I think my Mini is drinking brake fluid

[BruceK]

So, last weekend I took my car out for a drive.  Half a mile from my house it started running like crap - with white smoke coming out the tail pipe.  That lasted maybe 10-15 seconds.  I thought for a second I blew a head gasket.   Then the engine cleared its throat and the car ran great.   So I decided to drive it to see how it was doing.  I drove for about an hour with no issues - it ran great - and I brought it home just before it got really hot (the weather, not the car).

So fast-forward to 5 days later and this evening I was checking all the fluids on Mini and when I came to the brake system I couldn't see the fluid line on the brake reservoir (was my clean brake fluid really that clean?), so I pulled the cap off to check. Completely empty.  Dry.  And no leaking wheel cylinders or calipers either.  I'm very lucky I made it home with virtually no brake fluid.

So, the only think I can think of is that somehow the brake fluid was ingested through the brake booster vacuum line to the intake manifold - and that was the white smoke and crapping running I briefly experienced. 

This is my first Mini with power brakes, so where do I start?  The car has the power brake set up from the early 1990s. 

[MiniDave]

I think you put your finger right on it, it's sucking the juice out of the master cylinder on the booster. You can buy a rebuild kit but I think it would be smarter to replace it.....remember it did this once before, the big cloud of white smoke and ran like crap for a few seconds I mean.

To remove the master, simply remove the two brake lines, undo the cap and set it aside, undo the two nuts that hold the MC onto the booster...I think the master will come right off then.

[BruceK]

I think you put your finger right on it, it's sucking the juice out of the master cylinder on the booster. You can buy a rebuild kit but I think it would be smarter to replace it.....remember it did this once before, the big cloud of white smoke and ran like crap for a few seconds I mean.

Yup, you are right!  It did this back in the Fall and I couldn't figure out what the issue was.  Last weekend was the same symptoms. 

I was just looking at Mini Spares, they have a rebuild kit for £54 and a new booster for £105.  I think I will go for the complete booster.   Seems like a good opportunity to replace hoses and other booster parts at the same time.

[MiniDave]

Is that the booster and master cylinder? Cause it's the master that's leaking......

http://www.minispares.com/product/Classic/Brakes/Hydraulics/Master_Cylinders/GMC90376.aspx?041001&ReturnUrl=/shop/classic/Brakes.aspx|Back to shop

[94touring]

OR...ditch the booster. 

[BruceK]

Is that the booster and master cylinder? Cause it's the master that's leaking......

http://www.minispares.com/product/Classic/Brakes/Hydraulics/Master_Cylinders/GMC90376.aspx?041001&ReturnUrl=/shop/classic/Brakes.aspx|Back to shop

No, I was just talking about the booster.    I've got to study the system to become more familiar with it.  I was thinking of some sort of a check valve in the servo unit.   So I wasn't thinking the MC was leaking, but you must be right.

[BruceK]

OR...ditch the booster.

I kinda like the power brakes (when they aren't compromising the safety of the entire brake system that is).

[94touring]

Then I'd start by replacing the check valve.  Been awhile since I've really looked at a booster but I think your failure point in this situation may simply be the one way check valve. 

[MiniDave]

No, the brake fluid has to come from somewhere, it's coming from the master cylinder seals, and being sucked into the booster under the vacuum.

[jeff10049]

You need a master the booster should be fine.

Just stick a new master on and be done with it don't worry about any check valves or anything and no sense ditching the booster as it's not the problem.
Unless you don't want it but you'll still need a new master.

I normally try and remove any excess brake fluid from the booster soak it up with rags then hose it with brake clean and soak that up.

[94touring]

No, the brake fluid has to come from somewhere, it's coming from the master cylinder seals, and being sucked into the booster under the vacuum.

Suppose that does make more sense  :-[

[BruceK]

Thanks for all the input guys.   In looking at the new MCs for sale at various suppliers I see many come with a warning tag to only use DOT4 or DOT5 fluid.  And a description warning that is for the seals.  It's been a couple of years, but I think I used DOT3 in the system after I did my rear brakes (old familiar).  So this MC seal failure may be on me. 

[MiniDave]

DOT 3, DOT4 and DOT 5.1 are all compatible and will not cause damaged seals.....it was nothing you did, probably just a worn out 30 year old master cylinder.

DOT5 is silicone fluid, never use DOT5 in a system that's had the others in it without a complete flush or all new rubber components.

[BruceK]

DOT 3, DOT4 and DOT 5.1 are all compatible and will not cause damaged seals.....it was nothing you did, probably just a worn out 30 year old master cylinder.

DOT5 is silicone fluid, never use DOT5 in a system that's had the others in it without a complete flush or all new rubber components.

Oops I meant 5.1 not just 5.    Yeah I thought it was no deal using DOT3.     

But here's what Mini Spares says about the replacement brake MC:  "Ensure the correct DOT4 or DOT5.1 brake fluid is used as these cylinders are not designed to use anything else such as Castrol SRF for racing or any mineral fluid."

And MiniSport even sells the warning tag for that same MC as a separate item which says to use DOT4.  Much ado about nothing?

[94touring]

I use 3 and 4 in mine.  It's whatever I have on the shelf kinda thing.

[MiniDave]

Yes, the only difference between 3 and 4 is the boiling temp - 4 is higher as you would expect. What they're cautioning against is using mineral based fluids - brake fluid is vegetable oil based. Those two are NOT compatible!

I used 3 the last time I needed fluid on the green Mini, and in the mts of Tenn on the CMU 59 I boiled the fluid on some of the more aggressive runs.  4 or 5.1 probably wouldn't have done that - it was a first for me!

They also make racing brake fluid which boils well over 600*, but it's expensive and hard to find.

[BruceK]

Thanks for the explanation - I didn't realize brake fluid was vegetable oil based.  Kinda surprised that is mean enough to eat paint.

[jeff10049]

Today's brake fluid's  are mostly glycol based DOT 3, 4, and 5.1.

Also, mineral oil based brake fluids exist in grilling systems of Rolls Royce and some others.

And then there is the DOT 5 silicone fluid I put that in my mini when I restored it happy so far.

DOT 2 and lower or before DOT ratings were castor oil (vegetable oil) and alcohol bases or even straight castor oil.

As Dave said 3, 4, and 5.1 are compatible with each other all the others are on there own and not compatible with any other fluid that I know of.

Interestingly the Glycol based fluids often start as natural gas just like some synthetic motor oils.

[croc7]

Prior to replacing a master cylinder, I had never bench bled one in the past. Did it this time and saved myself a boatload of time and trouble.  Well worth the effort.