Hard brakes 89 ford sc hard pedal and no brakes when brakes applied hard. Normal pedal and brakes when applied normal.

_______power brake booster, check vacuum to booster and the check valve for brake booster. primary symtom is high hard pedal that requires greater then normal pedal pressure to stop car. testing booster= pump brakes several times with engine off to deplete stored vacuum. turn on engine with pushing slightly on brake pedal. you should be able to feel the pedal fade away a bit, and then become firm. But not hard. if you feel nothing at the pedal when engine starts. Brake booster is not working. Good-day! make sure vacuum is going to booster with engine running. it may just be a bad vacuum line or check valve.

Check all wheel sensors for broken wires, loose connectors or position of sensors in their mounting brackets.

That is a Vacuum Problem, You need to Check the vacuum lines @ the Brake Booster, while car is running Listen for Air leak While someone Presses On Brake peddle, Either the Booster is leaking Or 1 of the lines

Usually when there is air in the brake lines, the pedal goes to the floor.
If it is very difficult to apply the brakes, the problem may be the power booster.

There is never a test or reason to pump your brakes

Accomplishes absolutely nothing.

There is no pressure in a braking system at all,
until you SLOWLY apply the brake pedal.

Then less than 1" off movement in the master cyl,
will develop 600 to 1800 lbs at the wheels

Release the brake pedal, the system goes to zero,
IT DOES NOT HOLD ANY PRESSURE

To answer your question

You not suppost to press the pedal hard
The saying goes--you apply the brakes,
not force the pedal arm and pin into the
vacuum booster, as though you want to damage it

When your driving what happens ?
You have higher manifold vacuum and
thus your pedal is normal,why the VACUUM
brake booster

Not likely that the booster is an issue. The booster would not be able to overcome the brake fluid compression.
First open and inspect all brakes for leaks, adjustment, etc. Check all brake lines.
So if no leaks, did the fluid go low in the MC? If so, pad consumption would eat more than a MC reservoir of fluid, and not require MC replacement, but rebleeding and top up of fluid. If the pads are worn, and you replace them, often this will expel air back through the MC.

If you determine the MC is faulty, bench bleed the new one before install, or you will have lots of problems getting air out....

If there is a malfunction in the brake servo assist system, and your assistance is reduced, the brake pedal will become hard and you will need much more pressure on the brake pedal to stop the car.

It is possible that there is a vacuum leak in the brake servo assist system and you are only getting 50% or less assistance when you are applying the brakes.

You need to have the vehicle checked out without delay because if you lose brake assistance entirely it will be very difficult to stop the car.

What you are describing is brake drag. Brake drag occurs when there is still fluid under pressure left in the brake lines. When you apply brake pressure, brake fluid is pushed into the brake lines from the master cylinder, with a piston. During normal operation, when released, the brake pedal should return to a resting position as the brake fluid returns to the master cylinder. Brake drag may be caused by any of the following:

A) The master cylinder piston is not returning far enough for the brake fluid return into the compensating port.

B) Topping off the master with automatic transmission fluid, engine oil, or power steering fluid, can well the rubber seals in the brake lines. The mineral oil in these fluids can affect the composition of the rubber in the brake line.

C) The master cylinder push rod could be out of adjustment.

You problem could be caused by, but is not limited to, any one of these symptoms; each and every car is different, and some have problems that can stump the greatest technicians for weeks, so do not take this as your solution, but rather a place to start.

with the sound of air when applying the brakes, sounds like a bad power brake booster, located behind the master cylinder,large round peice