Can not find retaining nut for clutch master cylinder.

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

Clutch master cylinder is leaking out of piston cup.Detach from clutch pedal, disconnect hydralic line, and remove 2 firewall retaining bolts or nuts and pull out. Replace clutch master and bleed line at clutch slave cylinder.

In this car, there is no bleed port provided. The hydraulic clutch system is closed and is sold only as a complete set of master, reservoir, slave, and line. The set is priced at $130 at autozone.com. I have pasted the replacement instructions from autozone.com below. Please let me know if you have more questions.

See Figures 1, 2, 3 and 4
The master cylinder, pipes and slave cylinder are a complete assembly and must be replaced as a single unit.

1. Block the clutch pedal to prevent it from being depressed while the slave cylinder is removed from the transaxle.
2. Remove the air cleaner/intake duct assembly.
3. Disconnect the negative battery cable first, then the positive cable.
4. Remove the battery hold-down retainer.
5. Remove the battery from the vehicle. Store the battery in a safe location, but do not place it on a concrete surface for any long period of time or it will discharge.
6. Remove the battery tray. Note that one battery tray mounting fastener is only accessible through the fender well.
7. Rotate the slave cylinder about 1 / 4 turn counterclockwise while pushing toward the bell housing, in order to disengage the connector and remove the cylinder from the clutch housing. Remove the slave cylinder bracket retaining nuts and pull the assembly from the studs.
8. If equipped with ABS, remove the brake master cylinder-to-power booster mounting nuts. Move the master cylinder off of the mounting studs and slightly toward the engine, being careful not to bend or kink the brake lines.
9. Remove the master cylinder pushrod retaining clip from the clutch pedal pin and disconnect the pushrod from the pedal.
10. Rotate the clutch cylinder about 1 / 8 turn clockwise and remove it from the instrument panel. Remove the hydraulic assembly from the vehicle.

To install:

1. Install the clutch master cylinder into the dashboard with the reservoir leaning toward the driver' fender. Rotate the cylinder about 1 / 8 turn counterclockwise to lock it in position.

When installing a new assembly, the plastic retainer straps should remain in place on the slave cylinder, to ensure that the actuator rod seats on the release fork pocket upon installation. If reinstalling an assembly, be sure to position a new plastic retainer strap onto the end of the pushrod and attach the straps to the cylinder.

1. If equipped with ABS, place the brake master cylinder into position on the power booster mounting studs. Install and tighten the master cylinder-to-power booster mounting nuts to 20 ft. lbs. (27 Nm).
2. Slide the slave cylinder onto the clutch housing studs, then install the nuts and tighten to 18 ft. lbs. (25 Nm).
3. Insert the slave cylinder into the clutch housing with the hydraulic line facing down and rotate about 1 / 4 turn clockwise while pushing it into the housing.
4. Lubricate the clutch pedal pin with silicone grease, then connect the pushrod to the clutch pedal and install the retaining clip.
5. Install the battery tray. Tighten the battery tray mounting fasteners to 89 inch lbs. (10 Nm).
6. Place the battery into the battery tray. Be careful that the battery terminals do not short against any metal during the installation.
7. Install the battery hold-down retainer.
8. Connect the positive battery cable first, then the negative cable.
9. Install the air cleaner/intake duct assembly.
10. Remove the block from behind the clutch pedal and, if equipped, properly enable the SIR system.
11. Start the engine and check the pedal for proper operation.

Fig. Fig. 1: Block the clutch pedal to prevent slave cylinder damage, should the pedal be depressed while the cylinder is removed from the transaxle


Fig. Fig. 2: To remove, rotate the slave cylinder about 1/4 turn counterclockwise while pushing toward the bell housing


Fig. Fig. 3: This plastic retaining strap must remain on the slave cylinder to ensure proper seating of the actuator rod against the release fork

Fig. Fig. 4: Rotate the clutch master cylinder about 1/8 turn clockwise and remove it from the instrument panel

HYDRAULIC SYSTEM BLEEDING The clutch hydraulic assembly has been filled with fluid and bled of air at the factory. Do not attempt to bleed the hydraulic system. While the unit does not require periodic checking, it must be serviced, when necessary, as a complete assembly. The system is full when the reservoir is half full.
Only DOT 3 brake fluid should be added to the system. If the fluid level drops, inspect the system, including the slave cylinder, for leakage. A slight wetting of the slave cylinder surface is normal. Fill the clutch master cylinder reservoir with brake fluid. Be careful not to spill brake fluid on the painted surface of the vehicle.

It part of the brake master cylinder.

The brake system and the clutch master cylinder have the same fluid tank, look at the brake master cylinder fluid tank, on the right hand side about half way up is a hose that goes over to the clutch master cylinder, if fluid level is not above this hose the clutch will stop functioning. If the clutch slave cylinder, mounted on the front side of transmission bellhousing, leaks then the clutch will stop functioning. The slave cylinder has a bleed screw, check these items and fill and bleed as neccessary. Hope this helps, let me know.

Removal:
1.From inside the cab, carefully pry the clutch master cylinder pushrod and retainer bushing from the pedal cross-shaft lever pin.
2. Disconnect and remove the interlock switch.
3. Remove as much fluid from reservoir as possible and place shop rags under the master cylinder to catch leaking fluid.
4. Disconnect hydraulic line from the master cylinder and plug lines and outlets to avoid contamination and excessive fluid loss.
5. Remove the two nuts retaining the clutch master cylinder.
6. Installation is the reverse of removal exept after installation, bled the system.

get a new one.

Make sure it is full of brake fluid first. If it is, check for leaks around the clutch master and slave cylinder. The master cylinder is mounted on the firewall below the brake master cylinder and towards the driver side. Follow the line to find the slave cylinder on front of the transmission. If you find no leaks, either the master and/or slave cylinder is bad or the clutch fingers are broken.

fluid in master cylinder for clutch?

There are 2 master cylinders on the escort, one for brakes, and the other for the clutch. You put the fluid in the master cylinder.