1981 Ford F100 brake system

Have an F100 with brake issues. Replaced master cylinder. Replaced the parts on the back. Bled lines. No leaks. Have a used brake booster. Brakes still mushy.

Mechanic tells me the vacc boost may be going out. He also tells me a bad vacc boost can damage the engine.

I agree that a bad vac system COULD be affecting the brakes, but I had a vacc boost die before and the brakes were merely hard to press. This is mushy.

So. Will the vacc boost actually affect the engine and could mushy brakes be the fault of the vacc boost?

I’ve never heard of a bad vacuum booster causing mushy brakes. Causes that I have experienced or heard of:

  1. Air in the brake lines.
  2. Bad brake hoses.
  3. Bad master cylinder.
  4. Bad wheel bearings.
  5. Leaking calipers.

No, a vacuum leak should not cause engine damage unless it is major and allowed to persist for quite some time.

How do the brakes feel when the engine is off, after the first few presses? If harder but still mushy, then I think that would rule out the booster.

Calipers on rear were replaced.

Do not see signs of leaks.

Engine off, brakes are hard.

Wheel bearings, from last time I got tires, seems to be in good shape. Although I don’t see how that would affect the brakes in this case.

Bad wheel bearings allow the wheels and rotors to wobble. This wobbling pushes the pads apart and they require more fluid than they should to apply the brakes. I have experienced this.

aha. Wheels solid tho. No wobble

Presumably you mean that the rear wheel cylinders were replaced?

You can’t necessarily see a wheel wobble from a bad wheel bearing.

If the flexible brake lines at the wheels are all original then I’d suspect that those need to go. If it were me and they were old I’d replace them even if that didn’t firm up the brakes.