Brake pressure issues

osborne

Well-Known Member
Posts
269
Reaction
84
I just put wilwood 6 pistons in front and 4 pistons in rear. I used the calculator to figure out the correct master cylinders for each. 3/16 steel line with braided lines at calipers. Wilwood bias valve hooked to rear M/C. I can not stop the rear wheels. I had it on jack stands with no wheels, idling in 1st gear and it takes all the leg pressure I have to stop the axle from spinning. I understand that the pads need to heat up to work well but I should be able to stop a spinning axle with minimum effort. Pedal is solid. Blead the brakes forward and backward (literally). Any ideas?
 

MARXICO

Well-Known Member
Posts
661
Reaction
470
This is my experience, I have tried everything including duel rear calipers and that didn’t work.

I started all over with front willwod 4 piston calipers and willwod 6 piston calipers in the rear .I have a ¾ front master cylinder and a 7/8 rear master cylinder my car 4500lb stops on a dime I couldn’t ask for anything more problem solved . Run as much hard line as possible and make your flexible lines as short as possible as the pressure expands the flexible lines and loses pressure, I also use a bias brake adjuster that hooks up to your pedals to adjust braking proportion from front to rear that I adjust at every race to compensate for hard packed or sandy terrain to compensate for front push or rear skidding .
 

MARXICO

Well-Known Member
Posts
661
Reaction
470
Also install check valves in line right after your master cylinder to help from having pedal pressure bleed off
 

osborne

Well-Known Member
Posts
269
Reaction
84
Sounds like you have exactly what I have except reversed. I guess I could add check valves, but dont think that is gonna solve my prob. I think I have an odd prob. here. There should be enough brake to stop a spinning axle without even tires on it? What the hell could it be?
 

skullver

Well-Known Member
Posts
103
Reaction
18
You can put a pressure gauge on the front/rear calipers and see what the pressure is, and go from there.
 

Dirtracer 619

Well-Known Member
Posts
1,064
Reaction
127
Sounds dumb. But you got to be real good at
Bleeding the brakes.
When bleeding does your peddle hit the floor?
If so. You will never get it bled. I ended up using a power
Brake bleeder. No problems since
Oh! And you do need the residue check valves


Sent from the RDC Mobile App. Get it for your IOS device todayo
 

Zambo

Well-Known Member
Posts
4,415
Reaction
2,270
Brake pressure is a function of the DIFFERENCE between the size of your mc and your brake calipers. If the surface area of the pistons in your rear calipers is half the area of the ones in your front calipers, then you need a rear mc that is half the surface area of the front mc to achieve the same braking pressure.

Achieving enough brake pressure is nothing more than creating mechanical advantage, like using a lever. A long lever creates a bunch of power, but you have to move it a long way. Think about how far your hand travels when jacking a car up a few inches...each stroke of the Jack you move your hand 2-3 feet and the Jack only moves a fraction of an inch. If you use a very short lever to exert force, it becomes super hard to move the lever....think firm pedal here. The pedal is firm because there isn't enough mechanical advantage to get the job done.

I would imagine that setting up a vehicle with drastically different brake sizes would be very difficult to engineer properly and also difficult to bleed by using the pump and hold method.
 

philofab

Powered by Optima
Posts
1,921
Reaction
407
This may be dumb but are your rear rotors thick enough for the caliper? What you are describing sounds like pistons at max extension.
 

skullver

Well-Known Member
Posts
103
Reaction
18
There is nothing stopping the pistons from coming all the way out, therefore max extension means they come out, or squirt all the fluid out if it isn't all the way out and the back of the piston is able to travel past the seal. But as long as they are sealed, and there is pressure, they will apply force.
 

Slippery P

Well-Known Member
Posts
2,225
Reaction
1,719
Actually, Philo's point is a good one which is often over looked.
Also what is the ratio of the brake pedal you are running this makes a difference as well.

I would also check brake pressures on all 4 calipers, Kartek sells the willwood gauge that screws into the bleeders. We have brake pressure sensors on our truck and see anywhere from 900-1400 psi depending on how hard the pedal is stabbed. This is a Brembo package, but those are typical pressures for most systems.

The masters are plumbed correctly front to rear? Maybe double check what size masters you should have.
 
Top