Rebuild your brake master cylinder

2 min read

QUICK FIX

Your two hour workshop

Renewing brake component rubbers should be part of your maintenance schedule, says Theo

THEO'S JOB CARD

MASTER CYLINDER REBUILD

Today, we will assess the condition of, then remove the master cylinder, clean it, strip it and assess the body itself for its suitability for reconditioning, the default always being to err on the side of safety and renew the cylinder or have the body professionally sleeved/reconditioned if in any doubt at all. New seals and other components will then be fitted prior to reassembly.

How tough?

You’ll need:

► Nitrile gloves, specs, PPE

► Bleed nipples (optional)

► Turkey baster or syringe (optional)

► Degreaser and methylated spirits

► Circlip pliers, screwdrivers, hand tools

► Brake fluid and rubber grease

► Brake bleeder

Specialists:

► Bigg Red, biggred.co.uk

Will take: 2 (HRS)

Will cost: 30 (£)

Cash saved: 400 (£)

START DIAGNOSE FAULT 45 MINS

A brake pedal that sinks or goes to the floor indicates master cylinder issues in the absence of any leaks, after bleeding.

TOP TIP Test the brake pedal with the engine running if servo-assisted.

Still unsure? Unscrew the brake pipe(s) from the master cylinder and fit bleed nipples in their place. Bleed the cylinder of any air and carry out the test. If the pedal still sinks, suspect the cylinder.

EXPLAINER

MASTER-CYLINDER FUNCTION

Brake fluid is incompressible and is the medium by which force from the driver’s foot, via the master-cylinder piston(s), is able to move wheel cylinder- and caliper-pistons at each wheel. It’s not a sealed system as it has to compensate for friction-material wear and fluid expansion, so at rest the seals uncover recuperation ports that open the circuit to the fluid reservoir. A residual-pressure valve may be fitted to a drum-brake master-cylinder to keep the wheel-cylinder pistons in contact with the brake shoes.

IMPORTANCE OF MAINTENANCE

Single-circuit systems have one piston and one circuit: if a seal/pipe/ hose fails, the footbrake is useless, the handbrake (secondary brake) being required to have at least 25 per cent efficiency as an emergency brake. Dual-circuit brakes have two pistons, each operating a separate circuit: if one fails, the other will still work (secondary brake), the handbrake is required to have 16 per cent efficiency. Glycol brake fluid absorbs moisture (hygroscopic) lowering its boiling point, requiring renewal at least every two years or soone,r but brake rubbers age and wear, too, requir

This article is from...

Practical Classics Magazine

23 November 2022

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