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 Posted: Oct 8, 2019 11:49AM
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The alcohol rinse is sometimes done when people are switching from DOT-3/4 to DOT-5.  I've done it on metal lines but nothing else.  The MCs and wheel cylinders I have rebuilt with new seals or replaced completely when changing to DOT-5.

Doug L.
 Posted: Oct 8, 2019 08:47AM
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I've put a myriad of different brake fluids in a myriad of different cars, and I've never had problems resulting from residual fluid contamination...

About the most I've ever done was use a turkey baster to suck out the reservoir fluid, top up the reservoir with new fluid, and bleed the system normally.

Running alcohol through the system? REALLY? That's right up there with painting my cat's toenails, in my book

 Posted: Oct 8, 2019 08:34AM
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Yes, hard to say as just synthetic was mentioned.

 Posted: Oct 8, 2019 08:19AM
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Seal kits are available for most MCs.  I don't know what a '75 850 would require.

See the page linked below showing what our host sells:
https://www.minimania.com/part/BMCSEALKIT-P/Classic-Mini-Brake-Master-Cylinder-Seal-Kits

I did not include any mention of DOT-5 fluid in my last post since you mentioned LMA and DOT-3.  If you are using or planning on using DOT-5 let us know.

Doug L.
 Posted: Oct 8, 2019 06:37AM
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If you mixed dot 5 with dot 3/4 that will cause problems requiring flushing

 Posted: Oct 8, 2019 05:26AM
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  Doug :

    Thanks for the tips. I guess I was hoping that between the two brand new master cylinders I purchased in the last year
one or both would be up to snuff. I got the bolts and some tape and will give it a try. If this is the problem can I buy a seal (or seals) and rebuild it myself or should I send the whole thing back to vendor?

 Thanks again
 Mini dave

 Posted: Oct 6, 2019 02:50PM
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If you were using DOT-3 without issue before, you do not need to flush with alcohol changing from DOT-3 to DOT-4.  There was a time when British cars used DOT-4 exclusively and you dared not mix/add DOT-3 to that system.

To test your new MC, plug its ports using clean 3/8-24 bolts with a bit of Teflon tape on the their threads.  Then pump the pedal.  If the pedal moves easily, something is wrong inside the MC.  If the pedal is "firm", the problem is downstream from the MC.

It sounds like your system was opened and drained.  Is that correct?  What method are you using to bleed the system?

Doug L.
 Posted: Oct 6, 2019 02:10PM
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 Forgetting my original brake fluid ( 75 Morris 850 twin leading shoe) was regular LMA and then mixed in new synthetic I now get foam in the master cylinder when I pump it up and no pressure. Replaced with new master cylinder, bench bleed, (with synthetic) and same thing.  I was told now I needed to flush whole system with denatured alcohol fuel. Or did I receive 2 bad master cylinders, or pump too hard bleeding? Is this alcohol the answer and If so will this fuel damage seals? Or do I need to go back to original dot 3 LMA fluid.

Thanks in advance!