hi Terry, It does sound like a duff master cylinder, (there is no spring on the rear piston) its a normal sliding caliper, when you bled the brakes did you bleed them in this sequence? L/H rear-R/H rear-R/H front-L/H front and you would need to do this at least three times.
The master cylinder is a dual channel device, the primary section is for the R/H front & L/H rear and the secondary for the others,when you apply the brakes the primary moves first and then due to hydraulic pressure it moves the secondary but if the seals on the primary section have failed/failing the secondary will only operate in the last bit of travel, but when you bleed them there is less fluid resistance and so the primary might work near to normal.
With brakes I would never trust a secondhand part unless it came with a certificate that it had been at least pressure tested or better overhauled.
nigel
July 2008
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