If brake fluid is such a good penetrant, why do bleed nipples and brake line fittings seize up so easily? (Actually, brake fluid is quite safe on rubber parts, but it destroys paint.)

Over many decades I've used various generic penetrating oils with limited success. I've also used WD-40 because it was handy, but wasn't much better. Then I tried PB Blaster and found it worked better than anything I'd tried before.

Now I'm using a 50/50 mixture of ATF and acetone and it seems to work even better than PB Blaster. I just mix it up in an old spray bottle. It's also cheaper than any proprietary penetrating oil.

Here is a quote from another forum:

"PB Blaster=ok. A 50/50 mix of acetone & ATF=Best Ever! Here's the cold, hard facts from the April/May 2007 edition of Machinist's Workshop. They did a test of penetrating oils where they measured the force required to loosen rusty test devices. Buy the issue if you want to see how they did the test. The results reported were interesting. The lower the number of pounds the better.

Penetrating oil . Average load .. Price per fluid ounce
None ................. 516 pounds .
WD-40 .............. 238 pounds .. $0.25
PB Blaster ......... 214 pounds .. $0.35
Liquid Wrench ... 127 pounds .. $0.21
Kano Kroil ........ 106 pounds .. $0.75
ATF-Acetone mix.. 53 pounds .. $0.10 "