Expert in Brake Hydraulics Needed

[133bus]

Had the same problem...I put 3 or 4 quarts of fluid through the front brake and still had to pump 3 or 4 time to get any pedel and as soon as I let the pedel go I would have to start all over again...Had a couple of machanics stop by and they said it was air in the MC,soooo I bled the master again with no success.After I just stood back and looked at my system I had on the car,I remembered that most of these setups are off of other vehicles and are no necessarily put in the same position as the original ones.So I looked at how the calipers were mounted and found that the bleeders were below the top of the piston...all I had to do was remove the caliper...turn the bleeder so it was the highest point and open it...and let gravity push out the air(can't push the pedel with the caliper off the rotor)....did this several time on both sides.....PROBLEM SOLVED.
Steve

[CSXDreamer]

Thanks for the tip 133 bus: I had done this previously when I had a new 1" bore master cylinder installed. My front calipers put the bleed hole in the cylinder at the 1:00 o'clock position, but I have to remove both pins to get the right rotation. I can keep enough of the pads on the disc to prevent over movement. I'll try this next.

[Bobcat]

FWIW ... I just jacked up the front of my ERA until the front caliper bleeders were vertical .... and I mean way up ( boards on the jack for more height ) !
Something else that gave me a little grief was getting all the entrained air out of the MC , until I talked to Doug at ERA . After gravity bleeding the brakes and MC , close all bleeders and VERY slowly push the pedal all the way down , hold for a count of 5 and then SLOWLY bring it back up and then rest for a few seconds before repeating . After about 6 of those , my pedal was rock hard . If you pump the pedal fast , all you do is entrain more air in the fluid . Working the pedal very slowly lets the air out of the system .
Also , make sure the MC bleeder is really at the highest point . You would be surprised at how little it takes to not get all the air out .

Bob

[elgecko]

Can someone give a recommended brake pressure at the wheel calipers.
What I don't understand is that wilwood said.......quote.....
"With a 5/8 bore master you should see about 1935 psi and with the 3/4"
bore, your psi will be about 1364 psi. This is based on the information you
provided for your 6:1 pedal ratio."
But.... people are saying 300-400psi at the wheel cylinder is fine but wilwood is saying
over a 1000 psi is a good reading.
The instructions with the gauge I'm using says don't drive a car with less than 900 psi.
I'm lost.
I do have a 5/8" front and a 3/4" rear MC setup planned.
Dave

[strictlypersonl]

Oh boy! Another opportunity to do the math...

Brake pad coefficient of friction, .4-.6. Let's assume .5 for this setup.

Radius of the center of pressure for the brake pad on an 11" rotor: ~5" (.42 ft)

Diameter of Grenada caliper piston - a pure guess: 2.3", which yields a piston area of 4.15 sq.in. Because this is a floating piston, the effective area is double that or 8.3 sq.in. (The piston effort is exerted on both sides of the rotor.)

Weight (under heavy braking) on each front wheel: ~900 lbs. > 900 lbs force at the contact patch for 1G braking

Radius of front wheel: ~1.1 ft.

Torque at the wheel required to generate 1G: 900 x 1.1 = 990 lbft. Let's round that to 1000 lbft. (We're doing a lot of back-of-envelope calculations here. )

So, the caliper must generate that much torque on the rotor/hub.

1000=.5 x .42' x 8.3 sq.in. x line pressure (psi)
or
1000/1.743 =574 psi line pressure required for a 1 G stop.

Ta Da!

(The big assumptions here are the area of the caliper piston and the pad coefficient of friction, which might change under heavy braking. If my guesses are off, it could change the answer significantly.)

[elgecko]

Dang....math was not my favorite subject. You are at the head of the class.
Sounding good to me.
Thanks

[lbperry]

Strictly Personal;
What equation are you using?
Why are you using the area of the caliper piston? Seems like if it is pushing on a brake pad you'd use the area of the pad.
Thanks,

[strictlypersonl]

I'm not using an equation. I'm using logic and physics.

I'm using the area of the caliper piston because that is what the fluid pressure is acting on. The pad transfers the piston force (area x pressure) to the rotor, regardless of the pad area. The only reason that a larger pad is better than a smaller one is that the heat generated per unit area is less, so the pad stays cooler.

[CSXDreamer]

I now have brakes.Thanks for all that replied and offered suggestions. I can push hard enough to lock up the front wheels. This is a 105 percent improvement over the origninal 9 inch rotors and Ford dovetail calipers that came with the factory build. I can remmeber pushing as hard as I could, getting cramps in my right calf muscle, and the car was barely slowing down. With no room between the firewall and the left inner fender liner, a vacuum booster or Hydroboost was out of the question. A custom engineered conversion kit took the rotors from 9" to 11", max for 15" wheels. The early 80's Camaro floating pin front brakes with 2 15/16" pistons was a big improvement and also a cause of a mushy brake pedal. The brake arm and pedal box were modified to bring the lug for the brake pushrod closer to the pivot point. These two changes gave me a theoretical 2.0 times mechanical advantage improvement in brake force. The trouble was the two new 2 15/16" caliper pistons took more fluid than the 1.0" master cylinder could deliver. The removed calipers had 2 5/8" pistons, so the theorectical fluid volume increase for the front is 25%, assuming the pistons in both types of calipers move the same axial distance. Increase in MC output, assuming the same stroke is 13%, and this was enough to supply the new fronts. A simple ratio comparison between old MC and caliper piston diameters with the new ones shows a modest 5% increase in braking force. With an adjustable brake pushrod, I now have a true 90 degree angle between the brake pedal arm and the pushrod when the pedal feels solid. Any pivot type mechanism like this (clutch pedal, rocker arm) has a variable ratio during its travel, and for a brake pedal, this is the way to optimize the force you generate with your right leg. Unfortunately, the brake pedal has about 3 inches of travel before it becomes really firm. I'd like to reduce this a little, and was wondering if anyone reduced their brake pedal travel using braided steel flexible brake lines?

[lbperry]

You can reduce the brake pedal travel, and increase your mechanical advantage, by moving the point where the pushrod attaches to the brake arm up (closer to the brake arm pivot point).
Depending on your brake pedal/pushrod mechanism, that may not be possible on all systems. And you have to make sure the pushrod moves linearly into the MC, and, above all, doesn't go over center so the pedal won't return.
I was having problems with my FFR braking system and had to redo it like this to get it to the point I could lock up the brakes.

[CSXDreamer]

lbperry, this is exactly what I did. I actually machined a new stud and installed it 5/8" closer to the brake arm pivot point or fulcrum. The hole from the original stud was filled by welding. To insure that the pushrod would engage the master cylinder in the same place (center), I had to lower the fulcrum 5/8" with respect to the pedal box. I did this by making new bushing blocks from mild steel, cutting new holes for them, and welding them in place using a 5" steel bar to keep them aligned. The old bushing holes were "patched up" by custom cut pieces of steel sheet metal that were welded in, maintaining the strength of the pedal box. I was able to use two of the original plastic bearings on the pivot shaft, but had to make two new ones from Nylon bar stock. I also did quite a bit of heating and bending on the brake and clutch arms to get the pedals functionally located in the narrow footbox. The orignal pedals were replaced with the available aluminum "AC" pedals. I left them "in the rough" for the friction they provide to the soles of the drivers shoes and did not insert the rubber doughnuts. The shallow areas around "AC" and the grooves were filled in with gloss black paint, accenting the "AC" and the ridges.