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Master cylinder on 83 Avanti

steve remick

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I recently checked my brake fluid in my 83 Avanti and it was slugs and liquid.

I cleaned the master cylinder and I noticed on the power brake booster it said to use Dot 5 (silicone) the manual says to use Dot 5.

The master cylinder cap said to use Dot 3

Which is correct?

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My guess is the brake booster is likely the reliable indicator, but is the master cylinder cap original to the M/C or did someone add that Dot 3 notice?

If the M/C has been changed at some point, the cover may have come with that notice.

Dot 5 should not harm a system designed for Dot 3. Either will work, but they cannot be mixed; the question is: what is currently in the system?

Since brake fluids seem to change/lose color after awhile, the purple color of Dot 5 is probably no longer an indicator.

Perhaps by taking a bit of fluid from the master and dribbling it onto a painted surface... Dot 3 will likely damage the paint if left on the surface a day or two. Dot 5 will not harm paint.

From the internet: "Another way to tell between DOT3/4 and DOT5 is to put some of the fluid in a glass jar and add a few drops of water.

Water will mix completely with the DOT3/4 but won’t mix and will remain as a separate blob in the DOT5."

As for preference, I use Dot 5 in my Avanti's. If switching from Dot 3 to Dot 5, you should remove all remnants of Dot 3 with a thorough Dot 5 flush of the entire system. I made the switch on my '71 when I did a brake system rebuild and was replacing all 4 cylinders.

BTW, what are "slugs"?

Edited by WayneC
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Avanti Motors started using silicone brake fluid in production about 1970, though it wasn't called DOT 5 until later. They may have put a sticker on the booster so anyone servicing the car knew what to use.

The master cylinder is likely a replacement as the DOT 3 designation may not have been in use at the time either.

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Do you know the shelf life on Dot 5? I have a old bottle from my other Stude's

I don't, I don't think there is one. Read the label and see if that says anything.

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I realize that there is much discussion regarding the relative benefits of DOT 3 vs. DOT 5. I recently upgraded my brakes to Turner. When I disassembled the old original calipers, I was amazed at the condition. After 36 years, they were virtually like new. The brake system had DOT 5 from the factory.

I had a similar experience with my '63 Avanti, after changing it over to DOT 5 many years ago. The only issue was the old pressure-operated stoplight switch. Changing to a mecanical switch on the pedal cured that, and had the additional benefit of faster operation of the brake lights.

As WayneC noted, the easy test to determine fluid type is to add a few drops of water to a sample.

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You might consider replacing your brake hoses if what is on the car are original. They can get soft and collapse internally and will still look good from the outside. Should that happen, you'll probably experience the car pulling when braking.

Even if you're not experiencing a problem now, a 30+ year old car should have the hoses replaced just because they're 30+ years old. Brakes are far too important to not keep in top condition.

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