Macca and Sweller are spot on - BRAKE FLUID!

When was your brake fluid last flushed/changed? If you can't answer that question, or it was more than two years ago, change it now! Even if it looks good and normal in colour, change it. You cannot visually tell the moisture content of brake fluid.

Brake fluid, as has been mentioned, is hygroscopic (water absorbing) which means that as time passes water diffuses through the hoses and seals and becomes trapped in the fluid lowering its overall boiling point. Although, strictly speaking, it is actually the trapped moisture that boils (at ~100C) then causes pockets of gas (water vapour) to appear in the system which are compressible, unlike the fluid itself.

4% moisture in DOT4 fluid will lower it's overall boiling point from 230C to 150C, which makes a BIG difference to brake fade. 4% water absorption can occur in as little as a year in damper climates.

Just as an aside, DO NOT use DOT 5 fluid as it is silicone based and does not mix with all the other glycol based ones. It can only be used from scratch in a new dry system. Confusingly, DOT 5.1 is glycol-based and fine. Also, never use fluid to top up or replace from an already open or old bottle.

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