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Brake prop valve... revisited

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Thumper, Aug 14, 2002.

  1. Thumper

    Thumper 1/2 ton status

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    Ok, the my rear discs have been working excellent. Lately, they have been working too good. It occurred to me that they arent working too good, but the fronts are working at less than optimal... like not at all. So, heres the question: The way I have my rear set up, I had to remove the rear lines completely from the prop valve on the frame crossmember so the discs would release. That was fine. The only drawback was the 'Brake' light in the dash. Liveable. Lately, the fronts have stopped workin. Can the prop valve still be f*ckin things up? My next idea is to remove the front system from it as well. The brakes are bled very well, and the pads are in good shape front and back. Any other inputs? I can lock the rears easily on gravel, but the fronts wont even slide a bit. This just will not do!

    TIA

    Mike
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Interesting problem, and I knew there were some later Impala SS mods for the prop. valve, so I searched. Unfortunately b-body.net is dead right now (basically) so all I could find was this: http://www.geocities.com/gtforman/Articles/brakes.html

    But your problem sounds like its the exact opposite! Sorry no more help, but this is what you'd *expect* to happen, not the problem you are having.

    FWIW, as my friend likes to say, being able to lock up the tires is not an indication in any way, of how good the brakes are. Obviously if in dirt, you are only getting the rears to lock up, something *(is* wrong though, and that could be dangerous going fast enough and having the rears lock up on you.
     
  3. DieselDan

    DieselDan 1/2 ton status

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    I'm perplexed by you statement "I had to remove the rear lines completely from the prop valve on the frame crossmember so the discs would release." Why wouldn't they release? The Metering valve should be holding pressure off the fronts just enough (100+/- PSI) so the rear <font color="red"> shoes </font color> can take up the slack. The Proportioning valve should be tailoring the rear pressure rise; maybe that part was FUBAR'd. If you're not running the rear circuit thru the combination valve, you might as well sht can it.

    How are your brakes on pavement? Are the rears still locking before the fronts? My guess is your master cylinder (the front circuit) is starting to quit . /forums/images/icons/grin.gif
     
  4. Thumper

    Thumper 1/2 ton status

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    I think maybe you missed part of my statement. I run 4 wheel disc brakes. When I installed the rear discs, they were dragging all the time. The info I got was that there was a check valve inside the prop valve that in the case of drums would hold the pressure so the shoes wouldnt pull too far from the drum. So I removed the rear lines from the prop valve. This allowed the calipers to release and sit where they were supposed to instead of dragging. These are working awesome right now. Its the fronts that arent working. My question is: could my running the prop valve this long (about 1 yr) without the rear circuit hooked up to it, cause the front system to shut down for whatever reason?

    Thanks for the input so far.

    Mike
     
  5. m j

    m j 1/2 ton status

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    I thought the residual pressure valves were in the master, guess not
    there is no point in having the combo valve in the circuit if it only has the front brakes in it.
    now that isnt to say I agree with removing at all.
    I have a cutaway image of a combo valve somewhere
    I would recommend tailoring a combo valve to work for you, in your case you just had concerns with the residual pressure valve being too high so change that bit and leave the rest functional.

    as for your new dilemma you need to provide more info
    does your front brake system generate pressure?
     
  6. alec78

    alec78 1/2 ton status

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    I had this exact problem on on old 74 chevy truck I had. (granted it was a stock disc/drum set up) This problem had me at bay for a month and a half. What happened was the brake hose (frame to caliper) had collapsed. I couldn't get fluid through them at all. They collapsed at the same time (imagine the odds) I wound up replacing everything from the master cyl to the calipers, except (of course) those f*#@'n hoses. I thought I would never see it again, but I have, (both hoses at the same time) on a different truck.

    Point being, I would make sure you are getting fluid flowing to the calipers. While doing that, see if you can turn the wheel with the brakes applied. If in doubt, get a brake pressure guage and check the front pressures.
     
  7. jimmy88

    jimmy88 1/2 ton status

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    Directly from the 1988 GMC service manual:

    "The combination valve is comprised of three sections, each serving a different function (Figure 7).

    The metering or hold-off section of the valve limits the pressure to the front disc brakes until a predetermined front input pressure is reached, enough to overcome the rear shoe retractor springs. There is no restriction to the inlet pressures below 20 kPa (3 psi) to allow for pressure equalization during the no-apply periods.

    The proportioning section of the combination valve proportions outlet pressure to the rear brakes after a predetermined rear input pressure has been reached. This is done to prevent rear wheel lock-up on the vehicles with light rear wheel loads.

    The valve is designed to have a bypass feature which ensures full system pressure to the rear brakes in the event of a front brake system malfunction.
    The pressure differential warning switch is designed to constantly compare front and rear brake pressure from the master cylinder and energize the warning lamp on the instrument panel in the event of a front or rear system malfunction. The valve and switch are designed so the switch will latch in the "warning" position once a malfunction has occurred. The only the lamp can be turned off is to repair the malfunction and apply a brake pedal force required to develop about 3102 kPa ( 450 psi) line pressure."



    Chilton basically says the same thing for '71 and up with disc brakes.

    Doesn't look like there is any residual pressure valve in the system, just the front "hold-off" section that accomplishes about the same thing.

    I've always wondered about the combination valve with the rear disc swaps, but you don't hear about problems, so I just figured it worked well enough. Since the proportioning valve is set up for drum brakes that are self energizing and would lock up with too much pressure, it would seem that just swapping to discs, without changing the proportioning section of the combo valve, would not allow enough pressure to the rear discs. I always figured on buying an adjustable proportioning valve when I do the disc swap.
     
  8. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    By pass the proporting valve completly. It was never designed to work with only one circuit connected.
    Willwood sells a adjustable proporting valve that you can install in the line for the rear circuit and use it to ballance the Front Rear pressure. If you wanted to get real slick you could install the valve in the dash
     
  9. Thumper

    Thumper 1/2 ton status

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    Ya, I took the prop valve right out. It seems to work much better now. Plus the idiot light doesnt light in my face any more! /forums/images/icons/smile.gif I was contemplating an adjustable valve too, but the system works very well right now. It seems in balance. Steve Frisbie had the same setup on his, and he put in the adjustable valve but ended up running it wide open anyways, so turns out its not necessary. Thanks for the input guys.

    Laters
    Mike
     
  10. Mr GearHead

    Mr GearHead Registered Member

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    I am glad it works better. Jegs sells a pressure holding valve you could put inline with the rear brakes to keep the shoes from retracting. If the fronts seem to lockup before the rears this could help. I think it holds 10psi in the line.
     

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