Temperature sensitive brake pedal

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71
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Ohio
Car is a 2006 Elantra. If the ambient temperature is below 70 degrees the brake pedal goes down 2" before encountering resistence. If the ambient is above 85 the brake pedal only goes down the appropriate small amount. This is for initial startup. After the car is driven about eight miles the brake pedal is normal regardless of ambient temperature. I'm thinking master cylinder where the seals work better when warmer but I've never heard of this happening. Anyone had any experience like this? Could it be anything other than the master cylinder?
 

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I should mention that the car does not have ABS and the brake fluid has not gone down even a fraction of a millimeter over the last month, so there are no external leaks.
 
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Slimy Mudhole
If you apply the brakes continuously and the pedal slowly goes to the floor then the seals of the master cylinder are on his way out. Other than that I have no idea
 
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Are you sure it's not the booster? Pump the "soft" pedal with the key off until it gets hard and see if that feels like your hard pedal.
 

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Originally Posted By: Darwin1138
If you apply the brakes continuously and the pedal slowly goes to the floor then the seals of the master cylinder are on his way out. Other than that I have no idea
It goes half way or maybe a little more than that to the floor. I think, but I'm not absolutely sure, that both the primary and secondary seals would have to be bad for it to go all the way to the floor. If only one set of seals was leaking then it should go about half way, I think.
 

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Originally Posted By: eljefino
Are you sure it's not the booster? Pump the "soft" pedal with the key off until it gets hard and see if that feels like your hard pedal.
That's an interesting possibility. I have full assist (stopping the car requires very little force on the pedal) both when the pedal is low as well as when it's high. I'm pretty sure that would exclude the booster, wouldn't it.
 

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Originally Posted By: Sawdusted
Could be a vacuum leak with the brake booster?
I have full assist with both pedal high and pedal low. I think that would eliminate the booster, would it not?
 

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Originally Posted By: Silverado12
Check rear brake adjustment first. Then bleed brakes.
I checked the drum adjustment and set both wheels to drag slightly. The pedal was normal for a day or two after that, but then went low again. Now i'm wondering if one of the self adjusters might be backing off, but that would not explain how the pedal can go from low to normal or from normal to low while just sitting overnight. I ran a full quart of fluid through the bleeders and got no air.
 
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I like more the theory about the self adjusters, check them out for rust and also the parking brake cable, in case they are OK then maybe as you said, only one seal of the master cilynder is bad, you will need to hook up a manometer to each wheel to see if they are developing the same hydraulic pressure.
 
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