Triumph Repair: Brake Fluid for Spitfire, spitfire 1500, seal materials

Question
Jim,
I have a 1979 Spitfire 1500.  I have experienced the brake petal traveling all
the way to the floor.  I can pump the pedal and regain braking power.  This
happens very infrequently, but I have a new master cylinder and plan to
replace the old one.  I have also read some articles to determine what type of
brake fluid to use.  One told me to use Dot 5 and the cap on the new master
cylinder says use only Dot 3.  I have also been told that if I use the wrong
type it will swell the seals or damage them.   Which is the best brake fluid to
use with my new Lucas master cylinder?
Thanks,
Bill  

Answer
Bill,

If you're not going to rebuild the entire brake system then I'd recommend going with DOT4 fluid, specifically Castrol LHA (Low Humidity Absorption) brake fluid.  

There has been a big debate about DOT5 (silicone) and it's compatibility with different seal materials.  I've known LBC owners who've run DOT5 for more than a decade without incident.

But if you're going to change to DOT5 you really should replace all the rubber bits at the same time.



Cheers,

Jim