2003 Ford F150 Brakes won't release

I would be looking at the check valve being jammed up first ? Brake Booster Both models are equipped with a dual-diaphragm vacuum booster. The power brake booster is a dual diaphragm, vacuum assisted power brake booster. The booster reduces brake pedal pressure and travel distance. The booster is located on the left side of the bulkhead in the engine compartment, between the brake pedal and brake master cylinder. The booster is divided into separate chambers by the diaphragm and will not operate if vacuum is restricted or if any of the vacuum related power brake components fail. The brake booster is replaced as an assembly. If the power assist fails, the brake system will continue to operate with increased brake pedal effort. See Fig. 4 . The power brake booster check valve is located on the front of the power brake booster. The check valve is positioned between the power brake booster and the power brake booster hose. It closes when the engine is turned off. In the closed position, it traps engine vacuum in the power brake booster and retains vacuum to provide several power assisted brake applications with the engine off.

PRINCIPLES OF OPERATION The ABS control module receives wheel speed readings from both front wheel speed sensors and the rear differential speed sensor, and processes this information to determine if an ABS event is necessary. The wheel speed sensor electrically senses each tooth of the ABS sensor indicators as it passes through the wheel speed sensor's magnetic field. The ABS control module continuously monitors and compares the rotational speed of each wheel speed sensor indicator and, when it detects an impending wheel lock, modulates brake pressure to the appropriate brake caliper(s). This is accomplished by triggering the Hydraulic Control Unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel returns to normal speed, the ABS control module returns the solenoid valves to their normal position, and normal (base) braking resumes. The ABS control module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS control module will do a preliminary electrical check, and at approximately 20 km/h (12 MPH), the pump motor is turned on for approximately one-half second. Any malfunction of the ABS system will cause the ABS to shut off; normal power assisted braking, however, remains.

Hello:
Thanks very much for your in-put:
I guess I should have been more descriptive. I have very strong vacuum on booster sidde of valve & valve is working, when I un-plug valve from booster (with engine off) I still have vacuum . My brakes are appling & holding themselves. I just don't know if the problem is in the booster or the ABS pump. The ABS pump (as all parts) is so expensive I hate to change parts I don't need to.

Thanks
Dale

** Problem Solved ** Bad Booster **
Never seen this before, I seperated master cylinder from booster, & when I held vacuum hose to booster it would extend plunger (thus, appliing brakes) & when removed plunger would retract (releasing brakes). Replaced Booster & brakes work perfect. Every booster I have ever seen fail would do just the opposite, pedal would be very hard & feel like very little brakes.
Maybe my experience will save someone else a lot of time.


Happy New Year
Dale

thank you very much for the information Dale we like to get the details of a problem and outcome !

Happy New Year
Happy Motoring
Dave H