[Darren69]

They (those rear masters) are prone to heat failure either from fluid heat or heat from the engine. 748/916 in particular since on those they are bolted to the engine direct. Just be sure leave loads of free play in the lever to piston adjuster and take it for a short run up and down the street to make sure it's not dragging even slightly enough to heat the fluid.

If it heats up enough and has no way to release the pressure it could lock the wheel and cause expensive damage to caliper and disc.