Foamy silicone fluid in master cylinder

OK, I replaced the master and slave cylinders on my '79 driver. I used silicone fluid. I bench bled the master and installed, with new line. After some driving, the pedal stared going lower to the floor. Opened the reservoir and the silicone is foamy. I have tried to rebleed the bugger and can't get the bubbles out of the system. I did recheck the line and did twist down one fitting, but still am getting the foaming.
Advice?

Patrick
Wayensville, N.C.
'79 driver
'79 next driver, soon...
'78 project
'77 project
'80 parts car
...<span style="font-style: italic">Spitfires</span>...

That's typical of silicone fluids in general. The foaming you're seeing is the little back bleed port squirting fluid up into the reservour when you start to push the pedal down. This squirt sprays up and drops down, entrapping air. The more shallow the reservour, the worse the problem will be.

Cold weather can/will make it worse as the air bubbles are less inclined to float up and burst.

Is your pedal better after bleeding? How heavy is the foaming?

I can't help but wonder if there's still something, either a line or possibly the master itself, that is reintroducing air into the system. How "new" was the new master cylinder?

The slave and master were purchased past summer from the big company in the middle of the countryalong with the connecting line. When I first bleed it after installing, I noticed gray dust in teh silicone. From manufacturing?

Patrick Langford
Wayensville N.C.

Spitfire squadron

Hmmmm....could be. Not sure what the grey dust might be....

Me either, but I'd be concerned if there were anything but brake fluid in the system.

Sure sounds like it's contaminated to me. I had lots of geysering going on in the reservoir when driving with a broken taper pin (had to pump the clutch up to make it work), but never saw ANY visible foam in the reservoir.

Or maybe not all silicone BF is created equal ?

What silicone fluid did you use?

I believe it's Cartel. Was rather pricey.

Did you let the silicon set before using. I f it is handled haphazardly,carried around, it foams. A lot of silicon changes require 2-3 bleeds.

RonMacPherson said:

What silicone fluid did you use?

Click to expand...

Don't recall the brand for sure, but whatever JCW was selling then. Might have been North American (same stuff TRF sells now), but maybe not. I've also used GE brand (which is NLA I believe) and never seen any visible foam.

Here's someone else's photo of a mix of DOT 5 & DOT 4 that was just "vigorously agitated" ... note the lack of foam.
(from https://www.buckeyetriumphs.org/technical/Brakes/Fluid/Fluid.htm)

Silicone brake fluid absorbs air over time after the bottle is opened or as it sits in the car itself. (That's better for some folks than normal brake fluid, which absorbs water which then rusts out wheel cylinders and caliper pistons.) Your bubbles may just be the result of having had an open container of silicone brake fluid sitting too long on the shelf before putting it in the car, especially if the bottle had been opened.

This absorbing air is the main reason that I went back to DOT 4 brake fluid about 20 years ago. As my brakes heated, the DOT 5 fluid would out-gas the air bubbles into the fluid at the wheels, and it would make the brakes drag, which created more heat and multiplied the effect. I noticed this effect after having the fluid in about a year. I figured that if I had to replace brake fluid once a year, I may as well use Castrol LMA and change it once a year, which would be plenty often enough to avoid moisture damage in the brake system.

The gray dust in the brake fluid may be oxidized aluminum from the walls of the old master cylinder. This aluminum oxide would have been deposited on the insides of the brake lines themselves, and might have been washed into the new fluid. Usually a black dust will mix with fresh brake fluid; this black dust is oxidized rubber from the flexible brake hoses and seals further downstream.

Scott

Gentlemen: since "Gray Lady" is going back together quickly, I have a decision to make. Every piece in the braking system is new, including all seals, lines, pistons, reservoir, etc. In other words everything that is in contact with the hydraulic fluid is dry and has never been in contact with fluid. SO, is there anything else that needs to be done (in preparation) of adding silicone fluid?

skucera said:

As my brakes heated, the DOT 5 fluid would out-gas the air bubbles into the fluid at the wheels, and it would make the brakes drag,

Click to expand...

Something else wrong ... even if the fluid did form air bubbles (which I don't believe), they would just push fluid back through the MC into the reservoir, rather than making the brakes drag. You must of had a misadjusted MC, or a sticking pedal, or a broken/weak pedal return spring, or something.

GE guaranteed their DOT 5 for 10 years or 100,000 miles ... I've run mine longer than that and even overheated the brakes a few times; but never had a problem with air in the fluid.

Some snippets from a report presented to the SAE back in 1981 :

Yikes, that not covered in my insurance.

I've used dot5 since the early 90's in my TR6,never had a problem. I do however think it takes a little more bleeding to get all the air out on a dry system.

angelfj said:

Gentlemen: since "Gray Lady" is going back together quickly, I have a decision to make. Every piece in the braking system is new, including all seals, lines, pistons, reservoir, etc. In other words everything that is in contact with the hydraulic fluid is dry and has never been in contact with fluid. SO, is there anything else that needs to be done (in preparation) of adding silicone fluid?

Click to expand...

Frank, the only thing I would do different is to lubricate the seals & pistons with a little DOT 5 as you assemble them into the cylinders. I doubt it's essential, but I always like to ensure that moving parts get a little lubrication. And I feel it particularly helps keep keep the steel rear brake pistons from causing electrolytic corrosion of the aluminum alloy cylinders.

Other than that, my only other precaution is to let the DOT 5 "breathe" a bit after filling the reservoir and before bleeding the brakes. Good time for a pause to enjoy your favorite beverage.

FWIW, I have DOT 5 in the Tiger and in the BJ8, and DOT4 in the 250, MGC and the BT7. No rhym or reason -- they just came that way from the POs, and I saw no reason to change any of them.

I will freely admit that I dispise DOT4 because of its paint-eating properties (the paint turned to goo as a result of old seals on my PDWA still annoy me to this day -- not an easy spot to repaint). But in truth, given that I mostly drive my LBCs in the city, I don't really notice any real difference in performance. They all seem to stop in reasonable distances, even if I'm pushing it a bit. I'm pretty cautious, and would be alarmed if I felt a huge change in pedal feel and pedal travel while driving, but I must admit that I can't tell you sitting here how the Tiger's pedal feels vs. the 250, or the BT7's pedal vs. the BJ8. Guess I could always do a test...

As an aside, I highly recommend Speedbleeders or EZBleeds when doing brake and clutch work. Makes bleeding brakes a one-person job, and *tons* easier.

Since 1990, I have used silicone brake fluid in my brake and clutch system. I replaced it as needed, but never drained the brake system completely and fully replaced it. I twice did the clutch line. During these last 17 years, I have driven 97,000 miles. I like Randall's suggestion of lubing everything with the silicone fluid as you put it together. I did that on both TR3As that I restored. In 17 years, the lubricating properties of silicone have kept all the cylinders and seals in perfect order. Rust cannot occur. The rear brake cylinders have 97,000 miles on them and the front calipers and pistons for the disk brakes have 178,000 miles on them from new.

On both cars when I filled the fluid, the cars were still under restoration and after I gently poured the silicone fluid into the reservoir, it was months later before the cars were finished and I think this time duration is needed to allow bubbles to rise out the system. Pour it in gently so as to not produce any air entrainment (bubbles) into the reservoir that you don't need and you don't want.

Frank - Your case is exactly like my two cases and I'd go for it again if I were you.

Race drivers don't want to use silicone because they will sometimes change brake fluid between races and they don't want to wait a month before the starter waves the green flag.