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Posted by: yeahmag May 10 2011, 05:29 PM

Bleeding the brakes the other night with my Motiv Power Bleeder and setting the rear venting clearance reminded me of how much I hate the rear proportioning valve. I can feel it (and hear it with the car off) and it drives me nuts. Would a Tee with an aftermarket proportioning valve give me better pedal feel?

-Aaron

Posted by: McMark May 10 2011, 05:34 PM

It's a fantastic and brilliant safety feature that is worth the hassle of keeping.

Get Chris Foley's master cylinder brace for a firmer pedal. http://www.914world.com/bbs2/index.php?showtopic=105636.

Posted by: yeahmag May 10 2011, 05:35 PM

I'm thinking I should probably tell you my current setup:

* Master cylinder brace
* Stock front and rear calipers
* Motul RBF600
* Pagid Orange Pads on all corners
* SS braided Lines on all corners

Posted by: McMark May 10 2011, 05:46 PM

What size MC? If you're just looking for firm, getting the 19mm can help.

Posted by: yeahmag May 10 2011, 05:49 PM

I believe it's a stock bore MC. I'm more opposed to the rubbery feel/give of the proportioning valve than anything... I like the power the smaller MC affords me though. I imagine that is exaggerating the give of the valve.

Posted by: McMark May 10 2011, 05:52 PM

You're not noticing give in the proportioning valve. It's open when it's open and snaps shut when it's triggered. Technically is not a proportioning valve. It's a pressure limiting valve.

Posted by: yeahmag May 10 2011, 05:54 PM

That's true, but it's spring operated and you can hear it working (and feel). At least in my car you can... I've always wondered if it is set under spec.

Posted by: McMark May 10 2011, 06:01 PM

Yeah, you can hear it in a quiet garage. I usually start the motor and drive the car to drown out the clicking.

Posted by: Racer Chris May 10 2011, 06:17 PM

There's no factory approved procedure, but it is adjustable.
I've been thinking about increasing the limiting pressure on mine (street car) too.

Posted by: Eric_Shea May 10 2011, 08:37 PM

As Mark stated... it doesn't proportion anything. It triggers when there's a certain amount of pressure in the chamber by manually pushing the valve body with a micro-switch in one end of it against that large nut you see on the other end. Otherwise it acts as a T.

So if it doesn't proportion anything, you'd have to ask yourself; why you would want to replace it with something that did?

An after-market proportioning valve will do what?



OK... we've waited long enough; it will "limit" fluid to your rear calipers unless... it's wide open and again, not proportioning anything. So, that was a big waste of money (unless you want less braking power in your car).

Your front and rear calipers were biased by the engineers who built the car. Porsche did this with all of their calipers. If you plot the ratios from front to rear they are basically identical model vs. model and year vs. year. You don't want to limit fluid going to them unless you're in a panic situation. You DO NOT want them locking up. This is exactly what that pressure regulator does.

I've never heard one, felt one or seen one go bad but... I just drive them and don't tend to worry about those things.

I would recommend you go with a 19mm MC as Mark suggests. That's a no brainer... come to think of it; so is taking out the pressure regulator.

Posted by: seez52 May 10 2011, 08:53 PM

hum, seems like I read an article way back when by Bruce Anderson, and he was trying to cure a spongy pedal in a 914 after using silicone fluid. Seems he discovered the factory proportioning valve ended up being the main culprit. He replaced it with an adjustable one, don't remember now which one, but at the time he seemed to be a big fan of them. Of course that was 20+ years ago, and maybe it's not kosher today.

Posted by: Chris Hamilton May 10 2011, 09:52 PM

You can replace the proportioning valve with a T from a 356 or 911. Bolts right in with the stock lines.

Combine that with some good new fluid and stainless steel lines and you'll have good brake feel.

Done that on my 914 and a few other autocross cars around here. Always a big improvement.

Posted by: Racer Chris May 11 2011, 04:08 AM

In my experience with aftermarket proportioning valves, they allow full hydraulic pressure to be transferred to the rear brakes until the system pressure reaches the adjustment limit.
At that point the valve reduces the rate at which the pressure increases in the rear calipers.
It is a very effective way to improve the 914 braking system over what the factory provided.

However, an even better way is to upgrade to a dual MC with bias adjustment. That way, no reduction is force is ever applied. Full system pressure is always available, proportioned between the front and rear by driver selection.

Posted by: 7275914911 May 11 2011, 06:19 AM

Could you do something as simple as going to rubber lines??

I have heard that the stainless can be mushy compared to rubber. I have stainless also but they went on with other brake upgrades so I can't really compare. Since you mainly AX heat should not be an issue?

Good Luck

Posted by: Eric_Shea May 11 2011, 07:06 AM

What brand? Most have a simple screw or lever adjusting the fluid.

Also, the factory valve limits once the pressure is reached and then, when the pressure equalizes in the chamber, it brings the rears back in. One of the first anti-lock brake devices.

The "main objective" is to keep the rears fully functioning yet never have them lock. The factory valve performs that function better than anything I know of to date. And remember, this is for a car that is insured (I presume) and driven on the street, not a full out race car.

Just say'n... aftermarket bias valves etc., is the wrong message to send to 99% of the guys reading this right now. The problem being, every one of them believes they're the 1% and they would know what they're doing (I'm a guitar player... I understand the mentality)

Posted by: sean_v8_914 May 11 2011, 07:26 AM

Pagid orange pads are not the greatest street pad, better suited for track. they take a bit to warm up and grab. a pad that grips cold will give you better brake feel.

the brake tee is the way to go if you are an experienced driver. on teh street we never really have teh opportunity to experience threshold braking over and over again. threshold braking takes practice but if you have trainned yourself to know that limit before lock up then the tee is a good swap.

Posted by: yeahmag May 11 2011, 08:20 AM

Just for reference this is not a daily driver, but mostly AX and weekend warrior. I am very experienced with threshold braking after racing competitively for 10 years and finally winning my class this year, that being said there is no need for unnecessary risk.

I've heard most of the feel issues associated with the stock P-valve is related to the volume of fluid inside of it. Just adding fuel to the flames...

Posted by: Racer Chris May 11 2011, 08:45 AM

It would be better in the Paddock since this specific discussion relates primarily to competition purposes.

I understand Aaron's complaint because I get the same feeling from the valve in my street car. I have a 19mm MC and I can imagine it feels worse with the 17mm because of the increased pedal travel. The volume of fluid displaced when the valve cracks open probably allows the pedal to just about reach the floor even when everything else is set properly.

Posted by: yeahmag May 11 2011, 09:19 AM

Hit the nail on the head Chris.

Posted by: Racer Chris May 11 2011, 09:40 AM

Click on the link to read a
http://www.stoptech.com/tech_info/wp_proportioning_valves.shtml

Posted by: Eric_Shea May 11 2011, 09:58 AM

Never in 30 years of 914 ownership... but, I'm not as smart as you'all.

I give up talking about pressure regulators. Carry on!

Posted by: yeahmag May 11 2011, 10:00 AM

Not to worry Eric. I like to debate to get all the info I can. I'm also stubborn in that I really only learn by doing.

Posted by: sean_v8_914 May 11 2011, 09:17 PM

i do a lot of 914 brakes...never had a stock 914 that works correctly do that. it would not leave my shop that way. something else is wrong

Chris, thanks for teh great read

Posted by: McMark May 11 2011, 11:07 PM

That reminds me that the factory manuals have specs for brake pressure. I think it's time to invest in some gauges and do a little testing. I suspect it might be possible to detect bubbles via pressure readings.

Posted by: Woody May 12 2011, 06:38 AM

I am running stainless lines, 19mm master, and the factory valve. If I recall correctly I did one turn in on the adjustment screw. My pedal is very stiff and have never felt or heard the valve open up. Im very happy with my brakes.