Posted by Determined on May 04, 2019 at 06:57:05 from (216.130.212.201):
In Reply to: Ford 850 truck posted by Chief 83 on May 04, 2019 at 05:04:34:
You sound somewhat confident that air in the system is not the problem, air in the system typically will present itself as a spongy pedal or brakes that do not adequately hold. so what else you ask.
Think about your master cylinder, you have a bore and a plunger that is only capable of moving a fixed amount of fluid each time it is stroked.
So lets say for example (numbers out of the air) your master cylinder has a 1 inch bore and a 3 inch stroke, and it can move 24 cc of fluid per stroke.
If all 4 corners of your truck have 2 wheel cylinders (8 total) then you theoretically have 3 cc of fluid available for each wheel cylinder when you depress the pedal.
If each of your wheel cylinders have a 1 inch bore and 2 pistons (likely larger but this makes the math simpler) then you are dividing the 24 cc of fluid or 3 inches of stroke by 16.
This would theoretically work out to 3/16 of travel per piston.
Next imagine your brake shoes being out of adjustment to the point the piston in the wheel cylinder needs to travel 1/4 inch to cause the brake shoe to make solid contact with the drum.
Mathematically it would not be possible with one stroke of the pedal to achieve contact so the first thing you do is pump the pedal and the additional strokes deliver the amount of fluid required to fully apply the brakes.
Next you release the pedal and the return springs on the brake shoes do what they were designed to do, retract the shoes until they come to rest on the adjusters.
The springs do not know or care that they may be retracting the pistons 1/8, 1/4, or 1/2 inch, their only job is to retract.
Quote;(Take foot off and wait 20 seconds press pedal easily to the floor)
The adjusters job is to limit the return of the shoes, if your master cylinder is requiring multiple strokes to get a hard pedal then it is time to talk to the adjusters.
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