Brake Problem in Cold Temps

1997 B4 sedan, 375,000 miles, old new car to me with the usual rust. Brakes will act like I have a line leak when the temp dips to the mid 20's F. Brake pedal goes to the floor and minimal braking action, no problem with clutch pedal. When ambient temp get's back in the 30's F brakes function returns to normal. There is no appreciable loss of brake fluid.

First time this happened initially thought a brake line ruptured due to water in the braking system. Inspection revealed a very small drip on the rt rear brake caliber but brake fluid level was OK. Went ahead and thoroughly bleed all 4 corners and the clutch-pretty nasty looking old brake fluid. Everything seemed fine till the temps dropped again and then the same thing happened.

Would a fine hairline crack in the brake caliper expand in cold weather and cause this? Have not seen any more brake fluid on that rt rear caliper or any loss of brake fluid in the reservoir since bleeding the brakes. Any ideas on what the problem could be?

i go to grocery store, get regular sized baster, get a container to hold (old)brake fluid, get all the fluid out of M/C reservoir, dont get below (very bottom)level, into M/C, but get all that old fluid out. fill with fresh fluid, i use the synthetic stuff(semi-syn i believe). bleed brakes, and fill reservoir, finish by bleeding clutch, bleed each corner a good few times, i buy a quart of fluid for this.

as far as the drip / leak yo u have either trace and replace all or replace caliper(rear disks?) and you may have to replace hard line, or -either/or. check that all 4 of your disks have both good pad, and not rust on disks, also check disk surface that they wear evenly, are basically flat on wear surface, braking surface that is. other things you can check as well. when/if you replace hard rear brake line, prob best to replace rear flex line as well, i prefer the SS high-quality flex lines. have a flange wrench-somewhere- for this. (or two).

is it possible the caliper leak is bad/worn rubber, theres a way and kits to swap out rubber caliper parts, it doesnt sound like, thought id mention it.

I think your brake master is going bad. Typically on a 2 year fluid replacement the master is fine but after prolonged contaminated brake fluid with a sudden flush to new fluid the MC will die. I've seen it multiple times on systems that have been let go for too long, done it myself on a couple.

I would guess that you'll be fine for a while but then you will start to lose braking capability.

Steve

here is the issue you have, contaminates and what not still in your lines. and a busted master

With that crap in your lines, when you flushed, you pushed the brake to the floor didnt you. this caused damage to the orings in your master. I suggest taking it apart, replacing all the orings and going over things and if you find any pitting, replace it from a donor car from the scrap yard until you find good parts to replace. You sill have crap in your lines. i suggest buying 4 quarts of cheep AF dot 3 and going a major flush 3 times with heat cycles. I had a issue like this, but not in cold temps, when i changed the fluid on the van, it was black and tick as motor oil. it took a quart each line to flush that crap out over 2 years.

Now to your cold issue. if you have a crack in the rear brake cylinder, replace it, why go on with a damaged brake part.

Brake fluid is only good for about 2 years regardless of miles. its about the water content that leaks in from the rubber hoses, heat cycles, and what not.

You had crap fluid for probably 20 years now right? and your wondering why the brakes suck and have issues?

i have never seen brake fluid look bad unless it was older than 8-10 years.

good point steve, and ^ as well, gunk will be in your hard lines, esp. dont want (you) to take out and suddenly lose M/C. you can flush/bleed new fluid thru but it will change color, probably pretty quick. should have stayed up on it. i use baster every -less than 2yrs, on each car. suck as much fluid out of m/c reservoir, bleed a good few times, each corner, helps with anti-lock as well, no air gets in system, just like swapping out brake fluid, cause thats what your doing!

It's the master and the o-rings are just worn out. They don't seal well when cold because they don't deform and take up the gap due to being worn. When it's warm they do deform easier because the o-rings are able to take up the gap. But I'm betting it's just due to age. I've seen some pretty nasty old master cylinders that still worked fine before and after cleaning. For the price of them I'd recommend just changing it and not trying to rebuild it.

Abacus said:

For the price of them I'd recommend just changing it and not trying to rebuild it.

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$12 set of o rings vs $120 master! $50 if you want to try Russian roulette with a reman

And if the o-rings are that worn, how do you know the bore isn't?

Why risk it.

There are a lot of aftermarket MC's out there for less than $50 that do a fine job, that's one part that's in plentiful supply because it's the same one used on the all Mk3 4 cyl cars.

Steve

Mongler98 said:

$12 set of o rings vs $120 master! $50 if you want to try Russian roulette with a reman

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where are you paying 120 for a master, VW? you can get a good master cyl for about $40-$60 not reman and never had a problem with one. PP or autohausaz, or like supplier. never had a problem with getting aftermarket, just dont get a cheapie.
then again i find more of autohaus' stuff is that febi sht, kind of of the opinion thats acceptable chinese stuff, not my fav. taking a look at their (current) M-C, i cannt say its *that good. with that you may take a chance.

so far temps have not dropped to mid 20's this week and brake fluid is still full and brakes functioning normally. I am becoming convinced it is the MC. After bleeding the brakes I am quite sure we have all new fluid in the system. That R&R of the MC will have to wait till warmer weather but keeping a close eye on it. While at it will attend to that rt rear brake and find the little leak. Thanks guys for your input.