Diagnosing Common Brake Problems

How to Diagnose a Problem

  1. Disconnect the brake lines from the master cylinder while leaving the cylinder on the vehicle.
  2. Obtain solid plugs for the master cylinder outlets with the correct threads.
  3. Plug both master cylinder outlets and try the pedal. If the pedal is very spongy or goes to the floor the master cylinder is bad. If you have a good firm pedal then the master cylinder is fine.
  4. If the master is fine, connect the line to the front portion of the system and try the pedal again. If the pedal remains firm then the front part of the system is fine. If the pedal goes to the floor, the problem is with the front half of the system.
  5. If the pedal is fine with the front part of the system connected then connect the rear portion . If the pedal goes to the floor then the problem is in the rear
  6. Once you know which portion of the system has the problem then it will be easier to fix it.
THE EFFECT OF PEDAL RATIO AND BORE SIZE ON HYDRAULIC PRESSURE OUTPUT
Pedal Ratio
Bore Size
lbs input
PSI out
6:1
1 1/8
75
453
6:1
1
75
573
6:1
7/8
75
748
5:1
1 1/8
75
377
5:1
1
75
477
5:1
7/8
75
623
4:1
1 1/8
75
302
4:1
1
75
382
4:1
7/8
76
499
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