Brake pedal - hard at startup

[Chester11]

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.

 

[Alex_V]

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.

 

[mightymousetech]

If it is really hard before you start it, you may have a leaking check valve in the brake booster.

 

[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!

 

[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....

 

[meep]

either the check valve, or the booster is leaking down itself. -m

 

[bdcardinal]

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.

 

[JHZR2]

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.

 

[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.

 

[440Magnum]

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!

 

[Chester11]

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).

 

[i_hate_autofraud]

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