Brake Problem Question

[mike_hinson]

I'm looking for help on a brake problem. I have a 9 car with stock Type 1 brakes on it. Large wheel cylinders have been installed in rear and small ones in front. Car had CNC 5/8" dual master cylinders on it originally. Stock 5 bolt front drums and Thing drums in rear. Car has steel brake lines and steel braided lines at each corner. In the car's first two races, brakes have been marginal. I just installed a 3/4" master cylinder for the rear brakes thinking the increased volume would provide better braking in the rear. In addition, I added 2 lb residual pressure valves on both front and rear at the master cylinders. Took out yesterday to test and brakes suck. 3rd gear and tried to do test stop. Pushed as hard as I could and car slowed, but would not stop. Should I have put the 3/4" master cylinder on the front brakes? Any suggestions for the brake bias bar on the CNC set up? Currently it is set to make the rears come on first. I need to get this fixed ASAP. MORE race is this Saturday.
Thanks for the help,
Mike Hinson

[ShaneR]

In most cars you want about 70% of your braking in the front, depends on the vehicle but pretty standard, I would change the bias adjuster to have the front brakes working more or first for starters.

[Racbaja]

When we first started racing class 9 back in 98, we had the same problem. I can't remember what size master to run but you want the smallest one you can find. Put the small wheel cylenders on all four wheels and this should fix the problem.

[ntsqd]

Quick hydraulics lesson:

Pressure is force divided by area. At the m/c end you're trying to get the most pressure from the least force (pedal effort). If the m/c has a 1.0 Square inch piston area and you push on it with 100lbs you'll get 100 psi line pressure. Change the m/c to a piston with .5 Square inch piston area and push on it with 100 lbs you'll get 200 psi line pressure. So at the m/c the smaller the piston the greater the line pressure for the same pedal effort.

At the wheel cylinder end you're trying to get the most force from the least pressure (within reason). Here the psi is multplied by the piston area to get the force. If the piston has 1.0 Square inches of area and you supply it with 100 psi you'll get 100 bls force from the piston. If the piston has 2.0 Square inches of area you'll get 200 lbs force from the piston.

What you trade off to get this additional force is piston travel. The smaller the m/c piston or the bigger the wheel cylinder piston, the farther the pedal will travel.

[mike_hinson]

NTSQD,
Let me see if I follow you correctly. I need to put the smaller 5/8" master cylinder back on the rears plus leave the larger (front) wheel cylinders on the rear, thus giving me the most braking force at the rear. Do I need to something different like put larger front wheel cylinders back to the front?

I have always been told I need the larger wheel cylinders in the rear on a light car like my Class 9 because the fronts can lock up under hard braking with a corresdonding loss of steering control. The car as currently set has front wheel cylinders on the rear and rear wheel cylinders on the front. This would appear to maximize my braking force. Am I on the right track?
Thanks for the help,
Mike Hinson

[ChuckH]

A VW based racer needs more rear braking, because the front locks up and you loose steering control.

Yeah put the 5/8 MC back as the rear MC and the larger (front) wheel cylinders on the rear.

Another old trick is setting up the rear to use the wider front brake shoes.

[WannaB-class5]

chuck nailed it on the head but meant to say "Yeah put the 5/8 MC back as the rear nd the larger (front) wheel cylinders on the FRONT."

[TimHayosh]

NTSQD (Thom) knows what he's talking about. In my 9 car I believe I had dual 3/4" MC's and did the front-to-rear slave cylinder swap. A 5/8 MC for the rear system should be even better. I never used those residual valves. I think they are used to keep the shoes from being pulled to far away from the drum by the springs. I had Thing drums and shoes in the rear and Type 1 stuff in the front. It sounds strange, but I was able to go much faster when the brakes worked better! One other variable is the pedal ratio. This is a way to reduce the leg force, but you increase how far you have to push the pedal. Good Luck.

[Byrdman]

The 2 pound residual valve is for Disk brakes, the larger 8-10 pound is used for drums. Remember you've got to over-come the spring force. A 2 pound won't do it.

[DailyPedal]

Don't take this the wrong way but incorrect drum brake adjustment is a common problem with 4-wheel drum brakes. I mean after all its been 40 years since American cars came with drums all the way around.
First make sure you are using the correct adjusting screws (early flat, late angled) with the corresponding brake shoes. Second, it really takes two people to correctly adjust VW brakes-one to slam the pedal as the other clicks the adjusters-otherwise the brakes will never get close enough to the drum to provide enough force and will go out of adjustment very quickly. Adjust the shoes till they drag while helper slams the pedal a few times and the drag will be gone so adjust some more until tight then back off slightly.
This method is common to all drum brakes. It takes very little brake shoe surface area contacting the drum to give the illusion of tight adjustment. The pedal slamming simply re-centers the shoe in the drum to allow for proper adjustment.

[ChuckH]

chuck nailed it on the head but meant to say "Yeah put the 5/8 MC back as the rear nd the larger (front) wheel cylinders on the FRONT."

Nope i said it right, you need as strong a REAR brake as you can get on a buggy, to do that you need a big rear wheel cylinder.

instead of retyping im going to quote ntsqd
At the wheel cylinder end you're trying to get the most force from the least pressure (within reason). Here the psi is multplied by the piston area to get the force. If the piston has 1.0 Square inches of area and you supply it with 100 psi you'll get 100 bls force from the piston. If the piston has 2.0 Square inches of area you'll get 200 lbs force from the piston.

[ChuckH]

Then you take the rear drums to a brake shop and have them modified so you can put on the wider front brake shoes, makes turning brakes work much better two.

Yeah i meant to say turning brakes :p ;)

[mike_hinson]

Last night, I got the 5/8" master cylinder re-installed. This puts me back where I started. Front drums have been turned and crack checked. New Thing drums installed on rear due to spline wear. I also installed front wheel cylinders on the front wheels to get a little more stopping power. I plan to use the balance bar to adjust front to rear. I will bleed the system tonight and test stopping power. I appreciate all the responses and do not take anything the wrong way. I am relying on all the experience out there to help me solve the problem. Thanks NTSQD for the brake school. That's what I needed to get the juices flowing toward a solution. I'll keep everyone posted on my progress.
Mike Hinson