A bad master or slave cylinder. The entire master and slave is, at least if you buy it from the dealer, a sealed and filled assembly that is ready to install. I would guess the entire thing can be easily swapped in 30 minutes. I removed mine in one piece in about ten minutes and reinstalling the new one shouldn't take more more time than that. Unbolt, remove, position the new assembly, bolt it in. It's easier to do and far less complicated to do than it may appear.