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The Hummer Knowledge Base

Description of Brake Fluids - Dot 3, 4 and 5:

Description of brake fluids.

  1. DOT-5 is 'oil' based. Won't mix with DOT-3/4, or water. From the bottle is it usually blue/purple in color. After use, it often turns yellow.

  2. DOT-3/4 is 'water' (a glycol, very similar to anti-freeze) based. It will mix with water, but not oil. Usually yellow in color from the bottle, but may be clear. Usually clear after use.
Tests and notes:
  1. Try to mix a known DOT-3 sample with whatever you have. Use a small glass bottle or jar. Do the samples mix, and stay mixed? Of do they separate like salad dressing? If they don't mix, then the sample under test is probably DOT-5.

  2. Try to mix a small sample of the fluid in question with water. Does it mix easily, or not? If it doesn't mix easily, it is probably DOT-5.

  3. Try to mix a small sample of the fluid in question with a light oil. Does it mix easily? If yes, it is probably DOT-5.

  4. You cannot tell the difference between DOT-3 and DOT-4 after the fact.

  5. Despite claims, mixing DOT-3/4 and DOT-5 is not the end of the world. They were designed so that accidental mixing would not result in failure of either braking performance, or damage to brake system components. This should not be construed to say it is OK to mix them, just that if it happens, you don't have to replace everything. A clean out and flush is in order however if they get mixed.

  6. DOT-5 does not normally need to be replaced on a regular schedule. DOT-3/4 should be flushed every two to three years depending on climate and usage. Most locations dictate that DOT-3/4 should be flushed every two years!

  7. Hummers with non-ABS brake systems all came with DOT-5 fluid. It is (was) safe to assume that you had DOT-5 if you had non-ABS brakes. However, as time progresses (progresses? more like regresses!) mistakes by mechanics who don't know better have resulted in some non-ABS Hummers being filled, either fully or partially, with DOT-3/4.

  8. Hummers with ABS brakes MUST USE DOT-3/4 fluid. You should never, ever use DOT-5 in an ABS braking system because DOT-5 will aerate (form bubbles) which can cause total braking system failure. This happens when the ABS system is activated. If you suspect or know that your ABS equipped Hummer has had DOT-5 fluid added, it should be fully flushed ASAP.

  9. DOT-3/4 fluid is hygroscopic (attracts moisture) and the brake system should be sealed from ambient air at all times. Once opened, a container of DOT-3/4 fluid should be either completely used, or discarded.

  10. DOT-5 fluid hates water, and won't attract moisture. Once a container of DOT-5 has been opened, it can be resealed and safely stored without any ill-effects.

  11. DOT-5 fluid can be bought in gallon jugs for about $30 from several sources. PM me and I'll give you the contact or name of a supplier.


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