Right and wrong hydraulic fluid

LHM is a mineral hydraulic fluid, which must be used in vehicles with hydromatic suspension, such as Citroens, Rolls-royce Shadows, etc. Using DOT in these systems will wreck the seals, resulting in a expensive repair bill

DOT3/4/5 are synthetic, and are suitable for use in normal brake/clutch systems. Systems designed for DOT are not normally suitable for LHM, and using LHM may result in seal failure. Whilst not an expensive repair on a vahicle such as a Series or 90/110, brake failure is not something you want to happen whilst youre driving.

DOT3 is a rather old spec now, but more than suitable for older cars such as Series/90/110. DOT 3,4 and 5 are backwards compatible with each other, so a system designed for DOT3 will run quite happily on DOT4 or 5. However, if a system is designed for DOT5 then do not use 3 or 4. DOT5/5.1 is a very high spec oil and is unlikely to be required in any normal car over about 5 years old, it is designed for high performance use. DOT4 is usually suitable for most cars. Certainly it's not worth paying DOT5 prices if your car doesn't need it.

Alex

Thankyou, very useful info. I'll make sure it is DOT from now on. You live and learn.

David

But get it absolutely correct though! DOT [US Department of Transport] 3, 4 or 5.1 are compatible. DOT 5 is NOT compatible with these, so beware. Most brake fluids are DOT 4 or, rarely 5.1.

LHM is seldom used in car brakes though it is commonly used in heavy plant vehicles and tractors, though these may sometimes use ATF or Super Universal oil.

Using the wrong fluid in your system is extemely costly and ultimately dangerous.

Huw

On or around Mon, 11 Oct 2004 16:34:15 +0100, "Huw" enlightened us thusly:

I was about to say that. well done DOT for screwing up the system.

DOT 5 is (AFAIK) Silicone fluid used in racing brake systems. DOT 5.1 is the next in the sequence of DOT 3, DOT 4 etc. The main thing is that as you go up the DOT numbers, the spec of the fluid in such areas as boiling point and resistance to hygroscopy (absorbing water) increases - some modern brake systems may get too hot for DOT 3 to handle, and you *really* don't want to boil the fluid halfway through an emergency stop...

from memory, about the only thing that uses LHM is most of the hydropneumo Citroens (some use LHS - Liquide Hydrolique Synthetique, as opposed to Minerale for LHM) which use it in the suspension and the brakes - the systems are combined - and (some) Rolls-Royce which have a power-braking system on similar lines, I believe.

Putting brake fluid in an LHM system knackers it just as well as LHM in one designed for normal brake fluid, from what I hear. You can, in an emergency, use different fluid in a Citroen, but only for a short time and it should be flushed and replaced with LHM ASAP.

quite.

I tend to go with DOT 4 in everything these days, although you can still get DOT 3 it ain;t common and it ain't worth it.

Mind, some very old brake systems use Girling Crimson fluid (I'm thinking S1 brakes here) and I'm not sure without looking whether those systems are compatible with DOT 3.

LHM was introduced to automotive use by Citroen in 1967? and has been used ever since by Citroen in their combined hydraulic systems (brakes, steering, suspension, gearchange) and a short while later was adopted by Rolls Royce, who were (are) building the Citroen suspension system under licence. (As far as I know, no other car makers use it) This is a mineral oil, and replaced LHS2, which was similar to conventional brake fluid, but higher viscosity among other differences. The change corresponded to a change from natural rubber based seals, O-rings etc to synthetic rubber. I think the main advantage of the LHM is that it is a better lubricant, although the synthetic seals are perhaps not as good as the natural rubber ones. Note that compared to most cars Citroen hydraulic systems operate at much higher pressures, and do not use any edge seals, but use all O-rings. The height control valves do not use seals, but are made accurately enough not to need them. JD

On or around Tue, 12 Oct 2004 06:17:13 +1000, JD enlightened us thusly:

sometimes, I wish i could combine a citroen with a land rover. The citroen hydraulic system, when in good condition, beats the hell out of anything else.

all sorts of cunningness is possible: for example, on the BXs I had, the front brakes are fed from the full-pressure hydraulics, but the back ones are fed from the return from the rear suspension - thus, the back brakes are applied proportional to the load on the back suspension. without any extra gadgets to get out of order.

and, with some modification and longer legs, and maybe bigger spheres, you could get a beaut of an off-road suspension setup with adjustable height etc. you can also run ABS systems without an extra pump, since you already have the high pressure system to work with. In fact, the normal non-abs brakes haven't got a conventional brake cylinder or anything, the brake pedal operates a valve which feeds pressure to the calipers.

of course, if you lose the hydraulics, you lose *everything*, but with suitable attention that rarely happens, and almost always gives some warning. The system has an "accumulator" sphere as well as the suspension ones, so even if the pump belt fails, you still have pressure for a short while, and as soon as pressure drops, that big red "STOP" light comes on on the dash.

Yeah, untill it goes wrong, then you're completely stuffed. At least with a broken leaf or coil you've got some chance of remaining mobile.

Alex

In message , Alex writes

Here we are again - who remembers the old Girling v Lucas problems.