Brake Fluid contamination from Penetrating Oil on bleeder screw?

Joined
Mar 30, 2020
Messages
66
Location
SF Bay Area, CA
Advance preparation for brake bleeding often includes the application of penetrating oil of some type (e.g., Kroil, PB) around the bleeder screw to assist in removal. In some cases, it's applied daily for several days in advance of the bleed operation to allow it to soak in and do its thing.

If penetrating oil really penetrates the screw threads as intended, won't it make its way into the caliper and contaminate the brake fluid? I read somewhere to avoid penetrating oil around brake fittings for this very reason. But I also read many forum posts and watched some videos describing this technique to deal with difficult bleeder screws. Is there any penetrating fluid that will not contaminate brake fluid?
 
The bleeder screws in tight to the base of the drilled and tapped hole of the caliper assembly, the tapered seat of the bleeder screw mates with the same taper on the drilled hole. It's metal on metal contact with design for no leakage. Penetrating oils are not endowed with super powers, they go path of least resistance and are chemically attracted to metal, but there is a limit to how far they can go.

The risk of contamination of the brake system from use of penetrating fluids is only possible if the bleeder was either not tight, or had significant damage to the taper in either the caliper or bleed screw. Basically, it would show signs of BF leakage before you even applied penetrating fluid to the exterior of this junction.
 
Last edited:
Agreed with KGMtech.
The penetrating oil will work through gaps in the mating male and female threads but will stop at the metal-to-metal interface between the caliper and the bleeder screw tip.
 
Even IF a minute amount of Penetrant would creep through it would be so minuscule that it wouldn't hurt a thing. There is much more to worry about than this, I can assure you!
 
welcome2.gif


I agree with the others.
And any flushing of the old fluid would remove any penetrating oil.

When you get done, smear some Silicone Grease around the threads of bleeder screws
and install the Rubber Caps to keep them clean.
 
I wouldn't worry about it one single little bit. Not a smidge.

This is not going to do anything, even if it could penetrate all the way past the bleeder screw, which it won't. Think about it - you're spraying penetration oil on it because it's rusted - it won't even get through the rust, never mind through the threads and into the brake fluid.
 
Thanks for the welcome and responses! My concern is that I will follow the application of penetrating oil with backing out the screw to perform the bleed. As stated above, I guess it will be okay as long as I maintain one-way suction/pressure on the brake fluid so there is no backflow into the caliper. I'll stop worrying.
smile.gif
 
Stop looking for new problems! ALWAYS try to spray your bleeders days in advance. Its simple, easy and helps a little.
 
Originally Posted by buck91
Stop looking for new problems! ALWAYS try to spray your bleeders days in advance. Its simple, easy and helps a little.


Yup, that's the answer right there.....
 
Originally Posted by Driz
Originally Posted by buck91
Stop looking for new problems! ALWAYS try to spray your bleeders days in advance. Its simple, easy and helps a little.


Yup, that's the answer right there.....


Actually, if you anticipate it's going to be an issue, apply a few drops of penetrant. That limits the amount, and avoids getting it everywhere in a big mess...
 
Originally Posted by JHZR2


Actually, if you anticipate it's going to be an issue, apply a few drops of penetrant. That limits the amount, and avoids getting it everywhere in a big mess...


Good suggestion, which also favors applying the penetrant using a few drops from an oil can, instead of an aerosol spray which tends to backsplash.
 
I had this same question about my boat. I used penetrating oil on the fuel tank pickup tube and it was orange/yellow. Coincidently the fuel that drained out had that same orange color. Lol.
 
Last edited by a moderator:
I doubt there will be any capillary action down the bleeder threads anyway. Your going to bleed the brakes anyway and that will flush anything present out to the bottle. Your best prep is to work a wire brush around the bleeder screw, give it a few gentle hammer taps and use a six point socket to loosen.
 
Last edited:
Okay, really you have think about this. Whats behind the bleed screw? Any capillary action will be long defeated by ANY braking pressure. And if the penetrant came contact with system fluid you would have loss of fluid by way of leak.
 
Back
Top