Brake master cylinder

01

Size the bore and match the brake circuits

Akebono describes the hydraulic chain of a disc brake: pedal force is amplified by the brake booster, converted into hydraulic pressure by the master cylinder and carried by brake fluid to the pistons in each wheel brake, which press the pads against the rotors. [3] US Federal Motor Vehicle Safety Standard No. 135, which covers light vehicles up to 3,500 kg gross vehicle weight rating, requires a master cylinder to have a reservoir compartment for each service-brake subsystem it serves. [1]

For split service-brake systems, FMVSS 135 allows the brake warning to be triggered by a pressure difference of 1.5 MPa (218 psi) between the intact and failed subsystems, measured at a master-cylinder or slave-cylinder outlet. Vehicles without a split system must also have an audible warning for gross fluid loss. [1]

Editorial calculation: a 22.2 mm (⅞ in) bore has a piston area of about 388 mm²; a boosted pedal force of 2,000 N on the push rod produces about 5.2 MPa (2,000 ÷ 388) of line pressure. Halving the bore area roughly doubles pressure for the same force but also halves the fluid displaced per millimetre of stroke, so bore and stroke must be matched to the caliper and wheel-cylinder volumes.

Editorial note: on replacement parts, match bore, outlet thread and port layout, and number of circuits to the original part number; changing bore changes pedal feel and brake balance.

Give the supplier the vehicle or machine model and OE part number, bore and stroke, single or tandem design, outlet threads and port positions, mounting flange and push-rod length, booster type, reservoir type (integral or remote) and sensor, brake-fluid grade, new or remanufactured, and quantity.

02

Check reservoir capacity, compartments, labelling and warning

FMVSS 135 requires total reservoir capacity at least equal to the fluid displaced when all wheel cylinders or caliper pistons move from new-lining, fully retracted positions to fully worn, fully applied positions, and completely separate compartments for each subsystem. Where part of the reservoir is shared by two or more subsystems, each partial compartment must hold at least the volume displaced by its master-cylinder piston over a full stroke. Losing fluid from one compartment must not empty another. [1]

The standard requires a brake-system warning when the fluid level in any master-cylinder reservoir compartment drops below the manufacturer's recommended safe level or one-quarter of the compartment's capacity, whichever is greater (one of the manufacturer's options for detecting gross fluid loss). It also requires a permanent label within 100 mm of the filler cap, in letters at least 3.2 mm high: "WARNING: Clean filler cap before removing. Use only ___ fluid from a sealed container", naming the fluid grade under FMVSS 116. [1]

FMVSS No. 116 defines the DOT 3, DOT 4 and DOT 5 grades, with minimum dry boiling points of 205, 230 and 260 °C, and distinguishes DOT 5 silicone-base from DOT 5.1 non-silicone-base fluid. [2]

Linked suppliers list brake master cylinders for forklifts and wheel loaders, servo master cylinder assemblies and automotive master cylinders. [4] [5]

03

Forklift, wheel-loader, servo-assisted and automotive master cylinders from four listed suppliers

The listings reviewed cover automotive tandem master cylinders, forklift and wheel-loader master cylinders and servo assemblies, listed as replacements by OE number; ask each for bore, outlet threads, reservoir arrangement and pressure test data.

04

Bore and pistons, tandem chambers, reservoir, level sensor, seals and mounting

For light vehicles up to 3,500 kg GVWR, three of these rows — chambers, reservoir and level sensor — are set by FMVSS 135 rather than left to the maker. [1]

05

Bore, stroke, burst pressure, reservoir volume and materials for a master cylinder quotation

No reviewed source publishes bore, burst-pressure or leakage values for these parts; the bore calculation above shows how bore sets line pressure, and FMVSS 135 fixes the minimum reservoir volume. [1]

06

Send the vehicle, hydraulic and fitment data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Vehicle: [make; model; OE number]
Design: [single; tandem; bore; stroke]
Ports: [threads; positions]
Mounting: [flange; push rod; booster]
Reservoir: [integral; remote; sensor]
Fluid: [DOT grade]
Condition: [new; remanufactured]
Quantity: [per part number]

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