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94 F350: turbo diesel..soft brake pedal..master cylinder..bled

Customer Question

'94 F350 turbo diesel, soft brake pedal, slowly goes to floor. Wooshing sound under dash when brakes depressed. Have changed master cylinder TWICE and bled system carefully, but same problem. Suspect booster (wooshing sound), but do not have hard pedal and appear to have good boost after pumping pedal twice. Pedal gets firm after pumping twice, but will still slowly go to floor. With engine off, pedal stays firm and won't drop to floor. Pulling heavy loads and need reliable pedal.
Submitted: 6 years ago.
Category: Ford
Expert:  john -aka eauto replied 6 years ago.

sounds like the vacum assist


  1. Loosen the hose clamp vacuum pump inlet fitting.
  2. Remove the hose from the vacuum pump inlet fitting.
  3. Remove drive belt by placing a 15mm box end wrench on the automatic belt tensioner pulley bolt and rotating it counterclockwise until belt can be removed.
  4. Remove vacuum pump retaining bolts and remove vacuum pump. NOTE: The vacuum pump is not to be disassembled. It is only serviced as a unit. The pulley is serviced as a separate item.


  1. Position vacuum pump and install retaining bolt. Tighten bolts to 24-31 Nm (18-23 ft lb) .
  2. Connect the hose from the manifold vacuum outlet fitting to the vacuum pump and install the hose clamp.
  3. Install belt by placing a 15mm box end wrench on the automatic belt tensioner pulley bolt and rotate the pulley counterclockwise. Position belt on pulley.
    • The vacuum pump is driven by the back (flat) side of the belt.
    • The BRAKE light will glow until vacuum builds up to the normal level.
    • If new belt is installed, recheck belt tension after running vacuum pump for five minutes.



Customer: replied 6 years ago.
No vacuum tester, but feel loads of vacuum at booster, and more importantly, no brake light comes on. With vacuum hose detached from booster, brake pedal is hard. When hose is reattached, pedal slowly moves to floor. Holding pedal to floor and reaattach vacuum hose and the pedal pushes backward up to previous hard position??? Don't understand how it could be the vacuum pump with this much response from the removal and attachment of vacuum line. And shouldn't "brake light" come on with loss of vacuum??
Expert:  john -aka eauto replied 6 years ago.

usually the light comes on with a hydraulic issue

have you checked wheel cylinders?

Customer: replied 6 years ago.

No, have not checked wheel cylinders, but have absolutely no leakage anywhere. Keep parked on concrete drive and also no noted signs of leakage when bleeding brakes.

Expert:  john -aka eauto replied 6 years ago.
You may have a wheel cylinder or rear brake issue. If the wheel cylinders are seized or leaking it will cause a issue. I would oul the rear drums and check ...if they are leaking the fluid is generally trapped in the brake assembly and wil not leak onto the ground
Customer: replied 6 years ago.
Brake fluid reservoir has always remained full, thus convincing me of no leakage.
Expert:  john -aka eauto replied 6 years ago.
they usually seap which causes the issues or seize causing issues regardless i think the rear brakes need to be accesed to evaluate the situation
Customer: replied 6 years ago.
thanks, XXXXX XXXXX & respond later
Expert:  john -aka eauto replied 6 years ago.
Customer: replied 6 years ago.
I find no evidence of rear cylinder leaks. I've given up on your expert advice, especially after telling me to change the vacuum pump without even testing for vacuum. I can't afford to just change parts at random, which is why I solicited expert advice. Thanks anyway.
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Category: Ford
Satisfied Customers: 15000
Experience:  ase tech over 15 years in the field
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