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General MG Brake fluid

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Bob Branson

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We all know what DOT3 brake fluid does to the live rubber seals originally used in BMC cars. While I'm still a good while from needing brakes, I'm wondering if the new rebuild kits and after-market wheel and master cylinders still use live rubber (not likely, methinks.) And if they use the synthetic stuff like American cars, is brake fluid still even an issue? I know Gastrol Girling Crimson is extinct, and I seem to remember reading that the silicone fluid is an acceptable replacement. I just don't want to ruin the new parts I have yet to buy. TIA
 
dklawson

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You will find tons and tons of threads on this board and others discussing the pros and cons of silicone (DOT-5) brake fluid. I won't be the one to open that discussion in this thread.

In response to your direct/initial inquiry, it is my understanding that rubber used on British cars or rebuild seals/components since the mid- to late-1960s is compatible with DOT-3 and DOT-4. However, for 100% compatibility you should replace all the rubber bits (including flex hoses, all cylinder seals, servo internals, and PDWA seals).
 
JPSmit

JPSmit

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As mentioned, there are many conversations and few conclusions on this topic. FWIW I use DOT-5 and have been pleased. The rubber is fine. The issue I had is that DOT-5 is very viscous and was "sliding" past the seals. If you go the DOT route - go to the store and shake all the cans and buy the thickest you can get.
 
Grantura_MKI

Grantura_MKI

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JPSmit said:
As mentioned, there are many conversations and few conclusions on this topic. FWIW I use DOT-5 and have been pleased. The rubber is fine. The issue I had is that DOT-5 is very viscous and was "sliding" past the seals. If you go the DOT route - go to the store and shake all the cans and buy the thickest you can get.
Click to expand...
NOT a good practice......ok, let's fill the fluid with air bubbles! Should make for easy bleeding of the system????
 
dklawson

dklawson

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Grantura_MKI said:
Should make for easy bleeding of the system????
Click to expand...

You are correct. You don't want to shake the bottle right before bleeding the brakes. However, letting the bottle sit undisturbed over night is sufficient to get the bubbles back out.

I personally have had no problem with seal leakage with any of the DOT-5 I have used. Until recently I bought Cartel brand fluid from Moss. The last time I bought I opted for a quart sold by a guy on eBay who was selling discount military grade DOT-5. It was almost identical to Cartel in appearance and thickness.
 
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bobhustead

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Wouldn't Boyles law or some similar principle drive the bubbles out of the fluid rather quickly on applying pedal pressure?Bob
 
dklawson

dklawson

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No. Boyle's Law won't drive the gas bubbles out. However, it does describe what happens with the bubbles.

If there are bubbles in the fluid and you apply pressure, the bubbles collapse giving the pedal the "springy" feel (high pressure = low volume). When pedal pressure is removed, the bubbles return to their normal size (low pressure = higher volume) The process repeats next time you apply brake pressure.

That's why it is very important with DOT-5 fluid to avoid bubbles as much as possible.
 
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