodles]

Did you follow arz's instructions to the letter?

 

[arknotts]

I think it wasn't done completely because you said the lines are mushy. That is what happens when air is in the line. Or it leaks, which is the same thing only different. ABS is bled the exact same was as conventional brakes. In fact, unless the unit is running, you are on conventional brakes. The solenoids are both in the open position for fluid flow in default. This is so if there ever was a problem, you would still have brakes.

Customers say a lot of things that seem to be way off base. It wasn't personal.

If you step on a normal functioning brake pedal, a valve opens allowing engine vacuum on top of the hydraulic assist to open to atmospheric air creating a pressure differential. That is normal. If the valve was sticking, they usually hiss when you hold the pedal.

OK, I'll try to bleed the brakes again tomorrow. It's just that I've gone through LOTS of brake fluid already and the lines have bled clean for quite a while now. I'm bleeding in this order: RR, LR, RF, LF. On the wilwood calipers I bleed the outer first then the inner one (though I have tried the inner first with no luck). On one of the bleeders in the front I get a constant flow of tiny air bubbles which I think is coming from the bleeder threads. Reason I think this is because if I don't open the valve as much or apply a little pressure to the threads they stop. I'll also try lightly tapping the calipers and ABS unit with a rubber mallet to hopefully loosen any air bubbles that are stuck to the inside surfaces.

Did you follow arz's instructions to the letter?

Yes, I followed these instructions very closely:

http://www.arizonaperformance.com/installs/mk3install.htm

Thanks for the help guys.

 

[arknotts]

Well I bled all four calipers again, and it felt about the same...maybe just a little better. Seeing as I still had a good bit of pressure in the pedal, even though I could press it down quite a ways I went for a quick drive. It seems to stop just fine, and I actually was able to bed-in the brakes according to ARZ's instructions. At the end of the bed-in process I was nearly able to engage ABS and still had a little bit of pedal travel left. I still think there might be a little air in the lines, but it is definitely safe to drive and I'll drive it a little while and maybe bleed them again later.

I compared it to the pedal on my IS300 and it's very similar. Thanks for the help guys, I'm just going to chalk this one up to some inexperience and being very picky/analyzing everything (especially when it comes to brakes ).

 

[Orion ZyGarian]

Air bubbles can be extremely annoying and a PITA to get out. You might end up going through a ton of brake fluid just to get a tiny bit of air out.

Also, the test of if a brake booster is good or bad is to push in the brakes while the engine is off...keep your foot on the pedal. Start the engine, and if the pedal doesnt descend slightly, the booster is bad. This is how I know I have manual brakes on my Miata DD.