The Evolution of Rolls-Royce and Bentley Braking Systems: A Deep Dive into Hydraulic System Bleeding
In 1967, the automotive world witnessed a noteworthy shift when Rolls-Royce introduced the Silver Shadow. Contrasting its predecessors' servo-assisted braking, this marvel embraced a high-pressure hydraulic braking system. While foundational components mirror the Silver Spirit, differences emerge in fluid types, accumulators, and valve bodies. Notably, the Silver Shadow excludes a Master Cylinder, an unconventional choice.
Hydraulic Mechanics: Brake Pump to Brake Disc
- The Central Unit - Brake Pump: Fluid from the reservoirs gets funneled through a protective shroud into the brake pump. This mechanism generates a high-pressure output directed into the valve body.
- Regulation via Valve Body: Attached to the sphere, the valve body governs the pump-produced pressure. The sphere houses a diaphragm with gas pressure on one flank. This configuration not only retains the pressure but also ensures a surplus for reserve.
- The Braking Action: Pressing the brake triggers the plunger within the distribution valve. As a result, pressure courses through to the calipers, activating the pistons. The ensuing compression makes the brake pads connect with the brake disc, bringing the wheels to a halt.
- Early Silver Shadow's Unique Setup: In its nascent phase, the Silver Shadow integrated a master cylinder. With four calipers upfront, two pairs of pistons manage diverse pressures - one from the high-pressure system and the other steered by the master cylinder. This redundancy operates as a backup mechanism, with the master cylinder contributing significantly to pedal feel.
In essence, redundancy and precise engineering characterize this system. A dual presence of brake pumps, valve bodies, accumulators, and distribution valves ensures reliability.
Bleeding the Hydraulic System: A Step-by-Step Guide
- Prepare the Reservoir: Begin with a pristine reservoir. Utilize the sight glasses to gauge levels by removing the covering panel. If the glasses are opaque, replace them and ensure cleanliness.
- Reservoir Filter & Hoses Check: Examine the reservoir filter for sediment buildup. Periodically replacing rubber hoses, ideally every 60,000 miles, is critical to maintain optimal pressure flow.
- Depressurizing: With the ignition on (and engine off), depress the pedal between 70 to 100 times. Once low-pressure lights activate, maintain the rhythm to ensure complete depressurization.
- Bleeding Sequence:
- Identify the bleed screw on the rear caliper associated with the master cylinder.
- Follow conventional bleeding for the master cylinder: depress the pedal, open the bleed screw to release fluid, close the screw, and release the pedal.
- With the engine running, maintain around 1000rpm until the pressure stabilizes.
- With a steady pedal press, initiate bleeding at each caliper. Monitor hydraulic reservoir levels religiously to avoid air infiltration.
- Note: On the front, there’s a dual reservoir setup. The primary compartment aids the front brake pump, hydraulic accumulator, brake distribution valve, and the front calipers. Additionally, always check the height control for potential bleeding if air intrusion occurs.
Subtle Reminders:
- Always prioritize safety and system cleanliness.
- Require parts or further assistance? Contact IntroCar's parts team for guidance : sales@introcar.com.
Authored by: Matt Duncan, IntroCar. Delve into the detail with our technical video. Your trusted source for Rolls-Royce and Bentley knowledge.
Full Video Transcript
More about bleeding the hydraulic system for early & late Rolls-Royce Silver Shadow, Silver Spirit and Bentley T series - or Power Braking cars.
In 1967, when the Rolls-Royce Silver Shadow was introduced, it had a completely different type of braking system from it's predecessors. The cars before that had a servo assisted braking system and the Silver Shadow had a high pressure hydraulic braking system.
The components on this system are basically the same as the Rolls-Royce Silver Spirit, but with different types of fluid, accumulators and valve bodies. There is also no Master Cylinder.
So, it all starts with the brake pump.
This is fed hydraulic fluid from the reservoirs into a little shroud that sits around it. The pumps produce high pressure into the valve body. The valve body is screwed onto the sphere and this regulates the pressure that's produced by the pump. The sphere has a diaphragm in it with gas pressure on one side, and that stores the pressure and allows a reserve of pressure.
The valve body then regulates that pressure into the system and it's stored there. When you put your foot on the brake this depresses the plunger in the distribution valve, allowing the pressure to flow through to your calipers, where the pistons are.
As the pressure goes into the caliper, the pistons squeeze, the brake pads then make contact with the brake disc and bring the wheels to the stop. The early Silver Shadow had a master cylinder also. There were four calipers on the front, all supplied with high pressure fluid, then on the calipers there are four pistons. One pair of pistons was supplied high pressure fluid from the high pressure system and the brake pumps and the other set of pistons was fed by the master cylinder... a bit like a backup system.
If you're just pressing and opening and closing a valve, that's not going to give you any pedal feel so the master cylinder provides the pedal feel.
There are two of everything except the master cylinder: two brake pumps, two valve bodies and accumulators, two distribution valves, one master cylinder on early cars, four front calipers and two rear calipers.
Bleeding the System
Always start with a clean reservoir. There are sight glasses in the reservoir. You can take the panel off that shows the levels - there are four screws on each side that allow the sight glasses to be exposed.
Drain the reservoir first and then clean the sight glasses or replace them. If they're no longer transparent, and you cannot see through them, then there is an o-ring behind the sight and it all needs to be completely cleaned.
The filter in the reservoir needs to be checked as you can get a gunk build up in the bottom of the reservoir which means that the fluid will not flow properly to the brake pump. Also, f you're bleeding the hydraulic system, you want to make sure that the hoses are in good condition. Rubber hoses tend tend to swell up internally over time. As a service item they should be changed every 60,000 miles. If not replaced then there will not be a proper flow of pressure through the hoses. So always make sure that your hoses are in good order and the reservoir is clean.
To depressurize the system, which can be done by pumping the pedal continuously about 70 to 100 times. Leave the ignition on without the engine running and keep pumping the pedal before long the red lights will come on to show that you've got a low pressure then keep going as there could still be some pressure in the system. After 70 -100 pumps it should be sifficient to bleed the master cylinder.
Leave the engine off as you don't want the pressure to build up again. You then check which bleed screw on the rear caliper is being fed by the master cylinder, apparently on early cars it's top or bottom, and it switches. So you have to make sure you are checking which bleed screw is coming from the master cylinder.
Once you know that you can bleed this like a normal master cylinder, which means you hold your foot down on the pedal, crack the bleed screw and the fluid comes out. Then, close the bleed screw and lift the pedal up. Then you're allowing the fluid to get pulled in through here, not back up through the pipe from the caliper.
You keep doing that until you're getting no air through. Once you know that the master cylinder system is bled out properly then you can start bleeding the power system. To do that you run the engine and allow the pressure to build up. Hold it at fast idle, which is about 1000rpm until your pressure is up. The workshop manual says to turn the engine off, but you can do this with the engine running.
As long as there is full system pressure, then you can bleed the brake system. Press your foot down on the pedal and leave it down, without pumping it. What that does is to push the plunger in the distribution valve, allowing the pressure to flow through to the calipers. Then go round to the calipers and crack off the bleed nipples one by one, allow them to flow until all the air is out.
Make sure that your hydraulic reservoirs are topped up. If you run dry in the reservoir, then air will suck back through the system. Do all four calipers on the front. One by one, allow the fluid to come through the bleed tube.
Be careful on the back as if you hold your foot down on the head and crack off the bleed screw too quickly you can actually cause the limiting valve to come in. So you just need to be careful and treat it a bit more carefully.
Notes
1. The front reservoir split into two parts - front and back, the front reservoir compartment supplies fluid for the front brake pump, the front hydraulic accumulator, the lower brake distribution valve, and the front calipers of the front brakes.
2. The height control also would need to be bled it's had any air introduced into it.
For more information please refer to the video...!
Matt Duncan, IntroCar