brake fluid

charles4d

Dot 3 synthetic brake fluid  okay to use in the old style master cylinder ?

 Master cylinder and wheel cylinder have all been rebuild by white post and

 never used yet.

Jon B

I believe that Dot 3, Dot 4 and Dot 5.1 are similar glycol-ether based fluids.  If master and all wheel cylinders are rebuilt, and the lines have been blown out with compressed air, I think you could use any of them.  (Just don't mix them afterwards,)

However, at this stage you might want to consider using Dot 5, which is silicone based.  This supposedly does not absorb moisture from the air, thus keeping it out of the brake system, where it could congregate in the cylinders and cause corrosion.  I have had silicone in my '37's brake system since the 1990's with no problems and no necessity to rebuild my cylinders.  I do, however, have a mechanical brake light switch.  Later Hudsons have a hydraulic type switch, and I've heard that the silicone can leak into these switches, and cause problems.

However, if your master and wheel cylinders were sleeved by White Post, you're in the clear, because these can't corrode as they could before being sleeved.  So you can use Dot 3, 4, or 5.1 with no ill effects.

RL Chilton

If you use silicone brake fluid, your stop light switch will fail and it won't take long.  Paul Schuster sells a replacement switch from a HD that is silicone fiendly.  Dot 3 will work fine, but as Jon said, does let moisture collect in the system, if it's not completely full.  Also, might develop problems on a car that is not used regularly.  

hudsonguy

I used to use DOT 5, but have switched to a good synthetic DOT 4. Valvoline, I believe. I still change the fluid in the master cylinder every year or two, to keep it as fresh as I can. A few minutes with the Mighty Vac pump is all it takes to do this.

I guess the reason I stopped using DOT 5 is that I live in a fairly humid area (Wisconsin), so I figure it is inevitable that some moisture is going to get into the system eventually. When it's in the 90's (temp AND humidity) everything that isn't 'air conditioned' is soggy!

Since Silicone (DOT 5) will NOT absorb this moisture, by it's very nature, then the water that does end up in the system will 'pool' together and could actually do more damage (rust) to that particular spot where it ends up. I'm imagining the DOT 4 actually absorbing this moisture, like we know DOT 3 does, and then can be easily removed when I swap out the master cylinder fluid.

At least that's MY logic! However flawed that may be....

charles4d

 Thank to all with your inputs