The driver's pedal effort acts on a pushrod entering the vacuum servo, a diaphragm unit bolted to the bulkhead. The servo uses engine vacuum drawn from the inlet manifold through a one-way valve to multiply that effort, typically by a factor of three to four. Its output rod acts on the tandem master cylinder bolted to its front face, which converts the force into hydraulic pressure in two independent circuits, front and rear. A shared fluid reservoir sits on top of the cylinder, sealed by moulded seals, with a large O-ring isolating the rear face of the cylinder from the servo diaphragm.
Four Master Cylinder Variants Across Production
The brake master cylinder changed three times, producing four non-interchangeable assemblies. Early MGF non-ABS cars used one cylinder up to VIN AD000505. ABS-equipped MGFs used a different cylinder up to VIN YD525453. From YD525453 onwards, a third cylinder was standardised across both ABS and non-ABS builds and carried through to the end of MG Rover-era production.
The NAC-era MG TF LE500 then used a fourth cylinder specific to the post-2008 revival cars. Ordering therefore depends on both model year and ABS fitment. Repair kits differ: the pre-YD525453 non-ABS cylinder has its own kit; the two later MG Rover-era variants share a common kit.
Reservoir Cap and Seals
The fluid reservoir is sealed by a plastic cap incorporating a float switch that illuminates the brake warning light when fluid drops below minimum. The cap also changed at the YD525453 breakpoint, with the later version fitted to all MGFs from that point and all MG TFs, this is the cap catalogued on the live page. The reservoir itself seats into the cylinder via two moulded seals that harden with age and weep fluid around the reservoir base, a symptom often mistaken for a failing cylinder. A reservoir-and-seal kit is catalogued specific to each master cylinder variant.
A separate cylinder-to-servo O-ring is included in both the reservoir kit and the servo repair kit, so it does not need to be bought twice.
Brake Servo, Pre- and Post-MY2005 Variants
The servo changed at VIN 5D639634, identifying the MY2005 cars. MGF and pre-5D639634 MG TF cars use one assembly; MG TFs from that VIN onwards use the later assembly, and the two are not interchangeable. The live page lists the later post-MY2005 servo. A servo repair kit covers diaphragm and internal valve overhaul.
A hard brake pedal points to vacuum or servo problems, but is often a split vacuum hose or failed non-return valve rather than the servo itself, the hose should always be squeeze-tested before condemning the servo.
Servo Mounting and the Universal-Fit Gasket
The servo is retained by four M8 flanged nuts onto bulkhead studs, with distance tubes acting as spacers between the servo backplate and the bodywork. The servo-to-body gasket was originally fitted only from VIN WD29376, but the catalogue guidance is clear: this gasket should be fitted to all cars whenever the servo is removed. It prevents moisture ingress corroding the bulkhead panel from behind the servo, a far more expensive problem than the gasket. The servo pushrod is retained by a clevis pin and R-clip catalogued as a single service item.
Handed Vacuum Hoses, Four Variants
The servo vacuum hose is handed by steering position and differs between MGF and MG TF: four distinct hoses cover MGF RHD, MGF LHD, MG TF RHD, and MG TF LHD. Four clip types hold the hose to the engine bay, the brake pipes, the engine, and (on MGF only) the Hydragas pipe. Always order against the correct model and steering position, routing is not interchangeable. A split vacuum hose is one of the most common causes of a pedal suddenly becoming hard, and should be inspected at every major service.
Diagnosis and Common Faults
A sinking brake pedal, one that drops slowly when held under steady pressure, is the classic sign of a failing master cylinder: internal seals allow fluid to bypass the piston. This is safety-critical because the brakes function under light use but fail under heavy braking. A repair kit works if the bore is sound; a pitted or scored bore requires a new cylinder. Fluid weeping around the reservoir base is usually the reservoir seals, not the cylinder itself.