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Tips
Tips

General Tech Tip for DOT5 Users

Geo Hahn

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Disclaimer: I have yet to try this but certainly will the next time I need to do brake work.

Background - One issue with DOT5 is that it holds air in suspension much longer that traditional brake fluid. The bubbles may be t0o small to see but if they are there they can compress and enough of them will affect the pedal feel and brake performance. I suspect this trait accounts for some of the unsatisfactory results often reported by those who try DOT5.

One thing I usually do with DOT5 is let is sit quietly on the shelf for weeks or months to give any microbubbles a chance to rise out. But that isn't always practical and anyway I wonder how long is long enough.

On another forum a British car owner describes using a glass jar with a metal lid that had a single nipple fitted to the top:

...just the one nipple to attach to the vacuum pump. I 3/4 filled the jar with fresh DOT5 from the FLAPS, screwed on that... lid and put the contents of the jar under 15-18 inches of mercury vacuum. Sure enough, microbubbles came out of the brand new brake fluid, probably just from transporting it home. I did this to the entire litre of new fluid before using it. The brake pedal is now the same as I remember it was using DOT3 and the entire job took no more than ten minutes.

The pump he shows looks like this:
handpump.jpg


https://www.avac.com/mityvac.php

Makes sense to me. Still requires a careful transfer of fluid from the jar to the reservoir but hopefully reduces the unseen microbubbles that shipping and careless handling introduce into the fluid.
 
Last edited:

TuffTR250

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Thanks for the tip Geo! Another tip I saw some place is to very slowly pour the silicone fluid down a screw driver blade that is down in the fluid, so when the fluid gets to the bottom it doesn't splash and form more bubbles.
Bob
 

Popeye

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Makes sense to me.

I was a "lab rat" in an astrophysics lab in college. We would "boil" air bubbles out of freshly mixed epoxy by putting it under a vacuum dome. I do not remember what level of vacuum we pulled.

I wonder where an average home person can get a decent vacuum pump? A vacuum cleaner?? Maybe harder than finding a source of vacuum is to make an airtight vacuum chamber...
 

TomMull

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I used DOT5 after some brake work recently. I wanted to avoid damage to the firewall from the other stuff and I always seem to spill it. I did experience some sponginess after bleeding but that worked out on it's own. The other issue is that it is not as slippery as the other fluids and requires polished surfaces on the cylinders.
Tom
 

dklawson

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I wonder where an average home person can get a decent vacuum pump?

The picture shown in Geo's first post is the Harbor Freight vacuum pump. It can be good but like so many other Harbor Freight tools you may need to tweak or rebuild it to make it work the way it should. In the case of the pump above, one of the valve functions is carried out by the large o-ring in the brass cylinder. Plan on replacing that with the (roughly) same size U.S. sourced o-ring. The o-ring you use will be a tiny bit bigger in outside diameter and in most cases this will allow the pump to work MUCH better than when you bought it.

I had an Equus brand vacuum pump from Summit Racing. It was all plastic. Each time I used it, it broke and I had to repair/rebuild something else. After the third failure I threw it out and bought the Harbor Freight pump which is almost all metal. Once I replaced the o-ring I found the Harbor Freight pump to be decent and I am happy with it.

Mityvac pumps have a good reputation and are priced to match.
 

Rut

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I use my wife's food saver vacuum for relaminating dashes, gluing strange shaped parts together and bet it would work for dot 5!
Rut
 
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