Brake fluid is one of the most safety-critical consumables on any car, and on a classic it deserves more attention than most owners give it. The fluid in the system is hygroscopic, absorbing water from the atmosphere through micro-pores in the rubber hoses and through the seal between the master-cylinder cap and the reservoir, and the water content rises steadily over time. As it does, the boiling point of the fluid falls, and on a heavy braking application the fluid in the calipers can boil, vapour-lock the line, and produce a pedal that goes to the floor with no braking effect at all. Renewing the brake fluid every two years, regardless of how much the car has been used, is the simplest and most important brake maintenance operation an owner can carry out, and a full change means draining and bleeding through every wheel cylinder until clean fluid comes through, rather than simply topping up the reservoir.
DOT4, the Standard Specification
DOT4 is the standard specification for the majority of the classic MG range, the MGB, MGC, MGB GT V8, Midget, and most contemporaneous British classics using it in both the brake and clutch hydraulic systems. DOT4 is a glycol-ether based fluid with a higher boiling point than the older DOT3 specification, fully compatible with the rubber seals used in the classic master-cylinder, slave-cylinder, and caliper assemblies, and the modern equivalent of the original Girling and Lockheed fluid that filled the systems when the cars were new. It is stocked in standard grade for normal road use, in a higher-specification React-type grade with raised dry and wet boiling points for performance and spirited road use, and in a competition-grade fluid offering the highest boiling points for track and hill-climb use. On the earlier T-types the clutch is mechanically operated and does not use hydraulic fluid, so brake fluid serves the braking system only on those cars.
DOT5 Silicone, for Storage & Concours Use
DOT5 is silicone-based and is not interchangeable with DOT4, as the two chemistries are immiscible and mixing them in a single system causes immediate problems with seal swelling and fluid separation. It has two specific use cases on a classic.
The first is long-term storage, as silicone fluid does not absorb water, does not eat paint if it leaks onto bodywork, and is genuinely stable over years of disuse, making it much easier to live with on cars laid up between events. The second is the concours-spec car where engine-bay paint protection is the priority, since DOT5 cannot damage the surrounding paintwork the way DOT4 will if it leaks. The trade-off is that DOT5 is harder to bleed cleanly, tending to entrap small air bubbles that DOT4 would carry through, so it is generally not recommended for cars used hard on the road, for cars whose system has recently been overhauled, or for any car with ABS, where the rapid cycling of the modulator aerates silicone fluid and compromises braking. Changing a system from DOT4 to DOT5 requires flushing every seal, line, and component clean of the old fluid first, and many owners renew the rubber components at the same time.
Clutch Fluid & Compatibility
On the cars where the clutch hydraulics share fluid with the brake system, the brake fluid specification is also the clutch fluid specification, and renewing the clutch fluid at the same time as the brake fluid is sensible practice, as the clutch slave cylinder seal is exposed to the same hygroscopic-degradation issues as the brake calipers. Mixing DOT4 from different brands is fine, as all current DOT4 conforms to a common specification, and DOT4 is an acceptable higher-spec replacement for DOT3, but mixing DOT4 and DOT5 is not acceptable under any circumstances. A copper-based brake lubricant or brake-rated silicone grease is stocked for use on caliper slide pins, pad backplate contact points, and shoe contact surfaces, as ordinary lithium or petroleum grease will attack the caliper's rubber boots and seals, and the brake tools section covers the vacuum and pressure bleed kits that make a clean full change straightforward.