1990 Ford F150 does the master cylinder isolate the front and rear hydraulics as would a single reservoir with two compartments or is that function served by the RABS valve?

NHTSA Vehicle Safety Recalls

• Recall Number
  16V345000
  Recall Date
  05/24/2016
  Component
  SERVICE BRAKES, HYDRAULIC:FLUID
  Summary
  Ford Motor Company (Ford) is recalling certain model year 2013-2014 F-150 vehicles manufactured August 1, 2013, to August 31, 2014 equipped with 3.5L engines. The master cylinder rear cup seal may roll, resulting in brake fluid leakage from the primary reservoir into the brake booster.
  Consequence
  Loss of brake fluid from the primary reservoir results in a complete loss of front brake function, increasing the risk of a crash.
  What Owners Should Do
  Ford will notify owners, and dealers will replace the brake master cylinder and, if necessary, the brake booster, free of charge. The recall began on October 28, 2016. Owners may contact Ford customer service at 1-866-436-7332. Ford's number for this recall is 16S24.

Bad master cylinder . What are you using a scan tool for ? No need Unless your Installing a new Electro-Hydraulic Control Unit (EHCU) or new Brake Pressure Modulator Valve (BPMV).
Hydraulic Brake System Bleeding (Manual)
Caution: Refer to Brake Fluid Irritant Caution in the Preface section.
Notice: Refer to Brake Fluid Effects on Paint and Electrical Components Notice in the Preface section.
Place a clean shop cloth beneath the brake master cylinder to prevent brake fluid spills.
With the ignition OFF and the brakes cool, apply the brakes 3-5 times, or until the brake pedal effort increases significantly, in order to deplete the brake booster power reserve.
If you have performed a brake master cylinder bench bleeding on this vehicle, or if you disconnected the brake pipes from the master cylinder, you must perform the following steps:
3.1. Ensure that the brake master cylinder reservoir is full to the maximum-fill level. If necessary add GM approved brake fluid from a clean, sealed brake fluid container. Refer to Fluid and Lubricant Recommendations.
If removal of the reservoir cap and diaphragm is necessary, clean the outside of the reservoir on and around the cap prior to removal.
3.2. With the rear brake pipe installed securely to the master cylinder, loosen and separate the front brake pipe from the front port of the brake master cylinder.
3.3. Allow a small amount of brake fluid to gravity bleed from the open port of the master cylinder.
3.4. Reconnect the brake pipe to the master cylinder port and tighten securely.
3.5. Have an assistant slowly depress the brake pedal fully and maintain steady pressure on the pedal.
3.6. Loosen the same brake pipe to purge air from the open port of the master cylinder.
3.7. Tighten the brake pipe, then have the assistant slowly release the brake pedal.
3.8. Wait 15 seconds, then repeat steps 3.3-3.7 until all air is purged from the same port of the master cylinder.
3.9. With the front brake pipe installed securely to the master cylinder, after all air has been purged from the front port of the master cylinder, loosen and separate the rear brake pipe from the master cylinder, then repeat steps 3.3-3.8.
3.10. After completing the final master cylinder port bleeding procedure, ensure that both of the brake pipe-to-master cylinder fittings are properly tightened.
Fill the brake master cylinder reservoir with GM approved brake fluid from a clean, sealed brake fluid container. Ensure that the brake master cylinder reservoir remains at least half-full during this bleeding procedure. Add fluid as needed to maintain the proper level.
Clean the outside of the reservoir on and around the reservoir cap prior to removing the cap and diaphragm.
Install a proper box-end wrench onto the RIGHT REAR wheel hydraulic circuit bleeder valve.
Install a transparent hose over the end of the bleeder valve.
Submerge the open end of the transparent hose into a transparent container partially filled with GM approved brake fluid from a clean, sealed brake fluid container.
Have an assistant slowly depress the brake pedal fully and maintain steady pressure on the pedal.
Loosen the bleeder valve to purge air from the wheel hydraulic circuit.
Tighten the bleeder valve, then have the assistant slowly release the brake pedal.
Wait 15 seconds, then repeat steps 8-10 until all air is purged from the same wheel hydraulic circuit.
With the right rear wheel hydraulic circuit bleeder valve tightened securely, after all air has been purged from the right rear hydraulic circuit install a proper box-end wrench onto the LEFT REAR wheel hydraulic circuit bleeder valve.
Install a transparent hose over the end of the bleeder valve, then repeat steps 7-11.
With the left rear wheel hydraulic circuit bleeder valve tightened securely, after all air purged from the left rear hydraulic circuit, install a proper box-end wrench onto the RIGHT FRONT wheel hydraulic circuit bleeder valve.
Install a transparent hose over the end of the bleeder valve, then repeat steps 7-11.
With the right front wheel hydraulic circuit bleeder valve tightened securely, after all air has been purged from the right front hydraulic circuit, install a proper box-end wrench onto the LEFT FRONT wheel hydraulic circuit bleeder valve.
Install a transparent hose over the end of the bleeder valve, then repeat steps 7-11.
After completing the final wheel hydraulic circuit bleeding procedure, ensure that each of the 4 wheel hydraulic circuit bleeder valves are properly tightened.
Fill the brake master cylinder reservoir to the maximum-fill level with GM approved brake fluid from a clean, sealed brake fluid container.
Slowly depress and release the brake pedal. Observe the feel of the brake pedal.
If the brake pedal feels spongy, repeat the bleeding procedure again. If the brake pedal still feels spongy after repeating the bleeding procedure, perform the following steps:
21.1. Inspect the brake system for external leaks. Refer to Brake System External Leak Inspection.
21.2. Pressure bleed the hydraulic brake system in order to purge any air that may still be trapped in the system.
Turn the ignition key ON, with the engine OFF. Check to see if the brake system warning lamp remains illuminated.
Important: If the brake system warning lamp remains illuminated, DO NOT allow the vehicle to be driven until it is diagnosed and repaired.
If the brake system warning lamp remains illuminated, refer to Symptoms - Hydraulic Brakes

You have a hydraulic clutch on your truck, check the reservoir for fluid, check the clutch master cylinder and lines for a leak, possibly line broke or need new master cylinder.

Yes, the reservoir is part of the Clutch Master Cylinder Assembly. Below is an exploded view of your Clutch Master Cylinder. The master cylinder mounts to the firewall in the engine compartment directly in front of your clutch pedal. Note in the picture the part marked "pushrod". This is what actually connects to the clutch pedal itself.

Bleeding the Brake System
When any part of the hydraulic system has been disconnected for repair or
replacement, air may get into the lines and cause spongy pedal action (because
air can be compressed and brake fluid cannot). To correct this condition, it is
necessary to bleed the hydraulic system so to be sure all air is purged.

When bleeding the brake system, bleed one brake cylinder at a time, beginning
at the cylinder with the longest hydraulic line (farthest from the master
cylinder) first. ALWAYS Keep the master cylinder reservoir filled with brake
fluid during the bleeding operation. Never use brake fluid that has been drained
from the hydraulic system, no matter how clean it is.

The primary and secondary hydraulic brake systems are separate and are bled
independently. During the bleeding operation, do not allow the reservoir to run
dry. Keep the master cylinder reservoir filled with brake fluid.

1. Clean all dirt from around the master cylinder fill cap, remove the cap and
   fill the master cylinder with brake fluid until the level is within 1/4 I n.
   (6mm) of the top edge of the reservoir.
   
2. Clean the bleeder screws at all 4 wheels. The bleeder screws are located on
   the top of the brake calipers.
   
3. Attach a length of rubber hose over the bleeder screw and place the other
   end of the hose in a glass jar, submerged in brake fluid.
   
4. Starting at the right rear proceed in this order left front, left rear and
   right front.
   
5. Open the bleeder screw 1/2 - 3/4 turn. Have an assistant slowly depress the
   brake pedal.
   
6. Close the bleeder screw and tell your assistant to allow the brake pedal to
   return slowly. Continue this process to purge all air from the system.
   
7. When bubbles cease to appear at the end of the bleeder hose, close the
   bleeder screw and remove the hose.
   
8. Check the master cylinder fluid level and add fluid accordingly. Do this
   after bleeding each wheel.
   
9. Fill the master cylinder reservoir to the proper level.

It is close to the firewall on the drivers side of the vehicle (most vehicles) it will look almost identical to the brake master cylender just a much smaller scale. Add your fluid their.

NO CK FOR WIRES BROKEN TO ABS SENSOR ON TOP OF THE REAR TWO WIRE TERN. & CK SENSOR IN REAR WITH OM METER

The right rear is the furthest point away from the master cylinder and will need to be bled the most. Have another person pump the pedal three times. On the third pump crack the bleed valve open while they keep pressure on the pedal. Close the valve and then they can release the pedal and start the three pumps again. Do this process until no air comes out of the line. Make sure that the reservoir stays full when bleeding so you don't get more air into the system.

This car uses the brake master cylinder reservoir for the hydraulic clutch. Check NAPA p/n390225. Nipple on right side of master cylinder reservoir is port for clutch.

Try using a pressure bleeder. It will force the air out. Works every time.