Dirty brake fluid---only in half of master cylinder

[cjohnson6772]

I put a new master cylinder on my truck about six months ago. I flushed the whole system and swtiched to DOT5 brake fluid. After finally getting my rear brakes bled (nightmare) the truck stops great. My concern is that when I pop the lid of the master cylinder the front fluid looks murky (purplish brown) and the back fluid looks nice and clean (purple). Should I be concerned? Why the drastic difference in fluid color? Is it simply because the front brakes are used that much more often (system has only been driven a few times a week for 6 months). Thoughts are appreciated.

[BMERDOC]

I would imagine that since you had to bleed the mess outta your rears you probably cleaned all the gook from the system in the process. The front brakes bleed so easy its possible the old crud in the lines and caliper stayed in place. Over time that stuff loosened up and migrated back into the cylinder. Flush the fronts again and keep an eye on it.

[fastwillie 696969]

dot 5 might be the problem ,you might want to change back to dot 3 or 4
Can DOT 5 Brake Fluid Be Used Instead of DOT 3 or 4?

Always refer to vehicle owner's manual for what the manufacturer recommends or warns against. As a rule, vehicles equipped with antilock brakes (ABS) should not use DOT 5 brake fluid.

DOT 5 brake fluid is silicone based. DOT 3 (standard) and DOT 4 (heavy-duty) are glycol-based. It can be distinguished from conventional brake fluids by its purple color (which comes from a dye).

Silicone does not absorb moisture. DOT 5 brake fluid does not become contaminated with moisture over time as conventional DOT 3 and 4 brake fluids do. Silicone is also chemically inert, nontoxic and won't damage paint like conventional brake fluid. It also has a higher boiling point.

Because of this, it is often marketed as a premium "lifetime" brake fluid. It is often used to preserve brake systems in antique vehicles and those that sit for long periods of time between use.

DOT 5 silicone brake fluid is also very expensive (costing four to five times as much as ordinary brake fluid), and it won't mix with glyco- based brake fluid (creating concern over sludging if all old fluid isn't removed when a system is refilled with silicone).

Silicone also has slightly different physical properties and compressibility, making it unsuitable for ABS systems calibrated to work with DOT 3 or 4 brake fluid.

[cjohnson6772]

I looked into it and questioned several people before switching. My understanding is that replacing is ok if you totally clean the system first. The thing I like about DOT5 is that it does not reek havoc on paint. Very important to me. I can't tell you how many times I've had to repaint inner fenders b/c of DOT3/4.

[Longhorn Man]

you know about how water will puddle in the lowest part of the brake system then since the dot5 won't absorb it then, right?

[cjohnson6772]

I did not know about that. It is beginning to sound as if I maybe did not get the entire picture from the people I spoke to locally. I can say that I have had the best brakes on the truck I have ever felt since I vacuum bled the system with DOT5. No problems have arisen so far except the dirty fluid in the cylinder. I am beginning to wonder if I should be looking for issues down the road.

[68gmsee]

Blackish gunk found in brake fluids may be caused by deteriorating rubber hoses and/or seals. If you see a lot of that in your brake system you're rubber based components need to be looked at -specifically the brake line front and rear brake hoses.

[swissarmychainsaw]

Originally Posted by cjohnson6772

I put a new master cylinder on my truck about six months ago. I flushed the whole system and swtiched to DOT5 brake fluid. After finally getting my rear brakes bled (nightmare) the truck stops great. My concern is that when I pop the lid of the master cylinder the front fluid looks murky (purplish brown) and the back fluid looks nice and clean (purple). Should I be concerned? Why the drastic difference in fluid color? Is it simply because the front brakes are used that much more often (system has only been driven a few times a week for 6 months). Thoughts are appreciated.

The reservoir has two channels one for the front and one for the rear.
I believe that the front feeds the front brakes. My best guess is that you have a leak or bad front rubber hoses. Replace them.

[Stocker]

Originally Posted by swissarmychainsaw

The reservoir has two channels one for the front and one for the rear.
I believe that the front feeds the front brakes. My best guess is that you have a leak or bad front rubber hoses. Replace them.

This thread is more than three years old. Hopefully the OP has found /fixed the problem before now. He just never bothered to post an update to his thread.

Incidentally, on some trucks the front reservoir feeds the rear brakes, and vice-versa. Not sure but I believe that applies to trucks with front discs, while all-drum systems are plumbed front-to-front and rear-to-rear.

[swissarmychainsaw]

Originally Posted by Stocker

This thread is more than three years old. Hopefully the OP has found /fixed the problem before now. He just never bothered to post an update to his thread.

Incidentally, on some trucks the front reservoir feeds the rear brakes, and vice-versa. Not sure but I believe that applies to trucks with front discs, while all-drum systems are plumbed front-to-front and rear-to-rear.

Good points Mike, I figured that someone would have an issue and find this thread and still get some help from it even if it is old!

Right after I posted this I opened up the manual and this is what I found;:

"The cylinder has two separate reservoirs and outlets in a common body casting.
On all 10 and G20 series vehicles the front reservoir controls the front brake system in the rear reservoir controls the rear system.
On all other 20–30 series vehicles, the front reservoir controls the rear brake system and the rear reservoir controls the front system (except on CA 30 with vacuum over hydraulic boosters where the front is controlled by the front booster and rear by the rear booster).

SO translating that: a half ton (and 3/4 van (G)) front reservoir controls front brakes.
On 3/4 and 1 tons the front res. controls the rear brakes.

PS I just ordered this manual from RockAuto for my 73 K10, it's a reprint of the factory manual. Pricey but "priceless"!