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The proportioning valve revisited ...

I've taken out my rear brake lines and calipers to rebuild the back half of the braking system. I was curious as to how the stock proportioning valve works and I disassembled it to get the crud out and check to make sure it's in good condition. I thought I'd take a little time to understand it and post the results.. So first here are some pictures with it apart:

So the way it works the best I can figure out, the brake fluid comes in the inlet port and enters the middle of the cylinder between the two seals on the piston. The fluid flows through the holes and out thorough a valve at the end of the piston. This valve is held open at low pressures by the force of the spring. At a high enough output (rear brake) pressure the piston starts to work against the spring which allows the valve to begin closing off. This regulates the output pressure and maintains a differential between front and back.

I took the only description of the pressure specs I could find and graphed it, shown here:


You can see the break point at 683 psi where the piston would start to move and regulate the output. The three lines are the specified tolerances of the valves pressure differential. The straight line would be no prop. valve with a Tee. I measured the factory compressed height of the spring and compared it to the maximum compression where the spring would start to bind. There is about .7 cm additional range it could be compressed before binding. The factory chose an adjustment screw length to be pretty close to the point where the spring reaches its limit. On mine, running the screw in flush with the locking nut gives about .5 cm, I estimated this increases the spring force about another 115 lbs and raises the break point up to about 927 psi nominal.

I've read about putting in a longer bolt. You can do this, but it will cause the spring to bind at some point. If you bind it then the valve will always remain open so it's essentially a Tee. You wouldn't want to count on having a precise break point where the spring is near binding.

What does this all mean? The hell if I know. But I think that with my front BMW calipers and V8 conversion I can tighten the spring to bias the rear more. It will be more than stock but less than a Tee. I'm reluctant to go with a straight Tee for a street car.

One other interesting thing. When I pulled the lines there was hardly any fluid in them. Proof that the previous owner had not bled the rears completely. At low pressures this prop valve is open but I noticed it does not allow a lot of fluid flow because of the small size of the internal regulating valve. I really plan to spend a lot of time bleeding to get all the air out of this system.
Old 08-27-2003, 11:01 AM
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Hi,
My car has BMW calipers on the front so I took the P Valve off and tossed it. I replaced it with a "T" fitting and it seems to work OK. It looks like you did a good bit of research. I think the "T" will work OK for you. I like to think that "simpler is better". It is also much easier to bleed the system without the P valve. Good luck.
Cheers, Elliot
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Old 08-27-2003, 11:32 AM
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Ellliot, thanks, if I get enough votes of confidence I'll go with the 'T'. I read through many past discussions on this topic and many put in a 'T' and others say the stock valve is fine just bleed it well. But I agree with you, a T is simpler and easier to bleed, and if it's not a safety concern then I'll go with it.
Old 08-27-2003, 11:39 AM
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Did you happen to notice if the proportioning valve has any method for adjustment? The reason I ask is because I recently did the 320i brake upgrade & now my front brakes lock up pretty easily. I would like to either adjust so that the rears get more pressure or replace with a T.

Vern
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Old 08-27-2003, 12:27 PM
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Yes, there's the hex screw on the end. There's a bit more range left for it to still act as a proportioning valve. Any more and you bind the spring and it should behave like a T. But I would either screw it in flush with the jamb nut and try it, or take it out completely and put in a T. Mine was factory set with some kind of epoxy, I had to break it loose with a propane torch to get it free. Might be just another reason to replace it.
Old 08-27-2003, 12:48 PM
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Thanks for the post. Its always good to understand how things work.

I recently installed a 320i calipers and a Tee. No problems either on the street or track, in the rain or dry. YMMV

Note on the conversion,
To hold the calipers on a mill for the machining, run a 1/2-20 tap throught the mounting holes and screw it to a plate which can be held in a vise.

Also, the Tee came from a BMW dealer for less than $15.
Old 08-27-2003, 01:41 PM
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The 'Tee' it is... Anyone need a nice looking powdercoated P-valve?
Old 08-27-2003, 01:51 PM
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Wow Guy!

Way to really "get into" your project. You can be the resident Proportioning Valve expert!!!

Thanks for the informative post.
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Old 08-27-2003, 02:02 PM
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Huh. Nice post!
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Old 08-27-2003, 03:40 PM
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Where exactly is that sucker mounted? Lower engine bay? I may do a 'T' as well as I'm putting on 911 calipers up front. Any pics or suggestions on the 'T' mounting, etc?

Thx
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Old 08-28-2003, 09:46 AM
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Jason

It's on the lower left side of the engine bay (behind drivers seat) and below the engine tin. I'm hoping the lines can be rebent slightly to mate to the Tee. Somebody who's done it will know.
Old 08-28-2003, 12:11 PM
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It's best to bend it with a tubing bender and for the brake lines they are pretty cheap. You can get them at Pep Boys etc. However small bends can be made by hand if you are careful.
Cheers, Elliot
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Old 08-28-2003, 02:18 PM
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Again, thanks for the post. very interesting!
Old 08-29-2003, 09:21 PM
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so, what are the repair parts numbers? i see o-rings and a gasket....
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Old 08-31-2003, 02:56 PM
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Good question! Haven't seen them in any parts catalogs. I can't even get all the replacement parts for a 19mm master cylinder let alone this valve. I gave the rubber parts a good inspection, they appeared to be in good enough shape so I cleaned them and applied a little silicone grease. The cylinder also looked good, I lightly honed it with some 1500 wet/dry paper. Then poured a little fresh brake fluid into it before reassembing. But it doesn't matter anyway, I decided to put in the 'T'. The experience satisfied my curiosity.
Old 08-31-2003, 03:20 PM
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The parts book does not list any parts inside the proportioning valve. Nothing is listed in the "repair sets" portion of the manual, either.

--DD
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Old 08-31-2003, 03:48 PM
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I removed the prop. valve and installed a "T" when building my v8 conversion based on others experience. This has worked out very well for my street car and I have no complaints. Using 911 M calipers/rotors/struts for the front and 914-6 capilers in rear with vented rotors, I can get it stopped faster from 100-0 better than any previously owned car.

I can't imagine adjusting prop. valve in its original position with a v8 installed. I can still see my "T" but can't imagine having enough room to get a wrench on it to perform any bleeding. My T is located right in front of the side mounted water pump.

Using a VW "T", the small amount of bending necessary can be performed by hand.

You will notice a 1-1/4" gap between the T and where it is mounted to lower bulkhead. I found various plastic spacers at my local Lowe's as well as new mounting hardware. I thought about just letting the metal lines support the T but a sturdy mount only took minutes for resolve.

John
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Old 08-31-2003, 05:35 PM
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I know they have different part numbers but is there any real difference between the proportioning valve for a 914-4 versus a 914-6? Anyone know for certain?
Old 05-21-2010, 03:43 PM
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I'm also doing the 19mm MC with the BMW front brakes. There was one mention of the BMW dealer as a source for a "T". Are there any other suggestions for sourcing a new brake line "T" to replace the proportioning valve?
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Old 05-21-2010, 06:58 PM
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The Six valve has a different "elbow" in the proportioning curve. The factory manual shows graphs of the two.

I believe that old Bugs also had a brake line tee somewhere that works for us.

--DD
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Old 05-22-2010, 07:03 AM
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