Adjustable Prop Valve

[HMeeder]

Quick question:

Did the 914 (specifically '72) come with an adjustable proportioning valve? My car seems to have one (looks like a 5mm allen head bolt held by a large retaining nut on the slim end of the valve.) I fully realize that I may not know what the hell it is I am looking at. Haynes doesn't mention an adjustable prop valve but that doesn't mean anything. I haven't had a chance to fool with it yet but the reason I ask is that I am autocrossing on Saturday and if that valve isn't stock then it means that I get dinged for 2 more points for non-stock brakes and I get bumped up to the next class. The difference is between being at the top of the street stock class or the bottom of the "stock" bracket. Kind of a bummer since it doesn't seem to do a damned thing for the brakes. Still mushy, can't lock the brakes on the slickest of surfaces. Just curious.

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Herb
'72 1.7 Tangerine 'Teen
'74 2.0 Red Rustmobile

[This message has been edited by HMeeder (edited 02-16-2001).]

[John Rogers]

The prop valve is adjustable, but not by the owner usually. It sounds like you have air in the valve and is hard to get out. I replaced the prop valve on both our 914s years ago with a tee and they brake great. Good luck.

[pbanders]

please ignore

[This message has been edited by pbanders (edited 04-20-2001).]

[DDS]

Herb and I seem to be working through the braking system together. I've been pondering that valve too, though my situation is a bit different. I know the 'T' works fine for most with stock brakes, but I've gone to SC fronts and stock calipers on the rear. (no class worries anymore) I'll be converting to the 19mm mc. I understand also that the stock p-valve can be made adjustable. I'm not flaring the fenders, so you know the tire size limits.
Given that I'll have much greater braking potential on the front, what should I do? Will I have a heat problem with the rears?
Thanks for any insights.
Dave

[racejoe]

Well, my $.02. My understanding is that the sole purpose of the proportioning valve is to keep the rear from locking before the fronts. So they retard the rear calipers if you will.

That being the case, the worst senario is with stock front calipers, or I guess even worse is if you upgraded the rears and not the fronts.

Anyhow, the more upgraded the front calipers are vs the rear, the less need there is for the proportioning valve.

That is how I understand it anyway.



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Joe
74 Yellow 1.8L 914
72 Tangerine NoL 914
http://pages.sssnet.com/jkaull

[HMeeder]

Howdy all:

Well, Thursday afternoon I found that my passenger side rear caliper wasn't working. That's when I found the prop valve, which I had never looked at closely before. After some nosing through the brakes I found a severe case of artherioschlerosis necessitating a quadruple by-pass. Ok, so the flex lines were waaayy clogged. I replaced with SS lines (happened to buy a set with the intention of upgrading the brakes in the future) and after proper bleeding the system worked great over the weekend.

I saw John Rodgers at the Qualcomm AutoX yesterday and he claims that replacing the prop valve with a "T" won't effect the points (which was the original reason for the question.) As it is, I am going to leave the car the way it is set up for a while and work on my driving skills. I will probably rebuild all 4 calipers soon but no upgrades. After replacing the lines, I had firm brakes again and had no problem scrubbing as much speed as I eventually built. (Heel and toe-ing is the skill that I really need to develop)

Final thought Dave: I'm thinking along Joe's lines. Without knowing more, it does sound like eliminating the prop valve is a do-able thing for you. It certainly will get rid of a potential air trap. Firm brakes are a good thing! I suppose that you could always try it and if it didn't work, you could replace it and only be out a few bucks and some time.

Good luck!

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Herb
'72 1.7 Tangerine 'Teen
'74 2.0 Red Rustmobile

[JTB]

I have SC calipers on the front and stock on the rear, along with a 19 mm master cylinder. I replaced my proportioning valve with an aftermarket adjustable unit (Garretson I think), and have it wide open. I found a sandy street and had a couple of friends help me. I would accelerate into the sand and then lock the brakes. The friends watched to see which end locked first. The theory was to lock the rears first, then back of the pressure just a little so the fronts would lock first. I kept giving the rears more pressure until there was no more to give. Still could not lock the rears before the fronts.

Should have saved the money and put in a tee.

[pbanders]

please ignore

[This message has been edited by pbanders (edited 04-20-2001).]

[DDS]

Things that make you go hmmmmm...

Am I understanding this correctly?

The stock and adjustable aftermarket proportioning valves simply limit the pressure exerted proportionally on the rear brakes. They are capable of adjusting rearward brake bias only up to the limit of what the system would do 'naturally'. (ie with a T fitting)

If this is true and since JTB couldn't get the rears to lock even with his adjustable valve wide open, then is it possible that the SC/stock combination cannot be optimized without some more sophisticated system that 'helps' the rears? (ie - proportional limitation of the front, or ideally of course, SC rear brakes as well)

Does this make sense?

Dave

[Va914]

I was watching a restoration show on TNN..(no jokes!)

They replaced the prop. valve and installed some kind of valve in the rear brake line. It's function was, upon hard braking, it would reduce the braking force to the rear..
I wish I could remember the name of the part.

Anyone else heard of this?

Thanks

[DDS]

Nope.

Hey JTB - what size tires are you running? Changing to big fat rears ought to require brake bias adjustment too. Also to Pbanders point, more traction means more weight transfer forward, means more rear lockup, so the dirt road doesn't tell the whole tale. Please LMK as I am in the process of building a car like yours brake-wise. Thanks! What else have you done with the car?
Dave
Dave

[John Rogers]

You might have seen a "residual pressure valve" and they keep some pressure in the lines which reduces the braking force to that part of the brake system. I have BMW 320 calipers front and rear and a residual valve in the line to the rear and no prop valve. All 4 wheels will lock at the same time. I have found it is more of the matter of how you apply the pedal pressure and not how much? If you stab the brake pedal the wheels will lock fast but if I press smoothly they never do? Good luck.