Forum thread about the question
http://www.rollsroyceforums.com/forums/26-sy-series-i-ii-1965-1980/279-silver-shadow-brake-issues-your-help-would-appreciated.html
Source: The Rolls Royce Forum - England

New disposable substitutes
The original SY accumulators have been designed to be serviced but they have become a rare item. So Flying spares sells new units "suitable for both SY and SZ" : See Here
Source: Flying spares website