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Brake pedal - hard at startup #4531888
10/02/17 08:23 AM
10/02/17 08:23 AM
Joined: Apr 2017
Posts: 163
PA
Chester11 Offline OP
Chester11  Offline OP
Joined: Apr 2017
Posts: 163
PA
Hi folks:

Slow motion brake job here - I have been messing with the brakes on an 07 Mazda 6i for going on a month. Replaced the front pads, brake fluid change / flush, and did couple bleeds to removed any air.

Now the pedal is hard before startup - which is different than before. After starting it drops slightly, and everything feels normal.

Does this indicate a master cylinder or vacuum booster leak, or did I just have a ton of air in the system?

For background - the pedal originally felt squishy under braking (like there was air in the system), and I did all this to improve the initial "bite." After a few bleeds and activating the ABS a couple times, it feels better, except for when I first get in and turn the car on.

Thanks guys - this is probably my 4th or 5th post on this - and the input has been helpful.

Re: Brake pedal - hard at startup [Re: Chester11] #4531957
10/02/17 09:42 AM
10/02/17 09:42 AM
Joined: Apr 2016
Posts: 1,198
Campbellsville, KY
Alex_V Offline
Alex_V  Offline
Joined: Apr 2016
Posts: 1,198
Campbellsville, KY
Having no knowledge of your particular car but speaking on general principles - it's
normal for the pedal to drop once engine vacuum comes in and increases the amount of assist via the booster.


To quote a friend, "Synthetics, man."

'85 GMC C3500, 454, 132K
'82 Mercedes 300CD, OM617 turbo, 173K (totaled 2018, in rehab)
'85 Mercedes 300TD, OM617 turbo, 217K (wife's)
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Re: Brake pedal - hard at startup [Re: Chester11] #4531966
10/02/17 09:56 AM
10/02/17 09:56 AM
Joined: Apr 2017
Posts: 1,481
Ontario, Canada
mightymousetech Offline
mightymousetech  Offline
Joined: Apr 2017
Posts: 1,481
Ontario, Canada
If it is really hard before you start it, you may have a leaking check valve in the brake booster.


Mighty Mouse Tech
BMW Tech, Former Acura Tech
2010 Civic Si Castrol 0W40, Redline MTL
2013 BMW 135i M Sport Edition Castrol 0W40, Redline MTL
Re: Brake pedal - hard at startup [Re: mightymousetech] #4531994
10/02/17 10:58 AM
10/02/17 10:58 AM
Joined: May 2016
Posts: 718
Canada
i_hate_autofraud Offline
i_hate_autofraud  Offline
Joined: May 2016
Posts: 718
Canada
Originally Posted By: mightymousetech
If it is really hard before you start it, you may have a leaking check valve in the brake booster.



Yup! I've seen leaking vacuum check valves on the booster before!

Re: Brake pedal - hard at startup [Re: mightymousetech] #4531998
10/02/17 11:01 AM
10/02/17 11:01 AM
Joined: Apr 2017
Posts: 163
PA
Chester11 Offline OP
Chester11  Offline OP
Joined: Apr 2017
Posts: 163
PA
Thanks...not seeing a check valve in any parts diagrams, maybe it's built into the booster?

I'm also looking around for leaks. Haven't found any yet....

Re: Brake pedal - hard at startup [Re: Chester11] #4532003
10/02/17 11:04 AM
10/02/17 11:04 AM
Joined: Feb 2007
Posts: 3,699
Southeast
meep Online content
meep  Online Content
Joined: Feb 2007
Posts: 3,699
Southeast
either the check valve, or the booster is leaking down itself.

-m


2018 F150
2015 crv (wifey!)
Re: Brake pedal - hard at startup [Re: Chester11] #4532006
10/02/17 11:07 AM
10/02/17 11:07 AM
Joined: Jun 2005
Posts: 11,612
Santa Barbara, CA
bdcardinal Offline
bdcardinal  Offline
Joined: Jun 2005
Posts: 11,612
Santa Barbara, CA
Originally Posted By: Chester11
Thanks...not seeing a check valve in any parts diagrams, maybe it's built into the booster?

I'm also looking around for leaks. Haven't found any yet....


The check valves are either part of the port where the vacuum line connects or part of the vacuum line.


2014 Ford Mustang GT Track Pack
1995 Ford Mustang GT

Ford/Mazda Parts Counter
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Opinions expressed are my own.
Re: Brake pedal - hard at startup [Re: i_hate_autofraud] #4532008
10/02/17 11:09 AM
10/02/17 11:09 AM
Joined: Dec 2002
Posts: 42,293
New Jersey
JHZR2 Offline
JHZR2  Offline
Joined: Dec 2002
Posts: 42,293
New Jersey
Originally Posted By: i_hate_autofraud
Originally Posted By: mightymousetech
If it is really hard before you start it, you may have a leaking check valve in the brake booster.



Yup! I've seen leaking vacuum check valves on the booster before!


That was going to be my guess. Ive had cars that do this after a short while, others that are soft after forever... Too long to assume that vac is maintained.

But nothing that OP did would point me to damaging the check valve or booster...

Which to me would point to air in the system that was removed.

Re: Brake pedal - hard at startup [Re: Chester11] #4532021
10/02/17 11:35 AM
10/02/17 11:35 AM
Joined: Apr 2017
Posts: 1,481
Ontario, Canada
mightymousetech Offline
mightymousetech  Offline
Joined: Apr 2017
Posts: 1,481
Ontario, Canada
Originally Posted By: Chester11
Thanks...not seeing a check valve in any parts diagrams, maybe it's built into the booster?

I'm also looking around for leaks. Haven't found any yet....


Usually part of the large vacuum line going to the engine.


Mighty Mouse Tech
BMW Tech, Former Acura Tech
2010 Civic Si Castrol 0W40, Redline MTL
2013 BMW 135i M Sport Edition Castrol 0W40, Redline MTL
Re: Brake pedal - hard at startup [Re: mightymousetech] #4532079
10/02/17 12:56 PM
10/02/17 12:56 PM
Joined: Feb 2009
Posts: 8,806
Texas
440Magnum Offline
440Magnum  Offline
Joined: Feb 2009
Posts: 8,806
Texas
Originally Posted By: mightymousetech
Originally Posted By: Chester11
Thanks...not seeing a check valve in any parts diagrams, maybe it's built into the booster?

I'm also looking around for leaks. Haven't found any yet....


Usually part of the large vacuum line going to the engine.


Yep, its usually in the fitting itself. It doesn't matter if the check valve leaks down SLOWLY, so long as it will at least hold for a few minutes. Leaking down overnight isn't a concern, IMO. But you DO want to be sure that it holds vacuum during any periods of wide-open throttle so that you have boosted brakes if you have to stand on the brakes right after a WOT burst. You don't want a delay while vacuum builds, or worse, no power brakes if the throttle sticks open!


'66 Dodge Polara & '69 Dodge Coronet R/T both 440/727
'08 Ram 1500 4.7/545RFE
'12 Challenger SRT8 392/6-speed
'99 XJ 4x4 4.0/AX15, '14 WK2 4x4 3.6/8HP
Re: Brake pedal - hard at startup [Re: Chester11] #4532113
10/02/17 01:59 PM
10/02/17 01:59 PM
Joined: Apr 2017
Posts: 163
PA
Chester11 Offline OP
Chester11  Offline OP
Joined: Apr 2017
Posts: 163
PA
Thanks guys - I think I actually found the check valve. It appears to be in-line within the vacuum hose, a little bit away from the brake booster. It's about 3 inches long, looks like a regular hose, but is reinforced, and has rusted clamps on either side. This actually works to my benefit because it is right on top winking at me.

I have been chasing what I think is a vacuum leak anyway, so if I can find the part, I may just replace it along with the clamps.

Finding the part will be the problem since I can't even find a hose diagram. (The ford part guys get all confused when I show up looking for Mazda parts).

Re: Brake pedal - hard at startup [Re: Chester11] #4532372
10/02/17 07:08 PM
10/02/17 07:08 PM
Joined: May 2016
Posts: 718
Canada
i_hate_autofraud Offline
i_hate_autofraud  Offline
Joined: May 2016
Posts: 718
Canada


Yeah, the bellows in the booster can have a small tear or fatigue crack allowing air to leak.


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