Need Replacement O Ring for my Stock Prop Valve: Lessons in Brake Modification (long)

[RockyTop]

I have a 97 GTS. As an easy and cheap fix to get more pressure to my rear brakes I had my Viper tech remove the O Ring in the proportioning part of the stock brake combination valve. Unfortunately, contrary to my instructions, he accidentally threw it away. Using modified rear knuckles that I recently purchased from John Archer, I had my tech move the stock Brembo front calipers and rotors to the rears and we installed a Mopar performance (i.e., Wilwood) adjustable proportioning valve on the rear brake line.

I am now getting rear lock up first (passenger side only, curriously enough) no matter how much I decrease the rear brake line pressure using the adjustable valve. I have no doubt flat-spotted the tire, which is no big deal as it was just about worn out anyway, but is surely exacerbating the symptom.

Russ at Archer's shop and the guy I spoke to when I called Mopar Performance's tech line think that I am probably exceeding the capacity of the new valve to reduce the rear line pressure due to the increased line pressure from the stock valve due to the removal of the O-ring.

Now to the point ! Does anyone have or know where I might get a replacement O ring without buying a whole new (or used) stock valve ? I called Archer, John B and Sean Roe and while all were very helpful (as always), none of them had one.

One other question: Any thoughts why only one rear tire would be locking up ? I could think of the folloing possible reasons: (i) differing tire pressures (I'll check); (ii) differing alignment setting (e.g., camber) - -unlikely since I just had it realigned before VIR and I stayed on the track and hit no curbs (at least not very hard); (iii) that side's pads may not yet be seated well - -again I think unlikely since I did some pretty heavy braking excercises trying to dial in the new prop valve; (iv) differing line pressures - -unlikely, but I may have the tech check the respective pad/line pressures using the tool they have just to make sure; (v) kinked brake line (I'll check); or (vi) dead or improperly tuned shock at diagonally opposite end (i.e, driver's front thereby permitting too much load transfer to the front) - -unlikely as I was testing in a straight line, I suppose differing front shock forces might cause one side to dip more. Your best guess ?

 

[Tom Glenn]

My guess on the rear lock-up is the extra weight of the driver and battery on the driver side rear tire keeps it from unloading as much as the passenger tire.

 

[kverges]

For one-wheel lockup, I would check corner weights, too. I used to have a similar problem in my RT/10 and no more due to coil-overs and precisely-set corner weights.

Contact me off-line and I will search my attic for the O-ring that came out of my '98 - I saved it and no longer have the car, so you are welcome to it if I can find it.

 

[Stephen Yap]

Dave is right. The Wilwood knob style proportioning valve should replace the stock proportioning valve. This probably means that you will also have to have something for the front. I don't know why using both the stock and the knob style prop valves will cause one wheel to lock up.

 

[Bob Woodhouse]

Rocky, David Tittle, Dilusi Motorpsorts is giving you good advice. From the limited amount of info (you never told us what calipers you installed in the front) it sounds like the O ring will do you no good. In fact I would pretty much bet on it. The stock system does not reduce pressure until 800#s of line pressure is present. Your issues seem to be under that pressure. If you have purchased some aftermarket front calipers then call them and ask them to do the math for you on what the piston sizing "should have been". I assume you are still running with the stock master cylinder. If you increase the piston size of the slave cylinder (caliper) by two times, which is basically what you just did at the rear, then you will get twice the clamping force. In relation to your front brakes I am thinking you may have gone beyond the ability of a prop valve to bring you a balance front to rear. If they can't do the math, E mail me.

 

[Bob Woodhouse]

One last thought on your single wheel lock up Rocky, all of the diagnosis could be thrown out if you have a brake hung up. Look for a stuck piston, cocked or poorly aligned caliper, uneven pad wear.

 

[dtittle]

Hey RockyTop,

Some will disagree but the car is incorrectly set up if you're using the two proportioning systems that you described. Fixing the O-Ring will not 'Fix' your problems.

Since the new knob style prop-valve SHOULD be doing ALL of the proportioning you need to completely bypass the rear proportioning block that's on the car. If you don't you getting messed up.

Setup is simple. The front line from the MC feeds the front brakes 100% and the rear line feeds the proportioned circuit. Do it this way and no problem

You also have to remember that you need a lot of fluid volume to the rear calipers. Using the factory O-Ring WILL NOT supply anywhere near the amount of fluid that you'll need and your proportioning will always be wrong.

Regards,
Tittle

 

[RockyTop]

Thanks so much to all who have helped with some excellent suggestions. Special thanks to Dave Tittle who spent 30+ minutes with me getting me up the learning curve. One suggestion that John Archer had re my one side lock up situation, the caliper may be misaligned due to mis-installation of the mounting bracket (i.e., one side mounted backwards). I'll check.

Once again my faith in the VCA, this board and its contributors is reinforced by the unselfish contributions of time and effort of some very knowledgeable folk.