Brake booster leak?

[Chester11]

Hey folks:

07 Mazda 6 here with 90k. I posted on this before - but checking back in for some advice.

After sitting overnight, I have no brake reserve.

I replaced a seized caliper - and have coincidentally been chasing a idle issue where I tested the brake booster.

All the brake pumping tests seem to come out okay. But, a vacuum test shows a slight decrease in pressure at the booster (like 2-3psi maybe over 20 minutes). It could be my equipment, my hand pump is garbage. The fuel trims seem ok, and plugging the line doesn't seem to fix the idle issue. (Which was my original concern - the bad caliper was just a wear issue, I think.).

I'm thinking of replacing the brake booster and the line to the manifold. (The line contains a check value, which seemed to seal okay when I tried to blow air through it, but sounded like a trumpet going the other way.).

(Interesting Mazda pricing - the booster is $44 new. The vacuum line is $95, and there appears to be no aftermarket. I'm trying to find a junkyard part).

I may have to pull the wiper motor to get to the booster. Should I do the brake cylinder while I'm in there? (The job is starting to get pricey). Thoughts?

 

[Lyman]

I had to replace the booster on my jeep due to a leak. It let itself be known via a whistling every time I let of off the brake pedal. Is the vacuum line rubber? I can't see a vacuum line being $95.
I'd leave the Master Cylinder alone if it's not having any issues.

 

[anndel]

Check the vacuum line first. If it comes to a bad brake booster, might as well replace the master cylinder, drain and replace the brake fluid then bleed on all wheels plus the ABS all at the same time. Good Luck

 

[Chester11]

it's mostly rubber with some steel on the manifold side. $94 is the discounted price from an online shop, my dealer wanted something like $130. I think the valve is shot based upon the trumpet sound, but the only was to know for sure is to replace it.

whatever the problem is, it's getting worse. I just drove it to lunch and there was no reserve left at startup. (sat for about 3 hours).

 

[zzyzzx]

This seems to be the type of thing that almost never goes bad. The one on my 1995 Escort is still the original. I would just get one from a junkyard.

 

[tookien]

On the push rod on the brake booster that meets the master cylinder, I would look at whether there is a rubber/plastic seal or adapter.

If you happened to loosen or replace the Master Cylinder, this seal can FALL OUT.

I had this issue before and I had no idea what it was because when I swapped the MC it must have fell out without me knowing.

This seal caused not only a vacuum leak at the brake booster, but would never allow me to bleed out the brakes, or it would just give me a false positive.

I had to change the BB because that seal was not sold separately. I didn't even know that seal was missing until I bought the new BB.

Anyways, your issue is probably different but I would watch out for simple things like this.

There is also a vacuum line to your BB you should look at. Check if you have adequate vacuum pressure.


If the BB is only 44 then buy it. I'm a fan of acquiring parts from the junkyard (except stuff like master cylinder, fuel pump, water pump etc...) but when it comes to brakes I'm a little cautious. The brake system is the most important thing in your car, IMHO.

 

[Chester11]

I hear you about the junkyard parts. I'm lucky to have decent place pretty close by - I don't want to spend $100 on a hose if it doesn't fix the problem. I would probably go new or maybe rebuilt for the cylinder.

 

[tiger862]

Check Master Cylinder for leak in rear as that is what eats seals on booster

 

[Lubener]

There likely is a check valve and a grommet involved for vacuum. I would check those before replacing the whole booster. Myself, after shutting down the engine, wait about 15 minutes and check to see if you get 3-5 assisted pumps before pedal goes hard. If you cannot get this, you need to look at the booster, check valve or the grommet it fits in because that is a safety issue The check valve should keep vacuum in the booster without it bleeding back into the manifold..If it takes all night to bleed down, that leak is minor and will have no effect on the way your engine or brakes perform.

 

[Chester11]

Thanks for all the help folks.

I only get 2-3 pumps out of the brake reserve. There is no obvious leak behind the cylinder, but I do hear a click when applying brakes (from the pedal / brake booster area inside the car). I figured the pedal mechanism needed to be lubed, but maybe this is part of the problem.

I'm trying to get a hold of the hose with check valve - it is surprisingly difficult to get - even from Mazda. The check valve is in-line, not at the booster. I thought about buying a ford valve from a fusion and trying that - it sits at the booster and not in-line..

One last question - should pumping the brakes 3 to 4 times drive the fuel trims lean?

I'm focused on the booster area because I swear I hear an air leak in this area - and do not want to have to pull the manifold to replace the PCV valve. This seems to be the most common vacuum leak on these cars.

 

[Lubener]

Originally Posted by Chester11
Thanks for all the help folks.

I only get 2-3 pumps out of the brake reserve. There is no obvious leak behind the cylinder, but I do hear a click when applying brakes (from the pedal / brake booster area inside the car). I figured the pedal mechanism needed to be lubed, but maybe this is part of the problem.

I'm trying to get a hold of the hose with check valve - it is surprisingly difficult to get - even from Mazda. The check valve is in-line, not at the booster. I thought about buying a ford valve from a fusion and trying that - it sits at the booster and not in-line..

One last question - should pumping the brakes 3 to 4 times drive the fuel trims lean?

I'm focused on the booster area because I swear I hear an air leak in this area - and do not want to have to pull the manifold to replace the PCV valve. This seems to be the most common vacuum leak on these cars.

If you are getting 2-3 pumps, I would look elsewhere. If you had a bad vacuum leak, your fuel trims would rise high on the positive side as the computer adds fuel to compensate for a lean condition.. You say fuel trims are ok, what are they? If your LTFT swings less than +/- 5, I wouldn't worry about it. When they get around +/- 10, you have a problem which likely would throw a code. The clicking you hear likely is the brake light switch. If your manifold vacuum is steady at 18-22 inches, you are fine.

 

[Chester11]

Last time I checked short and long were running single digits on the negative side.

At idle and in gear they cycle around zero (when rough idle happens).

Pumping the brakes can drive both to about +15 for a few moments, but no code.

I'll keep looking - but I'm about out of ideas other than PCV. I have sprayed water and solvent everywhere and can't find a vacuum leak. (The PCV is behind the manifold and I cannot spray it directly - apparently the hose cracks and causes a leak. I have not gone this route because the trims looked ok and seemed to rule out a vacuum leak).

I am going to replace the last motor mount and see if that helps.

 

[mk378]

Every time you release the pedal, the air that was let into the booster to generate braking power is pulled out to restore it to vacuum.

So, pumping the brakes lets unmetered air into the engine, completely expect it to be met with a fuel trim.

 

[Chester11]

Originally Posted by mk378
Every time you release the pedal, the air that was let into the booster to generate braking power is pulled out to restore it to vacuum.

So, pumping the brakes lets unmetered air into the engine, completely expect it to be met with a fuel trim.


thanks. that makes sense. I was tying it figure out why pumping the brakes would be different than holding down the pedal at a light.

 

[Chester11]

Update - I did a smoke test and was surprised to find no leaks. I figured there had to be a small one somewhere that would give me a feeling of accomplishment. I also disconnected the EGR, which did not make a difference or trigger a code.

I am still trying to find an in line check value - if anyone can point me to a source for a generic one, I would appreciate it. (I am trying to rule everything else out before replacing the mount).

I am also going to clean the MAF - I last did it about 3 years ago, I read that negative fuel trims can indicate that the PCM is attempting to compensate for a dirty MAF (I do have slightly negative trims). Throttle body has been cleaned. Other than replacing a bunch of parts, I am out of ideas - and may finally visit the mechanic.