Brett H. - 89GTA

I did the rear end raised on jackstands/rear brake test today, and the rear brakes cannot keep the wheels from turning in Drive at idle with me standing on the brake pedal. I'm not going to start another debate regarding the effectiveness of the factory rear discs. Instead, I would like clear up a few details concerning an adjustable prop valve installation.

I read the tech article, but unlike the author of the article I'm interested in keeping the stock prop valve in place as a distribution block for the front brakes. I would like to modify the stock prop valve to provide full pressure out the rear brake port, and then install the adjustable prop valve to regulate the pressure. I would also like to keep the brake warning light functioning as it does now.

To implement this, I assume that I can simply remove the rear brakes' plunger and spring from the stock prop valve. My questions are:
- After the plunger and spring are removed, can I reinstall the old end plug to close up the assembly? Or do I need to use a screw similar to what's mentioned in the prop valve modification threads?

- Do I leave the prop valve unmodified for the front brakes?

I am looking at an upgrade for the front brakes (probably the C4 mod), so I figure an adjustable unit will come in handy for that as well.

Kat

iTrader: (4)

Instead of trying to make hybird prop valve to cover up a problem. Why not figure out why the stock stuff isn't working first?

BTW have ya replaced the calipers recently?

Kat

Brett H. - 89GTA

The calipers have been replaced.

Rear brake bias, or the lack of it, is a common enough complaint with these cars even with the '89-up PBR rear calipers. I'm not covering anything up; I am instead addressing the issue head on.

After some more research, it seems that the screw in the front of the prop valve has a bleed hole in it that I would need to plug if I reuse it. Does the screw in the back of the prop valve for the front brakes also have a bleed hole? If it doesn't, could I get that screw off another prop valve and use it in place of the one with the bleed hole?

Chickenman35

Just toss the whole Factory unit. Not worth the time and effort muckin' about with it.


Get a simple brass Tee fitting. Some inverted flares and a flaring tool and the jobs done. Took me all of fifteen minutes.

Plumb in a WilWood ( or other mfg) adjustable unit and adjust to your heart's content. You will not believe the difference in how the car stops when the rear brakes are optimised. Like night and day.

Edit: Spelling

irocet305

Can you elaborate on that last post for the Wilwood install....please!??

Chickenman35

Most production cars do not use the full potential of the rear brakes....exceptions being cars like Porsche... or cars with ABS.

You can thank our clueless, lawsuit happy public for that ...along wth their high priced lawyers. Locking the front tires is considered " safe". Nice and stable...just so that your Grandma can drive it. Of course an properly educated driver ( Such as those on TGO ) would never even think about slamming on their brakes in the middle of a corner (that they took a just a weeee bit too fast) would they? Of course not. The car's gonna swap ends. So the Mfg's have to design their cars handling and braking potential to the lowest common denominator to keep the Ralph Nader types off their backs. Thankfully ABS alleviates this problem...and you can use the full potential of all four tires and brakes.

But getting back to our non-ABS equipped cars and their situation. It is surprising how much more rear brake bias you can use as oppossed to factory settings.

Watch any properly setup road race car. Do you notice that a lot of them "hunt" back and forth under braking. That is because they have the brake bias adjusted so that the fronts and rears are all working at 100% of their potential. All four tires are just on the verge of lockup...that will give you the absolute maximum braking of the car.

Under braking...lets say that the front tires are working at 99% of their traction capabilty ( before lockup ). Which car will stop faster?

1: Car " A" is setup so that front tires are at 99% of their capability...rear tires are at 60% of their capabilty. ( A common figure with production cars 25 to 40 percent of braking ability of the rear brakes\tires is not used ).

2: Car "B' is setup so that front tires are at 99% of their capabilty...rear tires are at 99% of their capability.

Doesn't take a Rocket scientist to figure out that car " B" is gonna stop faster.....a lot faster.

Lowering the car and stiffer springs tend to lesson front weight trasfer. So you can crank in more rear bias. Add wider and stickier tires and you can crank in even more.

With an adjsutable proportioning valve you can adjsut the brake bias to suit your driving style...and if the valve is installed inside the car, so that it is driver accessable, brake bias can even be adjsuted to suit different road and weather conditions. IE : wet or dry roads.

Gus_Mahn

Is there any way to keep the safety feature in the factory proprtioning valve? How badly are you screwed if you brake a line or hose with an adjustable? Can you still stop?

Brett H. - 89GTA

I would argue that you are safer with an adjustable prop valve if you lose a brake line. The safety feature is the dual master cylinder that implements independent front and rear braking systems. If you lose your front brakes, the rears will still work and vice versa. If the rear brakes are working that much more effectively due to an adjustable prop valve, they will be able to stop the car sooner than if the poor factory rear brake bias was left in place.

laiky

i too am interested in this exact same mod. but i would like to keep the brake warning light functional as well. i suppose you could just gut the stock prop valve and install the adj prop valve aft of it?

Chickenman35

Do not gut the stock proportioning valve . You will seriously compromise the operation of the system. The front and rear sections of the master cylinder MUST have separate fluid circuits. If you gut the inside ( of the stock valve ) the front and rear outlet ports are now in direct communication with each other ( IE: Fluid circuit ). The system cannot work this way.

Already been discussed here

laiky

ohh crap!! good point!! thanks!

xpndbl3

iTrader: (5)

chickenman.....can i get some pics of what exactly you did here.....maybe i'm a little slow to figure out what you're doing exactly.

Chickenman35

There's a TGO article by Rob Millet that explains it all....complete with picture. Just follow what he's done. Stock proportioning valve must be eliminated completely.

Article is here

xpndbl3

iTrader: (5)

hmmm....must have missed seeing this on that tech articles page. thanks for the link.

Gus_Mahn

Just curious, what is the issue with F-body rear discs? From what I'm reading standard rear disc don't apply with enough force. What about factory 1LE discs? My 89 Turbo Trans Am has factory 1LE brakes, which have never had the pads replaced. Since it was new, the rear brakes would lock-up during a panic stop. Is this typical? I'm thinking about adding the adjustable valve so I can dial out some pressure on the street and turn the rear pressure up at the drag strip to help build boost on the starting line.

Chickenman35

No..your situation is not typical. Normally, 1LE brakes have even more front brake bias than J65 cars ( 10.5" front disc and 11.8" rear disc ). Something else wrong with your system. A 1LE car will normally lock the fronts well before the rears.

The adjustable valve should do the trick for you. Pressure on the Wilwood units can be adjsuted from 100psi to 1,000psi. Should be more than enough to correct your problem.