My cabover Ford truck has brake issues. Hydraulic brakes. New master and hydrovac. Have good hard pedal after pumping pedal. Take foot off and wait 20 seconds press pedal easily to the floor. Same issue running or not. Have bled brakes a number of times. Just doesn't feel like air in the system. Power bleed almost impossible due to master cylinder configuration and location. Have fun out of ideas. Need to get hay and tractors moved. Any suggestions?
- Thread starter Chief 83
- Start date
Janicholson
Well-known Member
There may be more than 1 bleeder on the hydrovac. You will need a helper, or a stick from seat front to pedal (much less desirable) Jim
since you confirmed that there are 2 wheel cylinders on the rear they both should have adjusters on them , I know the older IH trucks have them, here is the fun part, both of them need to be adjusted, shoe may be hitting in front of drum and not the back , they can be a pain in the rear to adjust, shoe needs to hit drum front and rear of drum at same time, I learned this the hard way, they are very touchy but when adjusted correctly you will have good stopping power
Determined
Well-known Member
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.
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.
Steve@Advance
Well-known Member
When you replaced the master cylinder and Hydrovac, did you compare the new to the old?
The trucks have many different brake options and the MC and HV must be paired to each other and the volume of the combined wheel cylinders.
There should have been a tag on the HV unit identifying it. Might be worth trying to find it before it's gone to the rebuilder if not too late.
The trucks have many different brake options and the MC and HV must be paired to each other and the volume of the combined wheel cylinders.
There should have been a tag on the HV unit identifying it. Might be worth trying to find it before it's gone to the rebuilder if not too late.
Destroked 450
Well-known Member
- Location
- Harned, Ky
After checking brake adjustment if it still doesn't have a good pedal you still have air in the system.
These trucks have big brake lines and without a power bleeder to flow a continuous stream of fluid air will work it's way back up to a high spot in the system while you pumping the pedal for the next bleed.
It's hard to get all of the air out using the pump and bleed method.
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