The slipping is most likely caused by either a worn out/weak diaphragm, or very worn friction plate, worst is it may have oil on it if the rear crank seal is leaking.
The cables are a doddle to do around 15 minutes, put box end on first, then just lift pedal, this will cause the ratchet to open, then with thumb push ratchet foprward enough to slip cable end in, release pedal push a few times job done.
tabetha