Cold morning brakes

[LegacyPunk]

Were talking single digits maybe even negeative temps, on really cold mornings it takes a second for I get any power to the brakes. This morning I drove for a few minutes untill I got any brake power. Sometimes I have to pump them a little. Its like trying to brake when the car is not on. Is this any cause for concern or just another case of cold morning MFS quirks?

[mikec]

Stock brake pads? Upgraded pads will often need to be warmed up before they bite properly.

Thats the biggest reason everyone is warned to not run track pads on the street - you can't get them warm enough to work properly.

[vrg3]

Being unable to brake is not MFS; it's dangerous!

When was the last time you changed your brake fluid? Maybe you have some water in there that's freezing up.

Is it really just like trying to brake without running the engine? Like, you can brake but you just don't get power assist? Cuz maybe there's moisture in the brake booster vacuum line or inside it somewhere.

[LegacyPunk]

yeah it was like rolling without the engine on and trying to brake. So I guess this is a bad thing. I havn't checked the brake fluid in a while so that will be the first on my list tommorow. Is there such a thing as a brake system flush and refill?

[vrg3]

Well, you don't flush the system out with anything other than brake fluid. Basically you just bleed the system until all the old fluid has been bled out. There are a few threads in this forum about it, I think.

But it's important to differentiate between the hydraulic system and the booster. If the problem is in the booster, no amount of fluid service will fix it (though you should be changing brake fluid at least every 2 years anyway).

Maybe one morning under somewhat controlled circumstances you can try to operate the brakes to get a feel for how they feel when malfunctioning, and then when they start working right, unplug and cap off the brake booster vacuum hose and compare the feeling.

[LegacyPunk]

cool, thanks for the advice oh mighty car god.

[wind chaser]

So do you plug the line once it is off the booster, the place the line comes off, or both?

I have been having a similar situation with my brakes on cold mornings. Refreshing all the brake fluid seemed to help initially. It may have been just enough warmer to make a difference though. This week it is colder when I get in the car, and it is back. Mine feels very firm in the pedal without typical braking for coresponding pedal movement. I can "push through" the extra resistance and get braking. After 2-3 times of doing this, they feel normal again.
I will check out the booster.

Which models have the single stage booster, again? I might as well get better brake feel if I have to replace it.

Could it have anything to do with a valve keeping boost out of the vacuum? I have less vacuum on my boost guage than a couple of months ago that I havent been able to account for. I have surging part throttle full boost issues also. Could the brake vacuum (booster)and ptfb conditons be related?

[vrg3]

To disable the brake booster, you disconnect the hose from the booster and plug the hose. It's probably a good idea to plug the nipple on the booster too, since you don't want dirt getting into the booster.

From what I can tell, the single stage booster was used on non-ABS GC/GF Imprezas and BD/BG Legacies with rear drum brakes.

You can test the check valve in the brake booster hose pretty easily. Remove the hose from the car and make sure you can blow from the booster side to the manifold side, and not the other way.

[wind chaser]

vrg3, thanks for the tips.
Since I have normal feeling brakes after a few hard pushes, I assessed the check valve cold vs after the brakes began to feel normal. Cold: no flow either way. After pedal felt near normal: difficult at first from booster to manifold then easy. After it went through initially, it continued to flow easily booster to manifold and not manifold to booster.
Does anyone know at what pressure is that this valve opens? None of the Subaru dealerships in the area have the hose + check valve and are 5 days out to get one in. NAPA has a check valve available, but I don't know if something lik this would work. It would be nice to have good cold morning brakes consistently SOON.

[vrg3]

Huh, so you traced it to a bad check valve. Nice work! The check valve is the little bulge you can feel in the middle of the brake booster hose.

I guess over the years some crud built up inside the valve until it started sticking when cold.

The valve cracks open at a very very low pressure... maybe 10 mmHg?

Pretty much any brake booster check valve you can buy at an auto parts store should work, as long as you can reasonably fit it to the 11/32" hose that our brake boosters use. Try looking in the "HELP!" section with all those red blister packages maybe.

It's probably better to avoid valves not designed for automotive brake booster use.

For the time being, you might want to try just bypassing the check valve entirely. You could replace it with a 3/8" coupler. It seems to me that it should work as long as you're not expecting power assist while left-foot braking or when the engine is not running. The engine may stumble a bit when you brake hard, but it should be okay. Or maybe you could try sticking an appropriately-shaped PCV valve in instead.

I have a spare check valve from a Legacy. If you want, I could mail it to you for the cost of postage. I don't know how you'd install it, though... I guess you could try lubing it up and then pushing it in with a long rod.

Aren't there any junkyards near you? I'm pretty sure most cars have check valves in their brake booster hoses.

LegacyPunk555, you should try testing your check valve when it's cold too...

[wind chaser]

When talking to a local Subaru parts guy about ordering another vacuum line with the check valve, he said: "So you have some moisture built up in the valve and it wont work on cold mornings, huh?" He said it is more common on WRX's than on our cars.
I got to thinking about WD-40 originally being WD=water displacement formula 40 and wondering if it could help "displace" the moisture that is freezing. I took off the vacuum hose and sprayed it in one side and shook it all about, then the other. I reinstalled it in the car after the hokey pokey with the hose and it has worked fine two cold mornings since.
I will probably replace the hose anyway, but it further reinforces in my mind that the valve is the fault for my car.

[vrg3]

Cool. As long as the WD-40 doesn't start to damage the valve's rubber seal or the rubber hose, that seems like a decent fix.

I wonder if it happens more with WRXes because of their open hood scoops. More moisture would collect around the brake booster and start to make its way into the check valve.