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adjustable proportioning valve

Discussion in 'The Garage' started by 74kay5, Dec 23, 2005.

  1. 74kay5

    74kay5 1/2 ton status

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    I found this FAQ online about adj prop. valves. I have a 14 bolt and was going to go with disks and a willwood adjustable valve. But I also think the combination valve in my truck is bad. Even with drum brakes the rear tends to lock up. Can I bypass it all together? From what this is saying I would guess no?

    What does a proportioning valve do? A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions.

    What are the symptoms of a bad proportioning valve? If your prop valve has gone bad your rear wheels will lock up easily, especially on wet surfaces.

    What does a metering valve do? A metering valve or "hold off valve" is used in the disc portion of a disc/drum system to hold off the application of the front discs slightly allowing the slower reacting rears to catch up. This provides rear stability on wet surfaces and reduces excessive pad wear.

    What are the symptoms of a bad metering valve? Your car will nose dive and the front pads will wear too fast.

    What does a residual valve do? There are two different uses for residual valves. The 10 lb valve is used to hold a residual pressure to the drum
    brakes to give a higher firmer pedal. The 2 lb valve is used in the disc when the master cylinder is lower than the calipers to prevent back siphoning of the fluid from the master.

    What are the symptoms of a bad residual valve? The brakes will be very spongy and you will need to pump the pedal to get good brakes.

    What is a combination valve? A combination valve incorporates metering and proportioning into one valve providing all necessary valving for disc/drum systems.
    See below

    Should I use an adjustable proportioning valve? Not if you can help it. The adjustable valve will only provide the proportioning function and not the metering that is needed.
     
  2. DieselDan

    DieselDan 1/2 ton status

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    I'm guessing that you have a 14FF under a K5. The K30 that the 14FF came out should have an adjustable proportioning valve mounted under the bed. The valve is height sensitive to reduce brake force (increase bias) when not loaded, and vise versa.

    Obviously our K5's don't have this. I had the same problem when I stuffed my one ton axles on. The rear would lock up instantly in anything resembling a panic stop (not good). Sure got every bodies attention :D When I dumped the drums and went disc I didn't mess around. I found a late model (C30 I think) master that equal size reseviors front & rear (made for 4 whl disc). I ditched the stock combo valve and installed a Wilwood adjustable valve on the fender. The brakes are SUPER! Not grabby but progressive and strong. I've made about three adjustments so far in the past months getting street emergency braking perfect. This is something that can't be done with a non-adjustable proportioning valve, & I think it's real important. The chances of an off-the-shelf valve working right for your truck with what ever tires you maybe running this week, or taking in account your latest suspension mods (of the month) is pratically NIL. :wink1:
     
  3. muddybuddy

    muddybuddy 3/4 ton status

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    im goin from 1/2ton brakes to 1ton with rear disc, where can i get one of the proportioning valves?
     
  4. ntsqd

    ntsqd 1/2 ton status

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    I think there is some good info, some bad info, and some marginal info in that. Don't believe everything you read over there, things tend to get repeated as the gospel by ppl who have not bloody clue what they're talking about.
    I worked as an R&D Engineer for wilwood for two years. That was long enough to teach me that brakes look simple and can be extremely complicated.

    First off, I have seen ONE OE metering valve ever. It was plumbed into the rear brakes! That never made sense to me, but it was obviously original plumbing that hadn't been monkeyed with.
    As best as I can tell, use of metering valves died out with the invention of proportioning valves.

    Second, Every Combo valve I've ever looked at contained a fixed prop valve and a brake system warning switch. It's quite possible that a combo valve out there has a metering valve built into it, but I've never knowingly seen one.

    Think about the claim for the need of a metering valve and then think about how brakes are used.
    At high speeds there is a lot of weight shifted onto the front axle when the brakes are applied. A metering valve's delay of front brake application wastes this added traction in favor of the brakes on the axle with the least traction. How can this be Good?

    At slow speed a delay at either end isn't particularly desirable.

    The only "metering" valve I would consider using is this one from DPI. These go in the rear brake system and allow you to operate the rear brakes at more peak pressure than w/o. They do this by slowing down the pressure increase rate long enough to allow some of the weight that shifted fwds on the front axle to come back to the rear axle. So as the rear gains back it's normal carrying weight (& therefor it's traction) the rear brakes gain more braking pressure. It's not a complicated device and it is biased towards Cup cars and similar racers so not all other vehicles see the same degree of benefit.
     
  5. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I had the same problem with the one ton 14 FF that went under my K5. It had the monster 13x3.5" drum brakes...and they'd lock up the rears so easily it was unsafe IMO.

    I swapped in hydro-boost and a '83 C30 master cylinder with my rear disc conversion and couldn't be happier. I have not installed an adjustable prop valve, in fact I still have the stock combo valve. In a full on panic stop the rears start to lock just before the fronts do...and it will lock up all 4 39.5's.

    Pedal feel is medium firm.

    Rene
     
  6. camiswelding

    camiswelding 1/2 ton status

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    I used the OEM 4wdb master cylinder and metering valve from a p30 van... and the factory pro valve for the rear... I already have factory hydroboost

    Its nice when you can have it factory chevy

    cam
     
  7. DieselDan

    DieselDan 1/2 ton status

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    I'm not busting you out, tRusty (please don't get you long-johns in a bunch :D). But this is why I think the adjustable valve is much better, especially if your truck is a DD (like mine) and must venture out in the populated highways. The fronts should begin to lock up slightly before the rears, in a panic stop. With my valve almost wide open my brakes were good (much better than those damn drums), but I could still induce rear wheel lock. I've been slowly testing my way to braking perfection :grin: with my current "winter tires" (39/15.5Rx15 TRX STS) and I can adjust them even more when I throw a set of cut Wrangler RTs this spring.

    Just my .02 :wink1:
     
  8. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Well I have yet to have the microscopic rear bias be any sort of a problem. By the time I'm on the brakes that hard all 4 are locked. It's cake to threshold brake anyways...so lockup isn't much of an issue.

    Out of the 30 or so vehicles I've owned in my life my K5 currently has the most balanced brakes, and top 3 for pedal feel. I figure messing with them to gain that last .5% of performance is an exercise in diminishing returns.

    It's currently 58 F, the longjohns aren't anywhere in sight. :haha:

    Rene
     

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