Aftermarket proportioning valve and brake bleeding

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vwduud
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Aftermarket proportioning valve and brake bleeding

Post by vwduud » Tue Jan 11, 2005 10:40 pm

Proportioning valves - once you've seen one, you've seen them all - more than likely, not.

I am working on solving my spongy brake problem. I have rebuilt the front and rear calipers, replaced the flexible brake lines, upgraded to the 19mm master cylinder and completely flushed and replaced the brake fluid.

I have searched and read topics on the STF on the 914 proportioning valve, and the added difficulty in bleeding due to this valve. Some of the topics recommend replacing the stock valve with an after market valve. Initially I was looking for topics on cleaning/rebuilding the stock valve. After much reading, it seems replacing would be a better choice. Additionally, I am upgrading the front calipers to the BMW 320 calipers.

So, two questions:
Which of the following proportioning valves would be the best choice for a replacement of the stock, in the stock location:

http://www.jegs.com/cgi-bin/ncommerce3/ ... rmenbr=361
http://www.jegs.com/cgi-bin/ncommerce3/ ... rmenbr=361
http://store.summitracing.com/default.a ... ning+valve

With any of the above valves installed, can the system be bled with this valve completely open, for easier brake bleeding, then adjusted for brake bias?

raywarren
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Post by raywarren » Wed Jan 12, 2005 6:24 pm

If you are upgrading your front brakes to BMW 320 you could
just replace the P valve with a T fitting.

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raygreenwood
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Post by raygreenwood » Wed Jan 12, 2005 10:52 pm

No...you can't leave off the proportioning valve. It has nothing to do with the front brakes. There are seperate master cylinder brakes lines to front and rear, so regardless of what is done to the front..the rear will still need its proportioning valve...if it needed one in the first place. An explaination as to what the valve does?....when you step hard on the brakes...enertia pushes teh nose down...and the rear up...taking weight off the rear wheels. It now requires less brake pressure to stop...now that down pressure is reduced. Without the proportioning valve...the rear wheels have a bad habit of locking up...then giving you no braking. Contrary to popular belief...the stock proportioning valve does not add any difficulty to bleeding....unless....it is sticking. In that case it needs rebuilding. This is the exact same part # valve used on the 411/412. the parts are unobtanium. Almost any aftermarket, highly adjustable valve with the correct line connections will work. Then...you have to adjust it. The stock one was tricky to adjust. You need a gauge that goes to 700 psi...unless you want to adjust by seat of the pants...which is an all day affair. Ray

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raygreenwood
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Post by raygreenwood » Wed Jan 12, 2005 11:04 pm

Oh forgot to mention, that the reason these parts are not a hindrance to brake bleeding...is that they do not stop brake fluid flow. If it is a proper proportioning valve...what it does...is lets fluid from 0 psi...up to the pressure its set at...flow past. So the relatively low pressure of brake bleeding is no problem. It passes right through. The one you have...with the type 4 part #....works the same way. When you open it up, you see a plunger with a single cup...with a hole through it to let fluid pass. When you squeeze the brakes...the piston starts moving...but since it has a hole in it...its letting fluid pass. Inside of the piston is a nylon plunger....set against a spring in a counter bore. The outside end of that nylon plunger...opposite the cup end of the piston....goes up against a cap on the big fat main spring. That main spring is designed to exert...when properly adjusted....some 617 lbs of force against the nylon plunger. When the piston and nylon "teet" sticking out of it, movee far enough and exerts enough pressure against the mainspring...the nylon plunger snaps back agains its spring, effectively blocking the fluid flow through the piston. Since there is already presure in the line and the movement of the piston in the valve is increasing that....but the volume of fluid flow is now stopped, the existing rear brake pressure is still maintained as long as your foot is in it.....but it cannot increase any further. I hope this helps. Ray

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Racer Chris
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Post by Racer Chris » Thu Jan 13, 2005 9:34 am

Just do what Ray warren says and don't listen to Ray Greenwood. :P
Many people have replaced the proportioning valve with a tee after increasing the front brake caliper size and experienced no problems. Front calipers with larger pistons will move the brake bias forward and there will be less likelihood of rear lockup.
The OEM prop valve generally sucks, particularly when it comes to bleeding air from it. If you think one is necessary an aftermarket adjustable unit is fine (Tilton, SS Brakes Corp, Etc.).

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raygreenwood
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Post by raygreenwood » Thu Jan 13, 2005 11:51 pm

If you have no problems on a 914 with enertial shift...then the car probably did not need a proportioning valve in the first place. Front brake configuration has no bearing on enertial shift...laws of physics cannotbe denied. But...stiffer than stock springs and dampers DO have a bearing on enertial shift. They can nearly prevent body "dive"...which is the SOLE reason proportioning valves are installed....regardless of how big your front rotors are...or how many pistons you have. Rear end wight versus front...coupled with foward motion...is what creates the reduction in rear downforce when you apply the brakes. I can't disagree with leaving the valve off...if it works....but just want to set the record straight as to the reason the part is installed in the first place. Never the less...for some reason the factory felt the need to install the part...and they were damn expensive.

As for the bleeding of this part....disassemble one and see how it works. Like I said...if it was hard to bleed...it was defective, rusted or clogged. Its a straight through tube... unless the aluminum piston is frozen up...or the nylon plunger has swelled from water content in teh brake fluid..and locked in the closed position. A properly function part gives no problem whatsoever. In that respect....all of these parts no matte who makes them are capable of the same fault. They all work basically the same way. If you want one to play with...I have about fifty in a couple of buckets.
Just be sure you are absolutely sure...your brake system, suspension and car can handle not having this part. Yours is especially suspect...if you have upgraded to a larger diameter master cylinder..which then has increased volume and a faster pressure rise in both the front and rear.

Its very easy to think this part does nothing. Its only needed in panic braking situations...wherin...a car that needs one...especially a rear or mid engined car...will be at very high risk of swapping the a*s with the nose in this situation (theres that dang enertial shift again). Since that may only happen to the spirited driver once or twice a year........are you REALLY sure its fine just putting in a "T". Not saying it isn't...just that if you need one...its a great way to get killed.

:D 8) This ends my public service message....now brush your teeth and go to bed! Ray

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Dave_Darling
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Post by Dave_Darling » Fri Jan 14, 2005 5:54 pm

Putting calipers with larger pistons on the front changes the brake system's bias. The rears, effectively, do less work because the fronts are doing more. You still have less traction on the rear wheels under braking, but you are applying less braking force to the rear wheels when you install the BMW brakes. So the need for brake proportioning is lessened.

I cannot say that you don't need any proportioning at all. Most especially in today's litigious environment. But you don't need as much proportioning (reducing the pressure to the rear brakes) because you are effectively doing that by increasing the pressure to the front brakes.

--DD

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Piledriver
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Post by Piledriver » Fri Jan 14, 2005 8:36 pm

914s don't squat.

Been here/done this with BMW 2002ti calipers...
(SS lines, 19mm master, stock front clipers on rear. Handbrake?)

Had to move the fronts to the rear to get the rears to even work.

I haven't heard that is needed with the 320i calipers, but keep it in mind.

(BIG misbalance with the 4 piston calipers and all that pad area)

Sliding up to a stopsign with the fronts locked and the rears doing hardly anything is almost as bad as having the rears lock.

The 914 proportioning valve is a POS and makes bleeding by yourself virtually impossible. There are other ways to set the balance...
Like getting the calipers sized right.

Do the brake mod. The 914 can stop with the best of them. Suggests better tires.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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raygreenwood
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Post by raygreenwood » Fri Jan 14, 2005 9:46 pm

Dave...that explaination makes sense...that the frot is now doing more work....and the back is NEEDED less. But...the back is still braking at the same rate..because you have done nothing to lessen the flow of fluid by making larger brakes in the front...because the front and rear have seperate feed lines. Yes, I realize the secondary piston does not move very far until the spring stop on the primary engages....but it does move..and it happens instantaneously on panic braking.

But what does make sense as you and racer chris are suggesting...is that by having the front stop faster...you don't need to depress the brakes are FAR...therefore not addin as much pressure to the rear as before....if I understand what you are getting at.
The only remaining problem with that....is that it still doesn't save the rear from lock-up on panick braking.

Piledriver, if the car does not "squat"..then it probably never needed a brake pressure regulator in the first place. But...make no mistake...though the body may not dive...the enertial shift is there just the same. If traction is low....same thing happens....the rear passes the front. just want people to be careful in these decisions. The 914 may not need a regulator...but the 411/412 definately does. When mine first died...I installed a "T". I almost rolled it coming over the bridge into Oakcliff south of Dallas on wet pavement. I ended up rebuilding the regulator...but am looking for a better one I can buy parts for.Ray

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Piledriver
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Post by Piledriver » Sat Jan 15, 2005 1:28 pm

The car works great in the rain on those oil slick Dallas freeways after a hard rain, and I HAVE intentionally beaten it to death trying to get it to do something stupid. (when no one else is around of course)

All just to see if I really could get away without the proportioning valve. You can, if everything else is right.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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Dave_Darling
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Post by Dave_Darling » Sat Jan 15, 2005 3:38 pm

Pile, you get the most weight transfer on dry pavement, not on wet. Try dry pavement, warm-ish day, downhill. That should transfer the most weight off the rear wheels and give them (relative to the fronts) the worst traction.

--DD

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Piledriver
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Post by Piledriver » Sat Jan 15, 2005 5:46 pm

Oh, I have, every combination I could think of.
The fronts ALWAYS lock first.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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raygreenwood
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Post by raygreenwood » Sat Jan 15, 2005 9:42 pm

Cool...nice to know that the 914 is balanced enough not to need that valve. Its a pain. Its a pain...not because it did not work on the vehicles it came from...just that the dang nylon plunger was teh defective part. The nylon swelled due to the water content in old brake fluid. Thats what causes it to bind up and not work. Ray

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Piledriver
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Post by Piledriver » Sun Jan 16, 2005 1:00 am

I wouldn't ry it with the stock calipers, but with the bigger ones I am running, it works.

It would all depend on the specific setup, and you have to test it.

Would it be possible to replace those nylon pistons with something that doesn't suck?
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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raygreenwood
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Post by raygreenwood » Sun Jan 16, 2005 11:16 pm

I thought about that a while back. It could do fine with Delrin (maybe)...but torlon for sure. It would last forever. It could even be done with aluminum. This part is about 1" long...and has a variable shape...with the "teet" part about 3mm and the stem about 5mm.
When I rebuilt mine, I had a company machine a new seal...which is really small....on a CNC mill designed specifically for seals. Problem was, the materials they had on hand were limited. They had to be stiff enoughto be machined. So the Buna N...I had the seal made from, was about 80 durometer. This made it very stiff. I split 3 just trying to install them. They are about 12-13mm OD...and about 4mm ID :shock: ...and about 3mm thick. I got it together....and it still works..... :shock: but I had to sand the nylon pin to get a smooth function. Since its no better or worse than any other...save for the nylon materials, i will be replacing it with an aftermarket when I get this beast back on the road. It will have rear discs this time. Ray

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