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KEN
KEN, ASE Certified Technician
Category: Ford
Satisfied Customers: 27
Experience:  ASE Certified Master Truck tech. 13+ years exp.
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have very little brake pedal pressure after replacing the master

Customer Question

have very little brake pedal pressure after replacing the master cylinder and front disc brake calipers; pressure to rear brake cylinders is good (checked by pluging the output port to the front calipers at the master cylinder? builds some pedal pressure with engine off but once engine is craked i lose all pedal pressure? this
is a 1994 ford f150 pickup.
Submitted: 5 years ago.
Category: Ford
Expert:  john -aka eauto replied 5 years ago.
are the rear brakes adjusted properly? I would pull the rear drums make sure the wheel cylinders are not seized and adjust the rear brakes. Usually when the pedal is soft after front brake repair its due to out of adjustment or bad rear wheel cylinders
Customer: replied 5 years ago.

john, i don't think the rear brakes are the primary problem. the brakes were functioning

prior to replacing the master cylinder. i checked them and replaced the front pads and rear shoes about two months ago. i decided to replace the master cylinder and front calipers because the pedal pressue was slowly leaking down after applying pressure to the brake pedal.

Expert:  john -aka eauto replied 5 years ago.
have you looked at the rear brakes?
Customer: replied 5 years ago.
john, if the pressue to the rear brakes builds and holds ok with the port to the front brakes on the master cylinder plugged would this not indicate that the rear brakes are ok?
Customer: replied 5 years ago.
Relist: I prefer a second opinion.
Expert:  KEN replied 5 years ago.

Hi Frank,

 

I agree that plugging the port for the rear brake line at the master cylinder should eliminate the rear brakes as being part of the problem.

 

If I understand correctly, the brake pedal was going down gradually when the vehicle was stopped and in gear before you replaced the master cylinder and front calipers and now you have the same problem only it is worse.

 

Bleeding the brakes is critical and can be deceptively difficult. I've attached the proscribed procedure for your truck. It is pretty straight forward, but you must be absolutely certain there is no air left in the system. You may have to refill the master cylinder a couple of times before you get all the air out. When you stop seeing air come out of the bleeder, bleed it again through at least four pedal strokes.

 

I also agree that the adjustment on the rear brakes is most often a contributing factor. Plugging the line at the master cylinder prevents the rear brakes from contributing to the problem to a degree, but does not insure that they are properly adjusted nor does it guarantee that the problem is only in the front.

 

I hope this helps,

 

Ken

 

NOTE: Bleed primary and secondary hydraulic brake systems separately, bleeding longest line first on each system. Do not allow reservoir to run dry during bleeding operation. Never reuse fluid that has been drained from hydraulic system.

  1. Loosen master cylinder to hydraulic line nuts and wrap shop cloths around tubing below fitting to absorb escaping brake fluid.
  2. Depress brake pedal slowly forcing air trapped in master cylinder out at fitting.
  3. Hold pedal down and tighten fittings, then release brake pedal. NOTE: Releasing brake pedal before fittings are tightened will allow air to enter the master cylinder.
  4. Repeat steps 1 through 3 until air ceases to escape at fittings and brake pedal is firm.
  5. Bleed brakes as follows: RR, LR, ABS valve, RF, LF
    1. Pump pedal several times, then hold pedal down firmly.
    2. With pedal firmly depressed, open bleeder screw on one rear brake until pedal fades, then close bleeder valve.
    3. Repeat procedure until a continuous flow of brake fluid is released from bleeder valve.
    4. Repeat steps 5a through 5c on other rear brake.
  1. Bleed the Rear Antilock Brake System (RABS) valve.
  2. Bleed the front brakes using same procedure as for rear brakes.
  3. Check brake operation and ensure pedal is firm, road test vehicle.
KEN, ASE Certified Technician
Category: Ford
Satisfied Customers: 27
Experience: ASE Certified Master Truck tech. 13+ years exp.
KEN and 8 other Ford Specialists are ready to help you
Customer: replied 5 years ago.

KEN, I HAVE DONE ALL THE THINGS THAT YOU SUGGESTED AND I STILL HAVE THE SAME PROBLEM (VERY LITTLE BRAKE PEDAL PRESSURE, ESPECIALLY WITH ENGINE CRANKED)? COULD THE PROBLEM BE WITH THE POWER BOOSTER? I CHECKED THE VACCUM PRESSURE AT THE BOOSTER AND IT SEEMS TO BE VERY GOOD. I HAVE NOTICED THAT WHEN DRIVING THE VEHICLE THERE SEEMS TO BE SOMEWHAT BETTER PEDAL PRESSUR WHEN THE ENGINE IS REVED UP FAIRLT HIGH.

Expert:  KEN replied 5 years ago.

I think I may have miss understood the problem. I'm not sure I understand what you mean by brake pedal pressure.

 

Do you mean that the amount of pressure you must exert on the brake pedal to make the truck stop is excessive? That would indicate a problem with the power booster, especially if it improves with higher engine RPMs.

 

Or by brake pedal pressure, do you mean that the pedal still gradually goes to the floor when you step on it and the brakes don't hold?

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