The facts about Rolls-Royce and Bentley Hydraulic Brake Fluid options

At its core, the RR363 is akin to DOT4 brake fluid, but uniquely blended with a castor oil-based lubricant. This unique formulation was deemed essential for the Silver Shadows. Their intricate hydraulic systems, laden with numerous moving mechanical parts, require consistent lubrication to avoid undue wear.

The older one (RR363), is effectively a DOT4 brake fluid with a castor oil based lubricant. The reason you cannot just use normal DOT4 brake fluid is because the hydraulic system on Silver Shadows have a lot of moving mechanical parts. They need lubrication and if you don't use the right fluid you could cause excessive wear.

One option people consider is adding Castor oil to their normal DOT4 brake fluid (mainly due to the cost of RR363). The problem with that decision is that the oils have a different weight so what you will find in the reservoir is that they level out and you end up with castor oil at the top and brake fluid at the bottom (or the other way around) and it doesn't mix. They developed this particular RR363 fluid with a dispersant that keeps the castor oil suspended so that it's constantly in there lubricating the system.

One of the biggest problems with RR363 is that it is hygroscopic so it actually attracts water and gets moisture in it. What that does is that if you don't regularly change the fluid and flush it through, you end up with a lot of rust building up inside the system. It affects everything... so if you try to open an old caliper from a Silver Shadow your piston will be all rusty and horrible inside. Whereas with the mineral oil (RH15215), because it is basically oil, when you take it out there is no rust, they are usually much better preserved. So I think that is why they developed the mineral oil system.

The big problem of course is that if you put mineral oil in a Silver Shadow, or brake fluid in a Silver Spirit, you will contaminate all the rubber components - and there is a lot of them. So your whole hydraulic system, eg. reservoir, brake pumps, valve bodies, accumulators, g-valve, rams, height control valves, calipers, rubber hoses, everything will have to be removed, completely resealed and the whole system flushed if you contaminate it - and it doesn't take a lot. If even if a little bit of the wrong fluid goes in then the system is going to be contaminated and you are going to cause the rubber to swell up and lose brakes.

So when you open up an accumulator that had contaminated oil in there, the diaphragm goes from a normal size to almost like a balloon and goes soft. So the biggest "no-no" is to put the wrong fluid in your car. Always make sure when you are topping up that you have the right fluid. There are warning labels on the reservoirs but we have seen several recently where it has happened - and it does happen. With the Silver Shadow up to 1980 they use the RR363 and the Corniche obviously carries on from pre-1980 to after and then they go to a mineral system on the Corniche but it's chassis 50000 series early chassis numbers and then 17 digit chassis numbers. If your car has a 17 digit chassis number it will be on mineral oil but still check, always double check. So the mineral oil (RH15215) is in this newer bottle. These aren't the original bottle - it's a Comma version of the mineral oil because the Rolls Royce one or the Bentley one is currently on back order.

They used to have a castrol bottle (500ml) but now it comes in this one litre bottle and the castrol version is now also a Bentley branded version. You do need one of these special spouts if you are going to fill your reservoir up. The reservoirs have an awkward little filling spout. The early ones have a green cap. There are two reservoirs and the green cap and they have a lead seal with a wound wire around it to stop people from opening it up and pouring in brake fluid

The idea is that you can ONLY use mineral oil filling bottle to fill it up, that is to try to stop people putting brake fluid in it. So, you will need one of these filling spouts (filler tube assembly: RH15233) if you are going to top it up. You can't just buy a bottle and pour it in. Originally the bottle came with this filler tube assembly and if you look in the boot or trunk you may even have a spare bottle of mineral oil with a filler on it, which you can just use to decant the mineral oil into and use that to fill up the car. Mineral is a much better system just because of the rust and contamination, because when you open up a reservoir of a Silver Shadow a lot of the time it is full of gunk and sludge and that's why hydraulic services require you to flush it out regularly. People very rarely do that because it's very expensive.

The mineral oil system went all the way up through to the end of the Crewe era, before Bentley was bought by Volkswagen and Rolls-Royce was bought by BMW, then when that happened and the Bentley Arnage and Rolls-Royce Silver Seraph were introduced they move over to a more BMW based system, which uses this central hydraulic system fluid. So on the Arnage and the Seraphs you have again a pump that drives a high pressure fluid which does your suspension and your steering and your brakes. So that is why they call it central hydraulic system, because it does those three items. It is effectively power steering fluid but it is used in the suspension and brakes as well. It's not the actual fluid that goes into the calipers, it applies pressure to a brake booster which is then attached to a normal type of master cylinder, which when you put your foot on the brake, pumps DOT4 brake fluid to the calipers. So you have got a regular brake fluid system with a booster, a high pressure booster attached to it which gives you extra braking force. For cars earlier than 1965 you can use normal DOT 4 brake but it is recommended to stick to RR363!

That is all for now but please comment if you have any questions - we'll be happy to answer them for you.

Thanks for watching!

Matt Duncan, IntroCar