Originally Posted by
Burntboot
HB - Boosters practically never fail, when they do, the result is usually a massive vacuum leak from a breached diaphragm.
Check valves can go too (also very rare), but there are simple tests to rule both of these out (see Brenthlehr's post above)
After having been sitting for a year, I would be more inclined to do a full brake service then bleed the brakes again (don't forget about the 5th bleeder screw).
Nothing like seized callipers/sliders to make the brakes seem ineffective.
BB