KidPandJ

http://en.wikipedia.org/wiki/Proportioning_valve

Never looked for it on the SC, its under the hood inline with the brake lines on everyother car I've owned. Surprised you've never heard of it.

SCoupe

Clarify...I'm perfectly clear on what it is however, no one to my knowledge has ever documented how to alter or adjust one on an SC300 and reported back on the results.

I'm interested because my Supra TT brakes are more than capabable of locking up the front (you feel the ABS pulse), but never have I felt the rears in the same mode. I'd like to bump up the rear brake bias using the proportioning valve but I do not have the skills to know what I'm looking at, just skills to follow anothers instructions if they explain it.

KidPandJ

I'm definetly not an expert on them (or brakes). Like you, I just know basically what they do and that most cars have them. After your question searched and found this article. Its pretty good. Has alot of good brake stuff in general but specifically a good article on the proportioning valve. After reading it, it looks like factory proportioning valves in factory vehicles are non-adjustable. Maybe you want to look into getting an adjustable one for your car. I know that still doesn't help you find the one on our cars, but hopefully the article will help some.
http://www.stoptech.com/tech_info/wp...g_valves.shtml

SCoupe

That stoptech link is awesome.

ki_soarer

although the valve is the same, wouldnt the fact that more fluid is needed for the front callipers to push the larger pistons cause less fluid to go to the rears? i would think that would through off the ratio a bit. im sure its not anything the normal SC owner would notice, but perhapse those that track their car may. i dont know though, just a thought.

im prety sure the stock SC front brakes are about 11.4 inches or so... but again, i havent measured them. maybe i will tonight, just to know.

KidPandJ

I think what you're saying is right, but the valve doesn't control the amount of fluid it controls the pressure in the front lines vs the pressure in the rear lines. At a certain pressure level, it decreases the rate at which pressure builds in the rear lines and allows the fronts to increase in pressure. This is different than measuring how much the front brakes stop vs how much the rear brakes stop. The article posted above really does a great job, much better than me, of explaining the whole thing (and brakes in general).

ki_soarer

havent read it yet, but wouldnt more fluid change pressures? im confused.

KidPandJ

Technically, because the brakes are a closed system more fluid has to be forced into the line to increase the pressure. I was just saying that the proportioning valve doesn't control flow, it works off of pressure differentials. To better reply to your other post. Yes, bigger pistons (in your front calipers) would cause less pressure in the front brakes and the proportioning valve would then allow the front brakes to increase to greater pressures and this causes the front brakes to have more braking power than was originally intended by the manufacturer. Though like you said, I (and other daily driven SC's) probably don't notice the increase in front bias.

MJHSC400

I think the lone fact that the cars we are interchanging brake systems with are very similar in weight and power will cause the brake bias to be nearly the same, and I also think the 4 pistons are smaller ind. pistons than that of the 2 piston calipers--

I think that the pressure will be the same, as once the pistons are all primed, there is niether positive nor negative pressure in the piston itself, until the pedal is depressed--

The figures at play here are the difference in the volume of the master cylinder vs. the surface area of the actual pistons-- This ratio compounded with the ratio of the brake pedal's fulcrum/lever reduction force multiplication crap'ola will be the final deciding factor.

Understanding this, it may take a fraction more pedal travel to see the same "pressure" inside the caliper, but that pressure alone is not the only factor here.

The larger diameter of the rotor offsets this, and the pad surface area vs. the piston surface area vs. the pressure -- blah blah blah--

It will feel about the same from the pedal end, but the stopping power can see huge increases, as much as 20% or so--

The rears if left stock will still do their job, as the braking system is abs, and is capable of locking the wheel if needed-- this means that the primary worry is heat dissapation, so slotted/drilled rotors and high quality pads OR a normal driving style are in order here....

A caliper on a larger rotor travels farther distance on the braking surface before it gets back to zero, and this circumference difference can play a huge role in fighting heat soak, as there is a certain temperature that the pad and rotor should remain within to fight heat instead of promoting it-- surface area..

GalantVR4

OK, I have to pipe in on this. EVERY car with four wheel brakes has a proportioning valve. However, it is usually buried. Older, non-abs cars usually had the P-Valve right under the master cylinder or it was on or near the center of the firewall. Pretty simple deal a brass block with different sized orifices and a "shuttle" valve that also activates that "dual/diagonal" braking system if you blow a brake line. Which is why, when you're bleeding your brakes, you NEVER do a FULL STROKE and bottom out the master cylinder. Then the shuttle valve will go off center and trigger your emergency brake light on the dash. Wilwood makes an adjustable one that sells on ebay for about 50 bucks or so.

Problem is, on OUR cars, and most ABS cars, the P-Valve is built into the ABS motor/control box. (remember where all those brake lines go from the master cylinder?)

Now, on to the idea that you'll wreck the proportioning differences by upgrading calipers. Let's say you throw a set of supra or LS four piston calipers, or even Wilwood or Brembo four piston calipers on. (The Wilwood four piston calipers for sprint cars, brand new from summit, are 129 each and a LOT lighter than the LS calipers) You will, naturally, use a bit more brake fluid per stroke of the brake pedal, simply because the four piston caliper has more internal volume. However, if you kept close to the same PHYSICAL size caliper, it should be close, because the four piston caliper has smaller individual pistons.

HOWEVER, if you get the uber huge piston calipers like the BIG road racing Wilwoods, (as a sample, the TCI big brake kit on my Race Galant VR4 has calipers that are twice the size of the stockers in length as well as thickness, and its pucks are the same size as the stock twin piston) then you run into another, more serious problem -- your master cylinder can't push enough fluid to move the calipers! Which means, you don't STOP.

However, from what I've seen, you'll just get those "Mushy Brakes", no matter how hard you bleed them. This is because you're essentially bottoming out the master cylinder and just missing getting the brakes completely applied. The "MUSH" comes from just trying to stomp that last little bit on the pedal..it is instinctive to do that, but all you're effectively doing is a bit of firewall flexing. Or, you're pushing the plunger down the bore far enough that the bore isn't smooth. The bore has corroded because you weren't using that part of it with the stock brakes and now fluid is bypassing the seals on the plunger internally)

From what I've seen, we don't have a larger BOLT ON master cylinder. One with a longer stroke is not a good idea, because the longer stroke will put your pedal all the way to the floor. The only other way is to find a master cylinder with a significantly larger bore. Problem is, an LS400 and a Supra master cylinder will not bolt on. For some reason, probably a nod at firewall flex, Toyota used a THREE BOLT flange on the SC master cylinder, while using a TWO BOLT flange on every other vehicle they made. So unless you're good at making an adapter flange, nothing else would work. The supra master cylinder would be ideal..even better if there was a TRD m/c, but without the adapter plate, it isn't possible.

Now for the rest of the brake bias story. If you upgrade, normally you will not be using factory brake pads, right? If you do use factory organic or semi-metallic Advance Auto or O'Reilly OEM style pads, you realistically will only increase your clamping force by maybe ten percent.

The advantage to bigger brakes, more caliper pucks, and bigger rotors lies, as was stated above, in the fade resistance given with the increased surface area of the rotor. Plus the extra area and fluid capacity of the caliper also acts as a heatsink. But if you use a performance brake pad for the front, like EBC Greens or reds, or even worse a ROTOR EATING pad that makes the rotor glow cherry red after each stop, you WILL radically alter the brake bias.

Fixing that is fairly easy though, and you do not have to resort to dual piston rears. Just get a more aggressive rear pad. Personally I tune bias BY brake pad material selection so I get exactly the grabbing power and fade resistance I need. On a car that isn't well balanced, like a 67 camaro with a cast iron big block, you'd need that adjustable proportioning valve. However, in our cars, you don't.

So much for brakes 101..
Dave

ki_soarer

dave... you rock! that was very nicely put. time to make this thread a sticky. or at least that post.

SCoupe

Simple enough.