Clutch slave cylinders/seals and brake fluid can be almost as emotive as good old engine oil, everyone has an opinion. Well I’ve just changed brake/clutch fluid after about 15 months, usually I’d do it every 6 months or so, but somehow I’ve ended up getting a bit lax and let it slide. So for what it’s worth, here’s how things have worked out on my own Capo over the years.
I think everyone will agree that it’s the clutch fluid that takes the most punishment, followed by the rear brake. So here’s a couple of photographs of the inside of the master cylinder straight after opening it and a comparison of the fluid drawn off, with straw colour of new fresh oil from a sealed container. No black residue in the master cylinder and only a slight colour change in the oil with almost no cloudiness. The fluid drained from both front and rear brakes was almost like new, but it’s nice to keep the fluid fresh anyway.
In comparison the oil I used to drain at 6-12 month intervals from the clutch was cloudy with black residue in the master cylinder and the rear brake was frequently amber in colour, only the front brakes seemed to show little degradation – all this was while using DOT4!
Todays oil is DOT 5.1 and has done 27,143 miles over 15 months
I’ve been using DOT5.1 now for the past five years and the first (10 year 44K miles) clutch slave seal was replaced in late 2013, not because it failed, but because it seemed to be letting a little air back into the system after long (24Hr+) runs – enough to give an extra 4-5mm or so of free play at the ball end of the lever otherwise it seemed fine in normal day-to-day use. Seal replacement was purely precautionary. The replacement seal has been used with DOT5.1 its entire life (2½ years 45K miles) and is showing no sign of leakage or air ingress. So to date it’s cost me one seal (€17) versus going out and buying a €90 aftermarket cylinder … I know which mast I’m nailing my colours to!
And finally ….. always ALWAYS make sure the level of fluid in the reservoir is right. It is so damn easy to overfill this one. My prefered method is to drain off most of the reservoir fluid once the system is bled (don’t expose the ports) then refit the rubber bellows and use a syringe to inject fluid back in while watching the bubble. Leave quite a big bubble showing, because when you screw down the cap the volume (and bubble) shrink a little.