I found a bubble in a brake hose. The fitting connection to the hardline is super rusted and won't budge. Have soaked in penetrating oil for days and still no luck. I dont want to bust a hardline twisting on this thing. Can I use heat or will that just get sucked up by the brake fluid?
FWIW, there is a cold spray that freezes the part to separate it from the other part. I can only say it kind of works because what I was working on had already been soaked in PB. When i get to my shop I'll get the name.
Just recalled the product name
https://www.crcindustries.com/products/freeze-off-174-super-penetrant-11-5-wt-oz-05002.html
IMO, if the hardline is that rusty, you might want to replace it, or at least cut it back, re-flare it and put a new fitting on. But you don't want a hardline to fail on you in a heavy braking situation.
Where is the next threaded fitting and how hard would it be to replace that section of hard line?
If you're going to have to open the system anyway, you could let that circuit gravity-bleed empty and then heat it up without cooking your fluid.
is there such a thing as "heat-sink putty" that would keep the heat from going too far up the hard line?
in my eyes, it's already berkeleyed and needs to be replaced. You can't make it more berkeleyed by trying to salvage it. Pitter patter!
L5wolvesf said:Just recalled the product name
https://www.crcindustries.com/products/freeze-off-174-super-penetrant-11-5-wt-oz-05002.html
Freeze-Off works great! You will be surprised how good this stuff works for that frozen brake line. Just make sure to wear PPE, gloves and safety glasses.
If you heat the fitting without venting the system, the hose may explode from the increased pressure.
I've seen it happen.
If you're not using them already get a set of crowfoot flare nut wrenches. You can then use your breaker bar + cheater.
Heat at the hose end will make the old hose explode out of its terminus, even with the system drained. Who cares? its trash anyhow. you will startle, but its harmless if your eyes are protected. If any of the rust is present on the hard pipe replace it with some fresh ni-cop pipe.
Agree with others.....its already broken, correct repair is to remove the rotted portion entirely. Replace line if able, or cut and add fittings/flare as needed.
The fact that the fitting won’t budge at all is bad. Usually, I can get the fitting to turn, but it twists the hardline. If I catch it early enough, it seems like it can be slowly twisted back and forth, until it is no longer frozen to the hardline. This usually involves a lot of pb blaster.
Add more leverage. Then the threads will break off and you will have to fix it right. Speaking from personal experience. I would buy a new set of lines and just figure on replacing it all.
AngryCorvair (Forum Supporter) said:
is there such a thing as "heat-sink putty" that would keep the heat from going too far up the hard line?
Yes indeed. I've used it and it's pretty amazing.
Torch, small tip. Focus the flame on the threaded area. The hose will blow off at some point, so be prepared.
Heat heat heat. It will go. Never fails me in the salty north.
I've had good luck alternating heating with a small torch then spraying with penetrating spray.
Dab with rag the worst of the spray to limit the flame up.
Take proper safety precautions as always
Heat. Get the nut orange. The flex hose WILL explode out of its crimp.
The hose is already bad, just cut it with some diagonal cutters. Then, applying heat won't increase internal fluid pressure
In reply to stuart in mn :
It is not the internal fluid pressure that makes the hose blow off, it is the combustion of the hose material itself where it is compressed. A cut hose blows off the same as a connected one. Surprised the hell out of me the first time it happened. Now I am prepared, and I still jump when it happens.
If you heat the fitting without venting the system, the hose may explode from the increased pressure.
I've seen it happen.
I see it happen every time i forget that i've seen it happen! 
I've had very good success using the following method:
Use this type of locking pliers on your line fitting, they are <$10/ea online:
Break the female part loose from the male part and spin the female part off while keeping the male fitting stationary on its line. This is usually possible because the female part is going to be replaced and whatever is keeping it from spinning can be broken/cut off or otherwise destroyed until you can spin it. Then, when you have the nut exposed but still seized to the line, heat the nut up until glowing, and then hold an ice cube against the line near the nut. The ice will melt rapidly, but if you've got the nut hot enough and cool the line fast enough you get an audible clink/plink kind of noise of it popping loose. I clean the hard line right behind the nut with sandpiper or even a wire wheel so that there is a clean section that i can work the nut back over, and then clean the line right behind the flare so that the nut will spin reasonably freely upon reassembly. It's important to get the nut to be able to spin by hand so that your hand can tell the difference between crossthreaded and not when you put it back together.
It's kind of a lot of effort for turd polishing. Sometimes making a replacement line or reflaring is pretty dang easy. But if you want to try it, there you go.
I too have a stubborn brake fitting and just ordered one of those vise grips. Good luck with your struggle.
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