Jamie, are you losing any fluid from the clutch master cylinder? By small resistance I presume you mean the pedal feels soft?
If you're losing fluid from the master cylinder you have an obvious leak somewhere and air is getting into the system.
Most leaks occur from the clutch SLAVE cylinder as it does all the work .. If there's no leak from the clutch slave cylinder, suspect the seals inside. It's like a bicycle pump ..the seal inside wears and it won't pump..
You may be able to get an overhaul kit. I'm in the UK .. but here's an Ebay link for a slave cylinder overhaul kit just to show you what it looks like:
FORD CORTINA MK2 CLUTCH SLAVE CYLINDER REPAIR KIT SP2085
Here's a link to show you what the slave cylinder itself may look like (it may NOT be for your vehicle..)
CLUTCH SLAVE CYLINDER FORD FIESTA ESCORT
It's easier and quicker to fit a new slave cylinder than it is to strip down and replace seals with a kit. Be aware, after overhauling or replacing a slave cylinder you have to bleed it - remove the air.
The easiest way of doing this is to make sure the reservoir is fill of fluid, crawl underneath the car with a spanner/wrench, place a rag over the bleed point and get somebody to pump the pedal a couple of times while you undo then tighten the bleed nut.
If it's not the slave cylinder at fault- then it's the master cylinder. Master cylinders also have a 'piston' and rubber seals which wear/perish.
However, it's more than likely that it's the slave cylinder at fault