Brake pedal loses pressure in cold temps

Last fall when the temps got close to freezing, the brake pedal in my 2005 Forester got very soft and mushy. I could push it almost to the floor. Pumping it a few times while still in the driveway would gain some pressure back.

As the winter progressed and the temps fell further, I was having to pump them longer and more often to keep the pressure up. It was getting scary to drive because several times the pedal went almost to the floor and the car was not braking adequately when I needed them to.

I did take it to the dealer. They said they checking the braking system but couldn't find anything wrong (and they are a good dealer IMO). They said it might be the master cylinder but didn't want to replace anything until they knew exactly what was wrong.

So here it is fall going into winter and the problem is starting again.

Does anyone here have any ideas about what could be the problem?

thanks, Dee

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Yeah, you probably need to arrange for the mechanic to keep it overnight so they can experience the failure themselves on a cool morning.

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If it is not losing fluid it is probably the master cylinder. Soft pedal fixed by pumping is usually a sign of air in the system, but warm/cold weather would not change the symptoms.

Dave

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OK, there's definitely a problem. A mushy pedal means either: 1. the master cylinder is leaking internally. This usually results in mushiness at all temperatures. 2. There's air in the lines somewhere, or there's a leak (which should be obvious to a mechanic). 3. The pads are being pushed off the rotors, which means the master cylinder has to be pumped several times to get the pads back on the rotors. This might happen when its cold, but not in summer.

So the question is why? I suggest the following:

Check for yourself that the ABS is working. Find a quiet piece of road (dirt or gravel is good) and stomp the brake pedal. You should hear and feel a vibration and kickback in the pedal. If not (and this is unlikely in a 2005) , there's an ABS problem.

Take it to the dealer (who did not try very hard last time IMHO) and have them verify that all the caliper slides are free and lubricated. If your Forester has rear drums, have them pull the drums off and inspect the wheel cylinders and e-brake latches.

Check that the rotors are not warped, and the wheel bearings (esspecially the fronts) are not loose. This can cause the rotors to push the pads off.

Check that the brake pedal free play is correct. It should be 1-3 mm of pedal travel before you feel the master cylinder start to move.

If the caliper slides are OK, then flush the brake fluid out completely. This is NOT just sucking the fluid out of the master cylinder and refilling it: it's bleeding all four wheels until fresh fluid appears.

If the pedal is still mushy on cold mornings, replace the master cylinder.

SD

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Thank you! I have printed your suggestions out and will take them back to the dealer.

I forgot to mention that front and rear brake pads had been replaced and the rotors resurfaced about 6000 miles ago. I don't know if that makes any difference or not to what you have recommended.

Dee

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Might have something to do with it. Check for warped rotors (more likely after resurfacing) that is pushing the pads away. It only takes one. It's generally more cost efficient to replace rotors (esspecially the fronts) rather than resurfacing.

SD

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Thank you, SD!

Dee

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So I took the Forester back to the dealer with a checklist of items. They said the rotors were not warped, the calipers were fine, so they recommended bleeding the brake fluid out of the lines from the wheels. They said either moisture or air in the lines could cause the problem, or it could also be that the brake fluid itself was starting to deteriorate.

I got the car back and the brake pedal is definitely firmer and the braking performance is back to normal.

Thanks again, Dee

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Glad to hear that its OK now. The problem was air in the lines IMHO, and likely happened when the rotors and pads were changed. At a guess the brake hoses were removed to ease the removal of the calipers, and necessary double check bleeding after reinstallation was not done, or not done properly.

On a technical note, moisture causes the brake fluid to deteriorate, as brake fluid is an H2O magnet. The effect is that the brake fluid (the water IN the fluid actually) will boil when the brakes get very hot, causing loss of pedal pressure, as the steam is compressible, while the fluid is not. Deteriorated fluid of itself does not cause loss of pedal when the brakes are cold. It has other nasty effects if left too long, such as corroding the master cylinder surface.

SD

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wrote:

Another relatively common occurrence, particularly with drum brakes, was the rubbers got hard in the cold and allowed air to get in as the cyl retracted. Used to be a common problem with old VW and BMC cars in cold conditions.

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