Troubleshooting brakes

patman · Jun 13, 2017

Scout II Dana 44s, disc front, drum rear. Currently using a YJ or TJ booster/mc/pv and they're a bitch to use. Pedal is firm, but takes the strength of a bear to hold while on hill. Pedal doesn't seem to leak down, but I also keep getting air in the lines (plastic lines, so I can actually see them). Even after bleeding them, it's still pretty awful.

Is my MC too large? Is the PV necessary? All the write-ups I'm finding are deal with tons/4 wheel disc. Would a dodge 2500 MC still be the way to go?

I'm assuming the air bubbles are from extreme angles on my side, but it seems like it happens too easily... anyone else deal with this?

d_daffron · Jun 13, 2017

Hard pedal means you need a smaller master cylinder, the dodge 2500 master cylinder is definately not the way to go it has a huge piston and will make your pedal even harder.

Are you sure youre getting power assist? Does the pedal get any easier with the engine running vs with the engine off?

tiny · Jun 13, 2017

Plastic lines and a booster sounds like a bad combination. Sounds like your booster isn't working.

muddinmetal · Jun 13, 2017

Stupid thought, but are all your caliper bleeders facing up on all 4 corners?

clemsonjeep · Jun 13, 2017

I've been working with this on mine too. Of course I'm on yota axles but had a similar issue in that I had good brakes but the pedal needed everything I had to hold it still in 4 low 1st gear.

I was running a 1-1/16" master cylinder but just swapped to a 15/16" and still need to work on bleeding the system to give an accurate report.

From everything I read a smaller master would help. If not...then I'll be swapping another booster to check that.

patman · Jun 13, 2017

d_daffron said:
Hard pedal means you need a smaller master cylinder, the dodge 2500 master cylinder is definately not the way to go it has a huge piston and will make your pedal even harder.

Are you sure youre getting power assist? Does the pedal get any easier with the engine running vs with the engine off?

The pedal does get easier when the engine is on, but on steep hills it still rolls (in neutral) unless I'm putting a ton of weight on it.

I figured the MC on it now would be too small since I think it can from a jeep with 30/35 axles

patman · Jun 13, 2017

muddinmetal said:
Stupid thought, but are all your caliper bleeders facing up on all 4 corners?

Stock brakes in stock orientation, but yes, they are all at the top

d_daffron · Jun 13, 2017

Just remember piston size is a big factor in braking pressure and pedal travel.

If you put 100lbs of force on a 1" master cylinder you get 100psi
If you put the same 100lbs on a 1/2" master cylinder you get 200psi
However the 1/2" master cylinder will take twice as much pedal travel to move the same amount of fluid.

So if you need a softer pedal you need a smaller master cylinder. If you need less pedal travel you need a larger diameter master cylinder.

84mallcrawl · Jun 14, 2017

d_daffron said:
Just remember piston size is a big factor in braking pressure and pedal travel.

If you put 100lbs of force on a 1" master cylinder you get 100psi
If you put the same 100lbs on a 1/2" master cylinder you get 200psi
However the 1/2" master cylinder will take twice as much pedal travel to move the same amount of fluid.

So if you need a softer pedal you need a smaller master cylinder. If you need less pedal travel you need a larger diameter master cylinder.

careful we are talking about circles and cylinders here

area of a circle is pie x radius squared. . . . .therefore . . . . 1 inch diameter is 3.14 x .5 x .5 =.785 square inches. . . . now apply 100lbs to this area using unit conversion you get 127.4 pounds per square inch
area of a circle is pie x radius squared. . . . .therefore . . . . .5 inch diameter is 3.14 x .25 x .25 =.19625 square inches. . . . now apply 100lbs to this area using unit conversion you get 509.5 pounds per square inch

d_daffron · Jun 14, 2017

Yup you got me there, I was trying to keep the numbers simple for a simple easy to understand conversion to get the point across. I didnt even think about the square vs circle area difference part.