hot running rear hub

[tone]

ok so I recently fitted a set of replacement sealed RF hub bearings with his modified 50mm nuts, all torqued to 130 ft lbs.
Checked brakes not binding and handbrake free on release, but the driver side is running really hot after a few miles.

Now in a panic as I have a car event in a weeks time and don't think I'd get that far
Any thoughts or advice more than welcome

[Steve Simmons]

Other than brakes, I can't think of how one rear hub could be running hotter than the other. Maybe try loosening the brake on that side a bit extra and see what happens? Are the fronts normal temp? If they are hot also then I'd suspect the left rear brake isn't working like it should.

[tone]

After much head scratching and staring at the brakes for hours with the obligatory mug of tea, problem solved!

The lead shoe piston in the brake cylinder was not fully retracting after brake release causing slight binding, hence the hot brake drum after a few miles of running.

The return spring seemed ok and on removing the rubber gaiter I noticed a bit of corrosion on the piston, checked the nearside, seemed in better condition but will change both sides as a precaution.

So new brake cylinders and return springs [just in case their weak] ordered from my incredibly helpful and friendly MG Octagon Car Club spares department, Should be back on the road by the weekend.

I guess this is a problem of low use and damp UK weather.

[Steve Simmons]

This may be an argument for silicone fluid, since it doesn't absorb moisture and would therefore theoretically protect bare steel parts. I don't use it in my TC, but I do use it in other cars with good results.

[Ray White]

Personally, I would purchase non ferrous wheel cylinders.

[stephen stierman]

I have used silicone brake fluid in these old cars that sit all winter since it first became available, I guess back in the 70's. My TC is no exception. New wheel cylinders, MC, flex lines and steel lines. No problems, no issues, no sticky pistons.

[i.thomson]

Wouldn't the fact that silicon fluid doesn't absorb water leave the water to do it's worst wherever it comes to rest without the protection of having it locked up in the fluid which presumably largely keeps it away from the metal? After all silicon fluid doesn't stop water getting in to the system in the first place.
Ian.

[tone]

If using silicon fluid how clean does the system need to be of the 'old' fluid
And I kind of agree with Ian that water gets into the system anyway through condensation etc I'm thinking of an annual flush through with fresh fluid might be a better, albeit pretty tedious, way of minimising water in the system?

[stephen stierman]

The system needs to be disassembled and completely cleaned of old fluid and used rubber parts should be replaced with new. I have on occasion disassembled a silicone filed system for repair and never found water anywhere in the system. You can flush you old system every year, but you don't get all the old fluid out no matter how much you bleed and rebleed.

[Duncan M]

Back to the warm hubs... An inexpensive IR heat gun is a very handy item to have around. Did you know that a TC differential
only runs at about 120F or 130F near the drain plug? I like to check things after good long drives. Check for anything unusual. Recently I noticed the rear drums were at 115F and the front drums were slightly cooler. Oops, time to adjust the fronts out a bit.

[Steve Simmons]

Glycol (DOT 3/4) fluid is hygroscopic, which means water can be drawn in and absorbed from the atmosphere into the fluid itself. Silicone is not hygroscopic, and therefore will not absorb moisture introduced from the atmosphere. This means that in order for water to get into a system running silicone fluid, it has to physically enter somehow. The only place for that to happen is the breather vent on the MC. Once in, glycol fluid will disperse it throughout the system where it will corrode anything it can. In a silicone system, the water will settle to the lowest points, as it is heavier than the fluid. So you may get some localized corrosion/rust at those points but nowhere else. In theory the silicone will protect the metal from the water, but I can't say if this is true in the real world. I've found a few rust spots in silicone systems but I can't say for sure that this is what caused it.

I agree with Stephen, the system must be meticulously cleaned, removing all traces of the old glycol fluid, and all rubber changed, before filling with silicone. Alcohol, brake spray cleaner, etc all seem to work well. Anything that cuts brake fluid and sludge without leaving residue behind. Blow the lines and everything else with compressed air to get the remnants out, then allow to dry for a while to ensure you didn't get any moisture or condensation in there.

In a dry climate, silicone fluid can be run pretty much forever, although you will get a bit of black in there as the rubber parts wear. You can filter this out and re-use the fluid if you're handy, but it's easier to just flush with fresh. In a wet climate I would flush the system annually no matter which fluid I used. The one thing on a TC and other cars with similar MCs, is that when running silicone fluid, if water gets in, it might settle to the bottom of the MC reservoir. So if you suspect moisture is getting in then it would be worth sucking the bottom of the reservoir with a tube to see if there's water down there. If so, get it all out. With glycol fluid you can never get all the moisture out, but it won't settle anywhere in particular. What's left will just distribute itself throughout the system again, obviously at lower levels with mostly fresh fluid in there.

[Duncan M]

All that hygroscopic means is that if water somehow becomes present with the fluid, the hygroscopic fluid will eventually absorb it. By the same token, silicone fluid will not absorb it, and the present water will be isolated. That could mean it floats on the top or sinks to the bottom, or if it gets into the pipes the water will end up somewhere as drops or a section of water in the line. Hygroscopic brake fluid does not SUCK water out of the air, and it does not suck water through metal. The silicone marketers latched onto a false interpretation of what hygroscopic actually means long ago, and ran with it. What silicone peddlers never mention is the fact that DOT 5 is more compressible than DOT 3/4/5.1. More compressible is, of course, what you do not want in a hydraulic fluid. It is not much more compressible, but it is more compressible. Because DOT 3/4 is hygroscopic, it is often used as a miscible lubricating oil in certain water cooling systems.

[Steve Simmons]

I wasn't meaning to say that it literally sucks moisture out of the air like a sponge. Rather that as air comes into contact with the fluid, the moisture from that air will end up in the fluid, and then be dispersed throughout the system. I've never seen any silicone fluid manufacturers claim otherwise but if they did then it sounds like marketing hype to me. And it would be strange since manufacturers of silicone also make glycol fluid!

As far as compressibility, you will never feel it with a human foot. Most people who experience a soft pedal with silicone fluid didn't bleed it properly. Silicone has to be treated gingerly. If you agitate the bottle, you can introduce micro bubbles into the fluid, which take some time to work their way out. These air bubbles are what cause the sponginess some people feel in the pedal. I've run my TC with both types of fluid and felt zero difference in the pedal between them. Also no performance difference whatsoever. Ultimately I settled on glycol. Silicone has a higher boiling point than DOT3/4 but it's doubtful You will get your TC brakes that hot, so while it's an advantage, probably not a very big one.

I'm currently running four cars with silicone fluid and four with glycol. I have no complaints about any of them. Either fluid will serve you well. One of the main reasons I run silicone in some of those cars is that the master cylinder is mounted high on the firewall, over a painted surface. Especially on my MGA which are known for leaky MCs. If you run glycol in an MGA, it will eventually eat the paint right off the front shelf. And the brake fluid left behind will promote rust. That's why most MGAs have a lot of rust around that area, and white so many MGA owners have switched to silicone. On a TC, it really doesn't make any difference which one you use. The advantages and disadvantages are pretty equal in my experience. Glycol fluid is cheaper and easier to find, so that's a big advantage right there.

[Duncan M]

Steve, you seem to have an open mind on the subject. Here is the dilemma as I see it:

Using the description you stated above, on how moisture/water can get into a vented system-- as DOT 3/4/5.1 does not suck, an equal amount of water would get into either system, no matter which type of brake fluid is being used. One of two situations for a vented system, depending on what type of fluid you are using:

1. With DOT 3/4/5.1 that moisture will become evenly absorbed in the hygroscopic fluid. This type of fluid can carry moisture in it, and still function quite well. Simply bleed occasionally as part of routine maintenance and inspection.

2. With Silicone DOT 5, the moisture will stay isolated and not get absorbed. Nobody seems to know where the water goes, but it does not get absorbed throughout the fluid, and ends up stuck somewhere in the system. Bleeding would not show the water, as the water is stuck in various places.

Is DOT 5 ideal for a vented system like the TC? I would have to say no. As most TC's are not driven in the rain or left out in the open during rainy season, it probably does not matter a whole lot.

As for spilling brake fluid on paint, I have always found it cleans up just fine with a dry rag followed up with a water soaked rag. I cannot think of a defense for leaky master cylinder or leaky wheel cylinders or fittings, etc. Leaks in any (high pressure) hydraulic system should always be fixed ASAP.

In a sealed system, I suppose it does not matter much what type of fluid you use, as long as you can remember what is in there.

[Steve Simmons]

I don't have a strong preference for one over the other, I've just used both types for decades. They both have pros and cons.

The difference as I've seen it between moisture getting into DOT 3/4 versus 5 is that 3/4 will collect it directly from the atmosphere by physical contact. Dot 5 does not. This is why an open container of 3/4 has a shelf life (typically quoted as 6 to 12 months), while Silicone does not. In a humid environment, an open dish of 3/4 will end up contaminated by water moisture while 5 will still be pure. Water physically introduced will contaminate either system, obviously. The only real difference being that once the water is removed, you can continue using the DOT5 in the system whereas you want to fully replace all the fluid in a 3/4 system since the moisture is in the fluid itself.

My experience with water in silicone is limited since I'm in a dry climate, but what I've found is that it will settle to low points. In a TC this would be the bottom of the MC and the lowest points of the metal lines. You will see any water in the lines if you bleed the system, because it will be pushed out with the fluid. The difference in specific gravity isn't so great that the water can't flow with the fluid being pushed out. So after a flush, there will be no "trapped" water in most cars. Even here in the Southwest, I do find some moisture contamination in my DOT 3/4 systems after a couple years, but I've never found water in my personal cars equipped with DOT 5.

I do drive my TC in inclement weather, and for that reason primarily I run DOT 4 in it. The MC is underneath the car where it could be exposed to direct water ingestion, and it's easier to flush because of the area at the bottom of the MC. Whether that's a legitimate concern is debatable I suppose. But I agree, on a car driven only in fair weather there is little reason to choose one over the other.

The problem with eating paint on an MGA is that the paint will be ruined before you realize there is a leak. Another example of engineers designing something great on paper that didn't work out so well in the field. Body shops have made good livings over the years re-painting MGA firewalls. There are probably more MGA owners running silicone than any other model of MG, for that reason. My own A has a big ring of rust, so I finally switched it to Silicone about a decade ago to cut my losses.

Sorry, I guess we're straying a bit off topic here...

[Duncan M]

In the MG TABC the master cylinder is not in a place where a leak will damage much, of course. Just the little bracket shelf it rests on. Easily replaced.

Most any later LBC like Lotus, Jaguar, Triumph, MG etc. have the firewall to drip on. Use of a diaper is old school, but quite effective at saving paint and sheet metal. Check the diaper for wetness when you think of it, rather than wait for a drop to appear on the firewall. 3/4 takes almost forever to evaporate, so even a small leak will leave a wet spot on the diaper.

Any sales pitch for silicone brake fluid should include the warning about using in a car with a vacuum power booster. Use 3/4 if you have a vacuum booster, as any small leak in the diaphragm will not destroy your engine, like ingestion of DOT 5 can. Once again, nothing to worry about with the TABC.

Car painters hate silicone fluid as it makes prep prior to shooting more difficult. Shop contamination by silicone brake fluid is a real thing. Fisheyes...

[Steve Simmons]

The bottom of my TC is so oily, it doesn't matter what kind of brake fluid leaks onto it! Nothing can penetrate the grime layer to get at the paint anyway.

I've heard the old tale about silicone fluid turning to sand in the combustion chamber, but never felt it held water. Even if you ignore the common confusion between silicon and silicone, in order for brake fluid to be sucked into the engine, you would have to fill the entire booster with fluid, which is not only a MASSIVE leak but there isn't enough fluid in the entire system to fill it up. And I'm not sure there would be adequate heat and time anyway, before it was shot out the exhaust valve along with all the unburned fuel and carbon bits. For the record, I've run DOT5 in a vacuum-boosted MGC for 15 years with no issue. Not that a dataset of one is a definitive study, but I've never heard of anyone else having the issue either. Admittedly this is a lot of assumption and guesswork on my part, but also many years of personal use and observation. So take it as you will!

One place you CANNOT use silicone fluid is in ABS braking systems. The agitation will introduce micro-bubbles into the fluid and the braking will suffer greatly.

[Duncan M]

IMG_1037.JPG (36.67 KiB) Viewed 3438 times

The illustration is not mechanically correct and is incomplete, and it is just really to show the two points of failure required in a booster system to start sucking brake fluid into the engine. First failure point is at end seals-- 2. The piston rod slides back and forth against these seals, and can eventually start weeping. The weep continues and fluid runs along the rod until it hits the air seals at two way valve-- 3 at end of rod. The seals fail on the two way valve and the fluid mists (under vacuum) as it leaks out past the two way valve, and into the one way vacuum valve-- and on into the engine. The fluid does not need to fill up the booster before it starts getting misted and sucked into the engine from the one way valve at top of booster.

I have seen this exact failure on more than one XKE. It is a known thing. However, in the E-type system there is an additional safety measure. Inline, between the booster and the engine is an air tank. They call it a pressure moderator, or something like that. Capacity at least a quart. I have seen that tank (not in illustration) completely full with brake fluid. Presumably once it became full and continued leaking, it was sucking brake fluid into the engine. Outlet of tank to engine is on top of tank. No doubt about it, it does happen. In both XKE cases the people had been adding fluid to the reservoir for years, thinking it was OK because there was no fluid leaking on the garage floor, and the brakes worked. Keep in mind, most systems do not have that tank.

Those E-types had glycol brake fluid, and as I say it took years to fill up the tank. Brakes still worked. To fix it (or prevent it from happening) it is best to rebuild the master and the booster 2-way valve at the same time. Or, replace the master and booster together. For any car, it could be difficult to determine if there was a slight leak like that into the engine, as the MC fluid levels drop as the pads are used up. You would have to know approximately how much it should be dropping with mileage, and if it exceeds that-- become worried. Better yet, after you replace the pads and bleed, etc., never add any more fluid. When you next replace pads, the fluid level should return to the full level--before bleeding.

Either type of brake fluid getting sucked slowly into the intake side of the engine is no good. Whether silicone fluid turns to sand at engine combustion temp is up for debate. If it does turn to sand, I bet it could still take a long time to destroy the engine, You might never figure out why that engine lasted such a short time.

[Steve Simmons]

Well I guess you can't really help someone who continuously feeds brake fluid into the system without wondering where it's going! To fill the booster on my MGC to the point that fluid would start getting sucked into the engine would take a whole lot more than a quart, and there isn't anywhere near a court in the whole system! In any case, I guess this doesn't affect anything on a TC.

Whatever byproduct comes from burned silicone won't be sand though, that comes from silicon (silica). While silicone does have silicon in it, it's a completely different thing altogether and is a synthetic unlike naturally occurring silicon. Silicone is found in everything from brake fluid to contact lenses to implants (you know what kind). I believe that if it's heated enough, you will get a super fine powder left behind. Nothing abrasive like sand, more like talcum powder as I recall. I'd have to brush up on that to remember exactly what happens.

Interesting discussion, even if a bit off topic.

[stephen stierman]

Gents, like I said originally, I have used silicone fluid since it became available in the market, what late 70's? I live in Ohio which precludes using these cars over several months in damp, cold, salty, rainy weather. I have read what all the engineers say and write and draw about compressibility, etc. etc. this that and the other. It does this, it's bad for that and on and on and on. So I have used it for thirty years I guess on road going vehicles that were properly prepared for its use, new rubbers, disassembly, cleaning and it is just fine and does what it is supposed to do. The brake cylinders don't seize up during the winter, if you have a leak the paint is not destroyed. (Duncan, sometimes, you just miss it!) I have never seen water come out in the mix when I had to open up a system. I will continue to use it on these old cars because I have years of ACTUAL experience with the stuff, not just reading what people write about it who have never used it. What you use is your business, really do as you like, but based on my climate and my usage, and experience it is just fine in my old cars.