Frozen(?) brakes

[ross1]

A curious situation has manifested itself on my '00 528i
In freezing conditions ONLY the brake pedal is as hard as a brick and there is little to no braking action, all this while applying LOTS of force on the pedal.
This car is essentially new to me, it sat idle out doors for a couple of years before I rescued it and another year since. I've heard of the compartment where the booster lives flooding, it is dry with no evidence of prior flooding. The drain is clear.
I want to sell it but obviously it will need this repaired.
My thoughts are that somehow the booster does have water in it and is freezing up, but how? I'm going to try and examine it better, perhaps try dipping something into the booster to see if any water inside as it's indoors thawing out now. I have a spare booster so not such a big deal if it needs to be changed but curious how this could happen.
Am I way off? Has anyone else here experienced this?
Thoughts, cures?
OH, FWIW I did manage a couple thousand trouble free miles on it before the cold weather set in. NO sign of any brake troubles at all.

[Twistytee]

When was they last time you changed the fluid? Brake fluid is hygroscopic. It typically won’t collect enough water to freeze, but when the fluid gets older and absorbs water the viscosity increases and can cause the cold weather symptoms you describe. This also makes moving the fluid through abs lines more difficult.

[WBAD530i]

Pull the drivers side cabin air filter housing and see if you have water collecting around the brake booster. Clean this area out and make sure the drain hole isnt clogged. The chances are that you already have water inside your brake booster. This means when the water inside it is frozen and effecting your brake pedal.

[BimmerBreaker]

I would start with a simple brake bleed on all 4 corners

[JimLev]

A curious situation has manifested itself on my '00 528i
In freezing conditions ONLY the brake pedal is as hard as a brick and there is little to no braking action, all this while applying LOTS of force on the pedal.

Water in the booster froze.

[ross1]

The compartment is dry, first thing I checked. I was also wondering about the fluid but couldn't fathom that much moisture being present.
I'll bet you guys are right, will bleed(and get a good look in booster), will advise results.

[ross1]

Near certain there is no water in the booster. ran a cotton rope in and also a small tube attempting to suck out any water, both came out dry after several attempts.
Power bled all four corners, fluid wasn't dark but had some tint, weak tea I'll call it so some moisture present but far less contaminated than I've seen before with no consequence.
A warm week ahead so I'll not know for a while, thanks for the advice.
Anyone know how I can run the ABS pump on this Bosch 5.7 system without having proper tools?

[JimLev]

Anyone know how I can run the ABS pump on this Bosch 5.7 system without having proper tools?

Drive down the road and jam the brakes on. Better if you can do it with whe wheels on one side of the car in the dirt on the edge of the road.

[geargrinder]

Pull the drivers side cabin air filter housing and see if you have water collecting around the brake booster. Clean this area out and make sure the drain hole isnt clogged. The chances are that you already have water inside your brake booster. This means when the water inside it is frozen and effecting your brake pedal.

^^^^ I'm with the bad guy. Except - bad - he already said the bin area was dry... But that doesn't mean that its not already in there, or something got down the vac line into it...

[itsbrokeagain]

3rd person to vote for water in booster. Happened to me. Booster was rusted inside and a gallon of water came out.

[ross1]

Follow up
Booster was dry.
My poor description of the symptoms was misleading. Seems it was simply a lack of boost, that combined with rotors surface rusted from sitting outdoors.
Checked the check valve at the booster and while it would only flow one direction it was restricted compared to the spare I had on hand. Swapped them and all is well.
I don't understand the correlation to ambient temps, perhaps moisture in the check valve? Don't see it could persist in there and not be drawn out. Didn't think to do a post mortem on it before tossing it.

[edjack]

Thanks for the feedback, but don't understand your statement.

I don't understand the correlation to ambient temps, perhaps moisture in the check valve? Don't see it could persist in there and not be drawn out. Didn't think to do a post mortem on it before tossing it..

[ross1]

Thanks for the feedback, but don't understand your statement.

I don't understand the correlation to ambient temps, perhaps moisture in the check valve? Don't see it could persist in there and not be drawn out. Didn't think to do a post mortem on it before tossing it..

I had a spare check valve an hand which seemed to pass significantly more air when blowing through it. I exchanged it for the one on the car and voila.
Why temps would effect this part is beyond me unless it was full of water which it didn't seem to be. I didn't spend any time examining the old one which I regret as it remains a mystery.