Adjustable Proportioning Valve Location - Brake Guru's Help!!

[Phantom]

1977 280Z with 1981 280Zx master cylinder, !2.2" rotors with Willwood forged Superlights in the front and 300ZX rotors with 240SX calipers in the rear. Car currently does not have an adjustable proportioning valve installed and braking is siginificantly front biased.

I purchased a valve which I assumed needs to go into the front brake line but where exactly? With the 280ZX master cylinder it is like having two piistons -right? - the rear one supplies fluid to the front brakes and the front one to the rear brakes? Since they are mounted on the same shaft will the propotioning vavle installed in the front brake line squeezing down on the oil supplied to it also reduce the oil going to the rear because it will limit the movement of the piston? - or does it cause a pressure increase that backfeeds the rear brakes and allows less oil (pressure) on the fronts for a higher given oil pressure on the rear? I'm being told that I need two valves and I just don't buy that. Need some input from the braking guru's here.

In reading some threads on the subject I've seen that most of the valves have been installed in the rear line - with the stock valve gutted? since the existing front brakes are significaltly stronger than the rear is still the case? by gutting the stock proportioning valve does that allow both cylinders to provide oil to both front & rear and then you use the rear mounted adjustable vavle to dial the rear down a bit? I'm really in unfamiliar territory here and need some guidance.

Thanks guys!

[JMortensen]

I wouldn't put a prop valve in the front brake system. I ALMOST did that on my car when I had Toy front calipers and 280ZX rear disc. I had the same problem of much too much front bias. Now I'm glad I didn't. Here's a quote from Tom Holt on the subject:

 

Disclaimer - this is all coming from someone who has always relied on

a balance bar and never use a proportioning valve...

 

A balance bar will bias the pressure regardless of the input pressure

provided by your foot(feet?) while a prop valve will send more

pressure to the rear up to a certain threshold (depending on where

the knob is set) at which point it bleeds off pressure to the rear.

Examples: a balance bar will split the pressure 80%/20% at 1 pound of

pedal pressure or 100 lbs. The Proportioning valve will allow the

rear pressure to increase in direct proportion to the fronts up to

maybe 20lbs of pedal pressure at which point it starts bleeding off

pressure at maybe 50% of the fronts. I would assume this can be good

in changing conditions where you would use less force and have maybe

a 65/35 balance and as things dry and you can mash harder the bias

will go to maybe 90/10...

 

If any of you guys ever want to mess with calculating what will

happen with different master cylinder/caliper/rotor combinations I

made a spreadsheet a few years ago that lets you plug in all of the

parameters and see what sort proportions you end up with. It doesn't

consider proportioning valves, but will let you figure balance bar

ranges or just optimize for a single cylinder setup... It's on my

website at http://sth2.com/Z-car/Brakemath.xls and as a follow up

disclaimer if you follow the recommendations of the spreadsheet and I

was grossly wrong and you die, sorry, but there could be errors in

spreadsheet. I don't know of any, but no guarantees!

 

What I'm getting out of this is the harder you hit the brakes with a prop valve in front, the more REAR brake you'll get, since the prop valve isn't linear. I'm considering going to dual masters as a result of this response. Duals might be the best way for you to go too.

[Cody 82 ZXT]

Bill, what pads are you running?

[clarkspeed]

Typically the pro valve goes inline with the rear brakes. Reason is you never want the rear of the car to lock up first under hard braking. As gas goes down, rear of car lighter, less rear brakes needed.

 

I prefer to remove the stock valve and run each line from the MC independently.

[Phantom]

Here is what I am beginning to see. With the original proportionoing valve removed - both halves of the master cylinder would be providing fluid to both ends of the car equally. True? If that is the case then putting an APR in the rear line would then allow adjusting out any excessive rear bias. If the car is majorly front biased at that point then the sloution isn't an APR in the front line but a caliper upgrade on the rear? Thoughts?

[TimZ]

Here is what I am beginning to see. With the original proportionoing valve removed - both halves of the master cylinder would be providing fluid to both ends of the car equally. True? If that is the case then putting an APR in the rear line would then allow adjusting out any excessive rear bias. If the car is majorly front biased at that point then the sloution isn't an APR in the front line but a caliper upgrade on the rear? Thoughts?

 

First off - I'm with Jon on this - don't put a prop valve on the front.

 

Is the OEM prop valve still in the circuit? If so, then this could be the problem, and your idea above should work.

 

If you are still front biased after all this, then you will want to do something to increase the braking force for a given pressure in the rear. I'd try playing with different pads before changing out the calipers - this can make a pretty huge difference.

 

If you want to do a caliper upgrade, it might not be as straightforwarward as you might think. Because of our system's front rear hydraulic split, if you want to change the brake balance to the rear you will need a caliper with less piston area than what you currently have. So, for a given amount of fluid displaced by the MC, the piston will have to move farther - make sense?

 

You could also keep the current caliper and go to a larger rotor, but that doesn't sound particularly easy.

 

Try replacing the OEM prop valve with the adjustable one (in the rear circuit) first.

[JMortensen]

I think Tim is wrong on the caliper thing. I had the same misconception a few years back about drums (smaller wheel cylinder = more throw = more rear bias = WRONG) and Dan Baldwin and some others I can't remember showed me the light. Same thing for calipers. Bigger pistons = more force. Smaller pistons = more travel. This is true. The problem is that once pads contact rotors or shoes contact drums, then it is the FORCE that matters, not the travel. So you might try a bigger pistoned caliper back there.

[tube80z]

I think Tim is wrong on the caliper thing. I had the same misconception a few years back about drums (smaller wheel cylinder = more throw = more rear bias = WRONG) and Dan Baldwin and some others I can't remember showed me the light. Same thing for calipers. Bigger pistons = more force. Smaller pistons = more travel. This is true. The problem is that once pads contact rotors or shoes contact drums, then it is the FORCE that matters, not the travel. So you might try a bigger pistoned caliper back there.

 

Jon is right. You need to think about pressure and area for brakes. The pressure is fixed by the size of the master cylinder. If you have more area for it to work against then you have more force available. To increase brake force at one end you either need to go up in caliper piston size or down in master cylinder size.

 

I have wilwood dynalite 4 pistons on front and 2 pistons on the rear. I use a 7/8 front master and a 3/4 rear. I have an adjustable balance bar and proportioning valve. This is adjusted to get the most ot of the rear until you really stand on it and then you get a little more front bias. It will take a bit of work to get it et right. A set of caliper bleed guages or a set of brake guages plumbed into the car can be a big help. For pads I'm using performance friction 01 material. If I can't get the heat then I switch to 97s.

 

Cary

[johnc]

http://www.stoptech.com/whitepapers/brakebiasandperformance.htm

[Phantom]

I guess the simplest way to figure out where I am currently is to first remove the innards of the stock proportioning valve and then find out what happens. If the car stays front biased then I should start with stickier pads on the rear. If it goes to rear biased then I should put the APR in the rear line and adjust. Does this sound right to everyone?

[JMortensen]

Sounds right to me. I think that you still might have front bias after gutting the prop valve though, I know I did with my old setup. Then your next step IMO should be larger rear calipers.

[TimZ]

Okay - I'll buy that I was mistaken - I know that bias follows piston size for a diagonal split, since the pressures front to rear (before the prop valve kicks in) are equal by definition.

 

My thinking was that since we have a front/rear split, and the MC was still displacing the same amount of fluid for a given amount of pedal travel, the smaller piston in the rear should drive the pressures on that circuit higher (all else equal). I believe my mistake was that the MC has a relief valve that still keeps the front/rear pressures equal.

 

So I stand corrected.

 

On to gutting the prop valve - can you do this without tieing the front and rear circuits together? If so, then fine, but if you can't you'll be giving up a pretty important safety feature of having a redundant hydraulic system. I just removed mine altogether and plumbed in the adjustable prop valve.

[JMortensen]

I think the mistake is in the piston size/pressure corellation, not the relief valve. The best analogy I've heard is to compare the master cylinder to the front sprocket on a bicycle and the caliper to the rear sprocket. The smaller the front sprocket the more pressure or torque you can exert, the larger the rear sprocket the more pressure or torque you exert. So if you're climbing a hill and you need lots of torque, you go to a small front sprocket (small master) and the large rear sprocket (caliper with large pistons). If you are going downhill you don't need as much torque so you switch to the larger front sprocket (large master) and smaller rear sprocket (caliper with small pistons).

[Phantom]

Awww - I'll just put teflon pads on the front and then crank down on the proportioning valve in the back. Might up the pedal pressure a bit though.

Seriously though - thanks for all the help guys. If anyone else has any thoughts I'm staying open and thinking this through some more.