I have bdr#260 built by speed fanitix. bought it used 4 plus years ago and three days after delivery...no clutch pedal
(big puddle on the floor, called Reg, he sent a new one to me free of charge! and I bought the car used. Thanks Reg!!!), been dealing with this issue ever since, not the puddle on the floor, but a clutch pedal that feels good and consistent in operation.
I have used the "as supplied" mazda B2200 unit, honed it, re-built it, installed new ones. installed High temp brake fluid (500F), read all the forum threads I could find, insulated lines, measure temperatures, btw could never even come close to 500 degrees around the clutch stuff...still nothing working! two issue for me was the bleeder on the bottom (can not bleed properly with out removing), not working consistently and consistently having to work on it!
Found a mazda b2600 slave, mount ear location further back, bleeder on the top, and it has an "o"ring seal on the piston in addition to the "V" seal. Installed this…clutch pedal has never felt this good and consistent in operation from hot to cold…it is the same. And it has lasted 3 plus months with no issues…a first! And you can bleed the system from under the hood…yeaha!
btw, the "O"ring/"V"ring piston assembly will fit in the b2200 housing, just don't get the advantage of the bleeder on the top.
My conclusion on this clutch situation is...air was entering the system at the slave...how? don't know...but it solved the problem.