To prevent the fluid from slopping out of the cap get 2 or 3 of these rubber accordian looking parts from almost any Autoparts store to put inside the Girling cap. It will prevent the fluid from getting out and eating your paint. Actually I prefer Silicone so I don't have paint remover issues. Also someone had a good point not filling them up so full.
All of these have a perfect fit inside the Girling metal caps.
HELP! Part No. 42072
42072 Dorman Master Cylinder Gasket, Autozone $5.99, O'reilly, $6.99
42072 Motormite Brake Master Cylinder Reservoir Gasket, Advanced Auto, $6.39
BK 6751210, Brake Master Cylinder Gasket, NAPA brand Balkamp, $7.99
I guess the Kirkham billet can is ok if you like bling
I prefer the the cans that come with the Carbon Cars from Exact. You can be billet and have a very original appearance at the same time. It all depends on your taste I suppose.
Happy Motoring,
Richard