1991 Ford Ranger w/ABS Rear brakes lockup

If the brakes work but you have to push very hard, try checking the vacuum supply to the brake booster with the engine running.

check the sensors on each wheel of the brakes ,their is a sensor on each wheel so check the wires to these sensors are not broke or corroded first then report back as star trek is on soon, then i will explaiun how to test them forget the code reader as the ABS will reset itself as soon as it reads a signal from all four wheels its the only onboard computer that does

ANTI-LOCK BRAKE SYSTEM (and HCU-Hydraulic Control Unit)
General Information The 4-Wheel Anti-lock Brake System (ABS) is an electronically operated, all wheel brake control system. Major components include the power brake booster, master cylinder, the wheel speed sensors, and the Hydraulic Control Unit (HCU) which contains the control module, a relay, and the pressure control valves. The system is designed to ****** wheel lockup during periods of high wheel slip when braking. Retarding wheel lockup is accomplished by modulating fluid pressure to the wheel brake units. When the control module detects a variation in voltage across the wheel speed sensors, the ABS is activated. The control module opens and closes various valves located inside the HCU. These valves, called dump and isolation valves, modulate the hydraulic pressure to the wheels by applying and venting the pressure to the brake fluid circuits. Some models are equipped with a Traction Assist (TA) system. The TA system senses wheel spin upon acceleration, turns on the Hydraulic Control Unit (HCU) pump and applies fluid pressure to the appropriate rear wheel. Two additional isolation valves in the HCU will also close to permit fluid to flow only to the rear wheels. The TA system monitors TA usage to avoid overheating the rear brakes. If the system does sense brake overheating, the ABS module will inhibit TA operation until the rear brakes are permitted to cool down.
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SYSTEM COMPONENTS The anti-lock brake system consists of the following components:

• Vacuum booster and master cylinder assembly
• Hydraulic Control Unit (HCU)
• ABS module
• Wheel sensors
• Pedal travel switch

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On 1992-1994 and 1995-00 a scanner is used to perform the brake bleed. But on yours, I think it's different according to the manual.
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What do you mean change the cal. on the rear also? Do you mean Caliper?

Double check the rear shoes foir heavy glazing and wear. My recomendation would be to replace the rear shoes but to get rid of the issue, to have the drums turned(machined) or replaced.

abs malfunction buy a new unit

If you are talking about the back brakelights being on continuously, some Ford products have a sliding plastic brake switch under the pedal which sometimes needs adjustment after a brake job. If you are talking about dash lights, then it sounds like the ABS or proportionate valve is stuck off-center. Some vehicles have to be bleed in a factory recommended sequence which keeps that problem from happening.

Had a similar issue on an old Mazda pickup that was really a Ford. I found that the issue was with an ABS sensor mounted near the rear axle. When I opened the cover, I found a lot of corrosion. I scraped out what I could and sprayed it down liberally with some electrical contact cleaner. I did not have any ABS function to my brakes althought they still worked. 2 or 3 days after my cleaning efforts, the ABS and brake light went out and the ABS function returned to my brakes! I think a junk yard wanted a couple hundred bucks for that part. Check and see if you have the actual abs function (ie brake pedal pulsing during a skidding hard braking action). This may very well be your isse and it may be a very cheap fix!
-Ray

This isn't hard to do at all really and should be easy to procure the part from the local parts store. There is only one little screw holding it in

1. Raise and support vehicle.
2. 
   NOTE: Clean off dirt and foreign material that may have collected around the rear anti-lock brake sensor before removal. Remove the rear anti-lock brake sensor.

1 Disconnect the rear anti-lock brake sensor electrical connector. 2 Remove the bolt. 3 Remove the rear anti-lock brake sensor.

1. NOTE: Thoroughly clean the mounting surface. NOTE: Inspect the anti-lock brake sensor O-ring for damage; lightly lubricate the O-ring with axle lubricant. To install, reverse the removal procedure

That is just the ABS system at work. It is supposed to prevent your wheels from locking which often makes the brake pulsate, especially in wet weather or or snow or ice. The only way to really stop this is to disable to ABS by unplugging it and removing the fuse. This will make the ABS light appear in your dash, but your brakes will stop pulsating.

It could be a couple of things..Either you have a fluid leak and its getting on the brake pads making them grab or it could be the abs system itself.. There is a speed sensor on the rear axle which also controls the abs.. That can be changed easily and isnt too expensive.. Kip