Mineral oil / Dot3 seal compatibility

Fred Werring · Apr 4, 2018

Will the seals in a master cylinder made for DOT3 brake fluid work with a mineral oil brake fluid?

I know you cannot use DOT3 in a mineral oil system, was wondering if the reverse is true. Sources on the internet disagree, wondered if anyone had done it.

Been having some fun with the master cylinder in my Ford 8700.

If you look for a MC with "Ford 8700" in the description....well, the cheapest rebuilt I found was $395 plus core and shipping. Lot of them were in the $5-600 range.

It was pointed out in the Ford forum that a MC from a 66 Chevy C10 would work...and that can be had for $69 including the core. That's why I'm asking about the seals.

The post in the Ford forum is linked below if you're interested.

Thanks

Fred
Post in Ford forum

MarkB_MI · Apr 4, 2018

No. I know this for a fact from when my son put power steering fluid in the master cylinder of my '88 Chevy C-1500. I ended up replacing almost everything in the brake system: master cylinder, wheel cylinders, proportioning valve and ABS unloading valve. I was able to save the calipers only. The petroleum-based fluid caused all the rubber parts in the system to expand.

Bob · Apr 4, 2018

What is wrong with your master cylinder... does it just need seals or is it badly worn or rust-pitted?

I see FIAT doesn't offer a seal kit, but if it IS just like a light truck MC, there's only a cup and a seal in each bore, perhaps they could be found on-line in a mineral oil-compatible material?

A quick search turn up a company that's come up in several other threads like this, but I don't know if they sell direct or how their distributors would be to deal with.

http://www.precix.net/pdfs/Brakes_pdf.pdf

Email inquiries to: [email protected]

And there's "Cardone", which has a "repair and return" program, typically through local auto parts stores that are dealers for them. Contact them and see if they can rebuild with mineral oil-resistant parts.

Email: [email protected]
Or call: 888-280-TECH

As to the Chevy MC, what are the bore sizes in your cylinder?

With the Chevy, one side would be for brakes, and likely have a "back pressure valve", the other side for the clutch wouldn't, but parts could probably be swapped around.

<img src = http://oi66.tinypic.com/2z8ol6e.jpg">

Bob · Apr 4, 2018

Looks like the re-man cylinder # D3NN2140AR can be had for quite a bit less from the aftermarket than from FIAT.

Also, it appears kit # C7NN2004A will fit, about $200 from FIAT, $47 and up from the aftermarket, even on ebay and Amazon.

GOOGLE the part numbers.

Hope all the aftermarket suppliers are on the right "page" and are supplying mineral oil-compatible parts!

<img src = "http://oi68.tinypic.com/2mh5u8p.jpg">

Determined · Apr 4, 2018

From past experience DOT 3 fluid over time will do bad things to the rubber parts in a mineral oil system as well mineral based oils will do bad things to the seals used in DOT 3 systems.

With all these new DOT 5 and Dot 5.1 fluids that have come out maybe there is something compatible with both fluids/systems.

Maybe someone who knows the chemistry of the fluids can comment.

MikeJM · Apr 4, 2018

Please do not think that Dot 5 and 5.1 are related, they have very different chemistry.

Dot 3, 4 and 5.1 are glycol based fluids and are all hydroscopic, they are also not compatible with Buna-N or nitrile rubbers commonly found in mineral oil systems. Most of these brake systems utilize EPDM rubber parts.

Dot 5 fluid is silicone based which is hydrophobic, it has been around for a while but hasn’t been real popular, it has its own set of advantages/disadvantages. I can not comment on its compatibility with rubbers other than EPDM, but rubber/chemical compatibility charts suggest that it is compatible with Buna-N commonly used in mineral oil systems. I have not tested the theory yet.

Frank 41 · Apr 4, 2018

Mineral oil, itself, is excellent to seal lawn mower tires to the rims.

Fred Werring · Apr 4, 2018

If you look at the pics in the original post, you'll see what the bores looked like.

I spent what felt like forever going through them with a drill mounted brake hone, until they felt/looked smooth.

What bothers me is the pistons in the aftermarket rebuild kit (the pic in your other post is what I bought) would not return under it's own spring pressure. Orings that came with the kit were 1-1/8 OD x 1/8 thick, I replaced them with 1-1/16 OD x 1/8, now the pistons will move. The diameter of the groove in the piston still held the oring out far enough to seal...not leaking yet, but I don't trust it for the long haul.

That's why I was looking at the MC for the chevy...honed right instead of me with the drill hone.

And now you've got me wondering if the aftermarket kit has mineral oil seals...guess I'll find out.

Guess I could put the rebuild kit in the chevy mc...unless they used oversize pistons/seals after they cleaned up the bores.

And my bores are 1-1/8.

Thanks

Fred

Fred Werring · Apr 4, 2018

Not what I wanted to hear, but what I needed to know.

Thanks

Fred

Mineral oil / Dot3 seal compatibility

Fred Werring

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MarkB_MI

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Bob

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Bob

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Determined

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MikeJM

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Frank 41

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Fred Werring

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Fred Werring

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