What does the brake pressure controle valves do

When the PCM detects a continuous open, short to ground or short to voltage in the PC solenoid valve circuit, then DTC P0748 sets. DTC P0748 is a type C DTC.
PC - pressure control solenoid .
The powertrain control module (PCM) uses the pressure control (PC) solenoid valve in order to regulate the transmission line pressure. The PC solenoid valve consists of an electrical connector, a coil, an armature, a regulating spring, and a poppet valve. The PC solenoid valve is attached to the control valve body. The PCM compares TP voltage, fluid temperature, gear states, and other inputs in order to determine the line pressure appropriate for a given load. The PCM regulates the pressure by applying a varying amperage to the PC solenoid valve. The applied amperage varies from 0.1 amps for maximum line pressure to 1.1 amps for minimum line pressure. The PCM then monitors the amperage at the return line.
C0561
System Disabled Information Stored
EBCM
DTC C0561 in Antilock Brake System This code is a result of the transmission code .
The electronic brake control module (EBCM) disables the traction control when other electronic control modules set DTCs for components that effect the operation of the traction control system.

the brake lights will come on when the brake light switch senses air pressure in the switch
That will disconnect the cruise control as it reads that you have put the brakes on
The application pressure I take it is the pressure in the brake lines when the brake pedal is depressed
I would be looking at a leaking valve in the foot brake treadle valve

This can be caused by a defective anti lock brake system ABS for short, this year has active ABS, this means the system can apply hydraulic pressure to the brakes with the driver never touching the brake pedal, if the valve that controls this pressure to the wheel leaks or sticks the brakes will lock up. To confirm this release pressure at the wheel by opening the brake caliper bleeder valve at the top of the brake caliper with the engine off. If the wheel spins free after the pressure is released you will need the ABS checked and quite possibly a new ABS hydraulic control unit. This type of failure is very rare but it does happen

{: ) The ABS/TCS module (or the Electronic Brake Control Module- EBCM ) is located at the front left lower corner of the engine compartment, under the air cleaner housing. The EBCM is attached to the brake pressure modulator valve. See sample diagram:


1. secondary air injection
2. Electronic Brake Control Module
3. brake pressure modulator valve

The braking force of the rear wheels is controlled by electronic brake distribution (EBD). The EBD functions like a rear proportioning valve. The EBD system uses the ABS system to control the slip of the rear wheels in partial braking range. The braking force of the rear wheels is controlled electronically by using the inlet and outlet valves located in the HCU. The HCU is a Hydraulic control unit. The HCU consists of a valve body, pump motor, and wire harness.

Did they check for DTC (diagnostic trouble codes) in the system? if so, what were they?

Accumulators in the valve body store extra fluid released to the system for ABS mode operation. The pump is used to clear the accumulator of brake fluid and is operated by a DC type motor. The motor is controlled by the ABM.
The valves modulate brake pressure during antilock braking and are controlled by the ABM.
The HCU provides four channel pressure control individually to all the front and rear brakes. Each of the four channels control the rear & front wheel brakes individually.
During antilock braking, the solenoid valves are opened and closed as needed. The valves are not static. They are cycled rapidly and continuously to modulate pressure and control wheel slip and deceleration.
During normal braking, the HCU solenoid valves and pump are not activated. The master cylinder and power booster operate the same as a vehicle without an ABS brake system.
During antilock braking, solenoid valve pressure modulation occurs in three stages, pressure increase, pressure hold, and pressure decrease. The valves are all contained in the valve body portion of the HCU.


PRESSURE DECREASE The outlet valve is opened and the inlet valve is closed during the pressure decrease cycle.
A pressure decrease cycle is initiated when speed sensor signals indicate high wheel slip at one or more wheels. At this point, the ABM closes the inlet then opens the outlet valve, which also opens the return circuit to the accumulators. Fluid pressure is allowed to bleed off (decrease) as needed to prevent wheel lock.
Once the period of high wheel slip has ended, the ABM closes the outlet valve and begins a pressure increase or hold cycle as needed.


PRESSURE HOLD Both solenoid valves are closed in the pressure hold cycle. Fluid apply pressure in the control channel is maintained at a constant rate. The ABM maintains the hold cycle until sensor inputs indicate a pressure change is necessary.


PRESSURE INCREASE The inlet valve is open and the outlet valve is closed during the pressure increase cycle. The pressure increase cycle is used to counteract unequal wheel speeds. This cycle controls re-application of fluid apply pressure due to changing road surfaces or wheel speed.

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?

It is a brake load sensoing valve that applies more pressure to the rear brakes as you load the vehicle. It is $375 from tascafordparts.com. Get one from the junk yard or just bypass it.

usually just the lines are leaking and can be replaced.
Valve, Brake Load Sensor Proportioning The Villager has a brake load sensor proportioning valve (2B547) that is mounted on the vehicle frame and connected to the rear axle housing (4010) . When the vehicle is unloaded, the brake load sensor proportioning valve closes and partially reduces hydraulic pressure to the rear wheel cylinders (2261) . As the vehicle is loaded, the weight presses down the back end of the vehicle on the rear springs (5560) . This activates the brake load sensor proportioning valve which opens and supplies more hydraulic pressure to the rear wheel cylinders . When the vehicle is fully loaded, the brake load sensor proportioning valve is fully open and provides full hydraulic pressure to the rear wheel cylinders .
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There is no seperate pump. The following lines will give you more details

Braking force and the tendency of the wheels to lock up are affected by a combination of factors such as the friction coefficient of the road surface, and the difference between the vehicle speed and the road wheel speed. The ABS prevents the road wheels from locking up during heavy braking by controlling the vehicle's brake system hydraulic pressure.
During normal braking, as the rotational speed of the wheel falls, no electric current flows from the ECU to the hydraulic unit. The solenoid valve is not energized. The brake master cylinder hydraulic pressure is applied to the brake unit, and the ABS is not involved. However, even though the ABS is passive during normal braking, its control module is constantly monitoring for rapid deceleration of any of the wheels.
If a wheel-speed sensor signals severe wheel deceleration - which means the wheel is likely to lock up - the ECU sends a current to the hydraulic unit. This energizes the solenoid valve. The action of the valve isolates the brake circuit from the master cylinder. This stops the braking pressure at that wheel from rising, and keeps it constant.
If the sensors signal the wheel is still decelerating too rapidly, the ECU sends a larger current to the hydraulic unit. The armature moves even further and opens the valve. It opens a passage from the brake circuit. Brake fluid is sent from the brake circuit back to the master cylinder. Pressure in the brake caliper circuit is reduced so that the wheel is braked less heavily.
If the wheel sensors indicate that lowering the brake pressure is letting the wheel accelerate again, the ECU stops sending current to the hydraulic unit and de-energizes the solenoid valve. This lets the pressure increase, so that the wheel is again decelerated.
This cycle repeats itself about four to six times per second.
It is normal in an ABS for the valves in the hydraulic control unit to keep changing position as they change the brake pressure that’s being applied. These changes in position may cause rapid pulsations to be felt through the brake pedal.

BPMV = Brake Pressure Modulator Valve.

Part of the anti-lock brakes and traction control system.
The BPMV is located at the left hand front corner of the engine compartment, attached to the EBTCM (electronic brake control module).

CAUTION: For safety reasons, the brake pressure modulator valve assembly must not be repaired, the complete unit must be replaced. With the exception of the EBTCM ,no screws on the brake pressure modulator valve assembly may be loosened. If screws are loosened, it will not be possible to get the brake circuits leak-tight and personal injury may result.