Understanding the Lucas Girling Hydraulic Brake System
The Lucas Girling brake system shows up in a lot of British cars from the 1950s through the early 1980s. You will find them in MGBs, Triumps, Minis, and even some Jaguars. The system is fundamentally straightforward — a dual-circuit master cylinder feeding flexible lines to calipers or wheel cylinders, with Girling being the division that handled the actual brake components. The diagrams you see online vary in quality, and a lot of the ones floating around are traced from vague reproductions rather than original Lucas service manuals. I have spent more time than I care to admit tracking down accurate diagrams for these systems. The original Lucas publications are out of print and expensive if you can find them. Many sites offering downloads have either pixelated copies or diagrams pulled from conflicting model years. The only way to verify you have the right one is to cross-reference part numbers against the vehicle's build date and brake assembly code stamped on the master cylinder reservoir or backplate. For the MGB, the Lucas Girling system used a split-circuit master cylinder — separate circuits for front and rear. The diagram should show a 12mm feed line from the master to the front calipers and a separate route to the rear drums. If your diagram doesn't show that separation, it's probably for a single-circuit early model or the wrong car entirely. The 1968 to 1974 MGB GT had a different proportioning valve arrangement compared to the roadster, and the diagram needs to reflect that difference.
Triumph TR6 and Spitfire systems differ in master cylinder bore size and line routing. The TR6 uses a larger bore master — around 1 inch compared to the Spitfire's 7/8 inch — and the diagram should show correspondingly different line diameters and reservoir dimensions. I once spent three weeks troubleshooting a soft pedal on a TR6 only to discover the diagram I was following was for a Triumph 2000, which has a completely different suspension geometry affecting brake line routing. That wasted a week of work I could have avoided with a proper diagram cross-reference. The practical workaround I ended up using for ambiguous cases is to photograph the actual brake assembly on the car, note the casting numbers on the master cylinder and each caliper or wheel cylinder, and then match those numbers to Lucas parts catalogs. The diagram becomes secondary to the part numbers. Original Lucas brown paper parts catalogs are available through specialist suppliers and they list every variation by chassis number range.
Common Pitfalls When Working From These Diagrams
One thing most people miss is that Lucas and Girling were not always consistent about marking diagrams with revision dates. A diagram might have been updated mid-production without a visible change mark. The proportioning valve on later MGBs was relocated from near the master cylinder to underneath the car near the rear axle, and an older diagram will show the wrong position. If you follow that version, your hard line routing will be wrong and you will end up bending pipes to fit something that was never the correct layout. Another issue is the distinction between Girling and non-Girling components on the same car. Some models mixed Girling calipers with Lockheed or Bendix wheel cylinders in the rear. A complete system diagram that assumes uniform branding will confuse the pipe fittings because each manufacturer used slightly different flare angles and thread pitches. The Lucas official stance was that all components should mate, but in practice you can run into mismatched parts on refurbished or swapped assemblies. The diagrams also rarely show the bleed sequence clearly. On a dual-circuit system with a proportioning valve, bleeding the rear circuit first is often the wrong approach. The correct sequence depends on whether your master cylinder has a residual pressure check valve in the rear circuit. If it does — and most Lucas units for drum-brake rear circuits do — you need to bleed the rear wheels before the front, otherwise trapped air in the proportioning valve can make the pedal feel spongy even after a proper bleed. The diagram itself will not tell you this, but the master cylinder casting usually has a part number you can look up to confirm the valve configuration.
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What the Diagram Won't Tell You
A brake system diagram is a starting point, not a complete guide. It will show you where the lines go and what fittings connect to what, but it will not tell you about the torque specifications for banjo bolts, the type of sealing washers required, or the fact that Lucas recommended a specific brake fluid for their rubber seal compatibility. The original Lucas specification called for a DOT 3 fluid with specific rubber compatibility, and modern synthetic fluids can cause the original Girling seals to swell or shrink depending on their formulation. This is not something any diagram covers. The limitation of working from a diagram alone is that you also cannot see wear patterns or the condition of the flexible hose internals. Lucas Girling brake hoses from that era used woven nylon reinforcement, and the outer rubber casing can look fine while the inner lining is deteriorating and causing a restriction. This creates a situation where the brake on that wheel applies slowly compared to the others, leading to uneven wear and a pedal that feels inconsistent. A diagram cannot help you diagnose this. The only real test is to clamp the line temporarily during a brake application and see if the pedal firmness changes. If you are restoring a car and need an accurate diagram, the best path is still the original Lucas workshop manual for that specific model. These are available as printed reprints from suppliers like Haynes or as digital PDFs from specialist archives. Third-party diagrams found on free forums are useful for quick reference on general layout, but they should not be the only source you rely on when actually routing pipes or specifying components. The cost of a wrong pipe or a mismatched fitting is far higher than the price of an official manual.
