67 Brake Fluid Flush

[Thomas S.]

Just finished a total flush and bench bleed. Good results except after completing the job I notice a translucent suspension in the front master cylinder well. Fluid is clear (DOT3) in both wells, but when the brake pedal is applied you get a faint cloudy stream out of the port in the front cylinder. The master cylinder was not rebuilt.

Is what I'm seeing typical of a master cylinder piston derogation? Are the pistons anodized aluminum similar to caliper pistons?

[Keith B.]

what did you flush it with?

[Thomas S.]

Straight DOT3 brake fluid. Maybe I should have said fluid change. All wheels, all calipers, proper order, etc. Typical discoloration out of the calipers and eventually clear.

[Perry M.]

It sounds like the fluid is cloudy, meaning microscopic air bubbles in the fluid.

[Thomas S.]

That's certainly a possibility. Just don't understand why it didn't show up when I bench bled the MC. I couldn't get a good seal for a pressure bleed so I went the vacuum route using an air powered Mityvac with the auto fill in each well. I'm sure you the process. Don't know how air could get into the MC unless there's something internal introducing it.

I thought perhaps the piston was leaching which is why I asked if it was made of aluminum.

Thanks for the reply.

[Joe L.]

I thought perhaps the piston was leaching which is why I asked if it was made of aluminum.

Thanks for the reply.

Thomas------


The MC pistons are made of aluminum. However, I don't think that has anything to do with the "cloudy" condition of the fluid you describe.

[Duke W.]

Just finished a total flush and bench bleed. Good results except after completing the job I notice a translucent suspension in the front master cylinder well. Fluid is clear (DOT3) in both wells, but when the brake pedal is applied you get a faint cloudy stream out of the port in the front cylinder. The master cylinder was not rebuilt.

Is what I'm seeing typical of a master cylinder piston derogation? Are the pistons anodized aluminum similar to caliper pistons?

You did a bench bleed, but did not "rebuilt" the MC, which means you did not remove MC??? The description of your procedure is confusing.

For a normal Blake fluid flush the old fluid should be siphoned out the the MC reservoir(s) and fresh fluid installed. Then start bleeding with the SHORTEST (not longest) corner to the MC, which is the left front. This purges the old fluid from the pipe to the combination valve via the shortest distance. Then to the RF and then go to the right and left rear. Do 6 pumps of the pedal (using a helper to run the brake pedal,) and go around two to three times using at least full quart of fluid.

Use the command and response system:

Bleeder: "Push down" as he opens the valve.

Pusher: "Down", bleeder closes valve.

Bleeder: "Let up slow"

Repeat as required.

So exactly what step by step procedure did you use.

Duke

[Thomas S.]

I unfortunately let one of the MC cylinders run dry while installing the front left caliper. I searched the archives for some instructions and went forward based on the below. Both commented by John Hinckley.

https://www.forums.ncrs.org/forum/technical-discussion-archives/36243-72-master-cylinder-bleeding?p=416408#post416408

https://www.forums.ncrs.org/forum/technical-discussion-archives/15726-brake-bleeding?p=246966#post246966

The inside of each well was cleaned out. There was rust and some hardened gray material at the bottom and it was non-magnetic. Not a lot, but worth mentioning.

The MC was filled and purged of air using the instructions and that worked fine. Fresh DOT3, no bubbles, no discoloration. Once this was done I tried to pressure bleed the system but couldn’t get a good seal despite having 3 different adapters for that purpose. I had planned on replacing the fluid anyway and felt the pressure approach would be the best solution.

At that point, I used vacuum beginning at the right rear, inner and outer, the other side and on to the front. The left rear gave me some issues as I simply couldn’t get a good tubing connection on the bleeder. So I fell back to the break pedal method on those. As expected the fluid was a coffee color and eventual clear with no bubbles. On all 4 wheels

I used an autofill bottle adapter on the MC with the fluid coming directly out of the original bottle of DOT3. Went through a total of 6 bottles. At the end of the process I did a pedal test to check on the firmness and that is when I noticed the stream of cloudy material coming up from the front cylinder port (check valve?). I syringed it out and replace it with fresh fluid but every time pump the peddle some more cloudy stuff came out.

I can see from your comments that I should have reversed the sequence for the flush process. I suspect I will end up having to do this again so I will follow that sequence.

The gray stream of "whatever" is puzzling and that's why I though it could be aluminum from the piston. The substance does not rise to the top, in fact just the opposite, it seems to settle.

One other question since you are the "go-to" on a lot of things. The 2 bleeders on the MC - why can't they be used to bleed it as opposed to disconnecting the brake lines?

[Duke W.]

None of my cars have bleed valves on the master cylinders, but I believe those can be used to bleed the m/c in order to fill the bore with fluid, and this can probably be done on the car rather than on the "bench" in the case of a m/c replacement, but I'm not sure because I've never done it. I recently helped a neighbor replace the m/c on his '77 Corvette. I don't recall if the new m/c had bleed valves. We did bench bleed it off the car, and then installed it followed by a complete system flush with a quart of fluid starting with the left front and all went well.

One of my cars has a plastic reservoir with only one fill port. Internal baffling creates two internal reservoirs. The rear level is difficult to see, and I once inadvertently let it run dry. It took a lot of (pump the pedal type) bleeding, but I finally got it filled and free of air.

I take it you "bench bleed" the one reservoir because it ran dry.

You said there was "gray material" at the bottom of the reservoir that you cleaned out, and the contamination you see after the bleed is also gray, so it may be the same material.

Given the condition of the m/c you probably should have rebuilt it, but what I recommend you do now is drive the car and check the reservoir. If there is this gray contamination material siphon all the fluid out, replace with fresh fluid and repeat as necessary. If it keeps coming back, rebuilt the m/c, and also remove the pipe from that reservoir to the differential pressure switch and clean it out thoroughly or replace it.

Older cars that have not had adequate periodic brake fluid changes usually have a dark sludge at the bottom of the reservoir. I'm not sure what this is, but I believe they are tiny wear particles from the cup rubbers, and periodic flushes are necessary to remove these and potentially moisture contaminated conventional polyglycol brake fluid. Silicone fluid is less subject to moisture contamination, but if my theory is correct periodic inspection, like swapping the bottom of the reservoirs with a Q-tip should be done to check for this rubber wear particle contamination, and it should at least be cleaned out, or maybe a complete flush because similar sludge could be in the calipers/wheel cylinders.

Let us know how this situation plays out.

Duke

[Thomas S.]

I agree and have decided to rebuilt it or buy an original rebuilt. The gray stuff was probably suspended in the old fluid but since it was coffee color, it wasn't apparent. Still salt on the roads in NJ but I do own a commercial building with a a large lot and the brakes were fine. Not that a short drive would prove anything other than something very bad.

I think the moral to the story is to assume that if you have rust deposits in the MC, it should probably be removed cleaned the right way and rebuilt.

Thanks for your input.

[Joe L.]

None of my cars have bleed valves on the master cylinders, but I believe those can be used to bleed the m/c in order to fill the bore with fluid, and this can probably be done on the car rather than on the "bench" in the case of a m/c replacement, but I'm not sure because I've never done it. I recently helped a neighbor replace the m/c on his '77 Corvette. I don't recall if the new m/c had bleed valves. We did bench bleed it off the car, and then installed it followed by a complete system flush with a quart of fluid starting with the left front and all went well.

One of my cars has a plastic reservoir with only one fill port. Internal baffling creates two internal reservoirs. The rear level is difficult to see, and I once inadvertently let it run dry. It took a lot of (pump the pedal type) bleeding, but I finally got it filled and free of air.

I take it you "bench bleed" the one reservoir because it ran dry.

You said there was "gray material" at the bottom of the reservoir that you cleaned out, and the contamination you see after the bleed is also gray, so it may be the same material.

Given the condition of the m/c you probably should have rebuilt it, but what I recommend you do now is drive the car and check the reservoir. If there is this gray contamination material siphon all the fluid out, replace with fresh fluid and repeat as necessary. If it keeps coming back, rebuilt the m/c, and also remove the pipe from that reservoir to the differential pressure switch and clean it out thoroughly or replace it.

Older cars that have not had adequate periodic brake fluid changes usually have a dark sludge at the bottom of the reservoir. I'm not sure what this is, but I believe they are tiny wear particles from the cup rubbers, and periodic flushes are necessary to remove these and potentially moisture contaminated conventional polyglycol brake fluid. Silicone fluid is less subject to moisture contamination, but if my theory is correct periodic inspection, like swapping the bottom of the reservoirs with a Q-tip should be done to check for this rubber wear particle contamination, and it should at least be cleaned out, or maybe a complete flush because similar sludge could be in the calipers/wheel cylinders.

Let us know how this situation plays out.

Duke

Duke-----

1967-72 Corvette master cylinders did have bleeder valves but these are practically useless to bleed the master cylinders. The best way to bleed these master cylinders is to do so from the ports with a master cylinder bleeder kit. I think that GM finally recognized the uselessness of the bleeder valves and eliminated them about 1973.

[Patrick H.]

It's not difficult to rebuild your master cylinder if it's an original.

I did it on my 72. Raybestos and Dorman both make kits, though the Raybestos seems to only show up once a year. There are parts in each that seem "better" than the other. Looking at them online, the Dorman includes a spring that the Raybestos does not.

My MC had a slight bit of rust staining in the bore, and using Evapo-Rust I removed the rust and then polished the bore using my Dremel. This also removed any rust from inside each reservoir.
I then installed the rebuild parts that matched my originals, and since then no leaking!

[Thomas S.]

It's not difficult to rebuild your master cylinder if it's an original.

I did it on my 72. Raybestos and Dorman both make kits, though the Raybestos seems to only show up once a year. There are parts in each that seem "better" than the other. Looking at them online, the Dorman includes a spring that the Raybestos does not.

My MC had a slight bit of rust staining in the bore, and using Evapo-Rust I removed the rust and then polished the bore using my Dremel. This also removed any rust from inside each reservoir.
I then installed the rebuild parts that matched my originals, and since then no leaking!

I have one on the way from Lonestar and will end up rebuilding mine at some point. I don't believe I have a leak in . I like to have spares of everything. The top surface edges are pretty worn which is probably why I couldn't get a good seal on the pressure bleed fixture. There are ridges on the surface edges for better sealing an mine are basically gone - totally smooth and somewhat irregular. Probably due to rust removal from a prior owner.

The bore surface has to be very smooth or the piston seals will wear out. How did you use a Dremel tool for that phase of the rebuild?

[Patrick H.]

Buffing wheel on a Dremel extender, and Flitz polishing paste.

[David M.]

This tool makes bleeding effortless and its inexpensive. Bleeding should be on your service schedule every 5 years. Buy the various adapters for your daily drivers and bleed them every 20-25K.

Always keep brake fluid sealed and stored in a controlled environment(basement). Date the bottle and dispose in a year or two. DOT 3 & 4 are very hydrophilic(absorbs water like a sponge). Don't store fluid in the power bleeder.

Once you figure out what you are doing with the master put a gallon through the system. Inspect for leaks while the system is pressurized.

Yes the fluid could have been frothed from trapped air causing the white discoloration. System contamination floating on top of fresh fluid will give off the iridescent rainbow sheen. The system simply needs more flushing.

https://www.motiveproducts.com/collections/brake-bleeder-kits