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There is a possibility that you have more than one problem. Most times a whistle sound coming from a power brake system could be a vacuum leak in the booster, but a booster that was leaking vacuum most times would cause poor braking ability, which would cause a high hard brake pedal. A quick way to tell if the noise is the brake booster is to mash on the brake pedal, and have someone pinch off the vacuum line to the booster. If the booster is leaking your noise will go away and the brake pedal will get hard and start rising as the vacuum goes away in the booster.
Now to the sinking brake pedal. Your Ford van should have a rear abs brake system, which uses an ABS unit mounted to the left frame rail. A hydraulic brake system must have a solid column of brake fluid between the wheel cylinders and the master cylinder piston. The brake shoes and brake pads must also push against a sollid object, the brake drums and the brake rotors. Anything in the system that will compress will cause a sinking soft brake pedal. Things that can cause a sinking pedal, rear brakes out of adjustment, air in the brake system, an ABS unit with an internal leak, a master cylinder with an internal leak, soft brake pads that will compress with brake application, flexing brake calipers, binding brake caliper slides or a soft swelling rubber brake line.
Start ruling out the easy things first. Have someone mash the brake pedal while you watch the rubber lines. Do any of them flex or swell when the pressure is applied? Check the brake shoe adjustment of the rear brakes. The shoes should lightly brush the drums when the wheels are rotated. Pull the front wheels and watch the brake calipers while your assistant mashes the brake pedal. Are any of the caliper slides stuck? Can you see the brake caliper pistons moving in and out, see the caliper slides work smoothly, or see the caliper flex? Move to the brake master cylinder. Remove the brake lines, and screw a brake line plug in the brake line ports, then mash the brake, does the pedal fade?
There is no way to find the real problem without a proper testing procedure.