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Why am I losing brake pressure???

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by K5thatShmobs, Nov 24, 2002.

  1. K5thatShmobs

    K5thatShmobs 1/2 ton status

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    Just put on longer lines and went through the bleeding process as normal. I bled them three times until it was nothing but fluid coming out of all corners. The pedal is still sinking to the floor, and I dont know why. I checked the master cylinder and every line all the way to each corner, and I can find any leaks. Suggestions anyone?
     
  2. MJF

    MJF 1/2 ton status

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    Someone answer this. I am having the exact same problem.
     
  3. A&P

    A&P Registered Member

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    Try this. With the engine off, pump the pedal until you get firmness. This should occur. If not I don't know. If it does work, then bleed em' again. I don't know how you are used to bleeding the brakes, but try opening the bleeder first, then pushing the pedal all the way to the floor and while it's held there, close the bleeder. do this about five times for each wheel. If this isn't helpful, well, at least I tried...
    -Ben
     
  4. K5thatShmobs

    K5thatShmobs 1/2 ton status

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    Ok, I just went outside and bled them again... it was all fluid once again, and everything seemed normal. The brake pedal is getting great pressure when the the engine is off. Then, when you start the engine, the pedal pressure almost dissapears. Has anyone experienced this?
     
  5. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Did you let the master run dry?
    How old is the master?
    If the master runs empty it's an absolute pain in the butt to get clear.
    I had a simular issue this weekend and it was air trapped in the Combination Valve(CV) for the rear circuit.
    The solution was to loosen the rear inlet line. The master will gravity bleed. The air purged at the top of the CV. I let it drip for a couple minutes and tightend it back up and problem gone. Didn't even bleed the rears out.
    Sometimes on a old master cylinder when you go to pump bleed them they will fail. What happend is there is rust and crud in the MC. In the normal range of travel the seals were not getting into the rust but when no fluid pressure happend they MC made a longer stroke than normal and some seals hit the depositis and rust and failed. It can cause an internal leak that has the same symtoms as your experiancing.
    MC's are cheap. If purging the CV dosen't work replace the MC. Usualy under $35.
     
  6. K5thatShmobs

    K5thatShmobs 1/2 ton status

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    Wow, after doing some searching it seems that a lot of people have run into the same issues as me. Today Im going to try plugging the rear brake line going to the rear diff and see what kind of pressure I get. If I get good pressure, I know that my master cylinder and PV are getting good pressure. Then I know the problem lies in the rear somewhere... maybe I have to adjust the shoes out or look at the wheel cylinders. I think adjusting the shoes is a problem that gets over looked all-too-frequently when addressing poor pedal pressure. But hey, start with the easiest thing first, right? Ill let everyone know what happens.
     
  7. imiceman44

    imiceman44 1 ton status

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    If you didn't adjust the shoes, then do that.
    You are right, people tend to forget about that one, and I had the exact same problem in my 73.
    It was so bad that with hard braking the light would go on, and wouldn't go out until I went uphill and pumped the brakes lightly for half a mile.
    After adjusting the rears, all worked fine.
    And the pedal was firm again.
    /forums/images/graemlins/thumb.gif
     
  8. K5thatShmobs

    K5thatShmobs 1/2 ton status

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    Ok, I unhooked the lines to the drums in the rear leaving only the block... I sealed if off and had a buddy build a little pressure. I unsealed and he pressed down, and some fluid comes out... I did it again... nothing. No fluid whatsoever. Again... no fluid. It seems that no fluid is going to the rear lines. WTF?

    Ok, so I go up to the proportioning valve. I loosen the fitting that goes to the rear, and I get some fluid... Whats going on here? Sure, it seems that something would be blocking the line between the PV and the rear opening, but how can that be? It was fine when I jacked the thing up a few weeks ago. Im thinking that Im just getting poor pressure.

    Anyone... I need some advice here. I need to be done with this truck in two days for thanksgiving weekend for a wheeling trip @ hollister. Ahhhh!!!
     
  9. imiceman44

    imiceman44 1 ton status

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    Well, I would say one of 2 things and both happened to me:
    Did you have moisture or cloudy fluid in the MC?
    if so, then when you started bleeding, that got sucked in to the cylinders and water is your enemy, it will give you the spongy feel and block everything.
    And the other thing is when you do get moisture and try to pump, the seals degrade fast and you loose the pressure.
    What I would suggest is unhook the lines from the front and blow through the rear to lcear all the tubes, then drain the MC and fill it with new fluid and bleed it, then hook to the lines and try again.
    If no pressure, your MC is shot you need a new one.
    /forums/images/graemlins/frown.gif
     
  10. K5thatShmobs

    K5thatShmobs 1/2 ton status

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    Victory at last! It was the damn prop valve. Ok, heres a short summary of what happened for anyone that might experience this in the future... and theres been a lot that have.

    All along I have been getting very weak pressure to the rears when bleeding, with the fronts getting plenty. Because I could not bleed the rears properly, my pedal pressure was way too weak when the power booster was operating.

    -Ok, so I tried replacing the MC. Bench bled it, and bled the entire system endlessly. Nope. Same thing.

    -I bled the MC some more, checked all lines, tried bleeding the prop valve itself, and it all checked out with no air anywhere. Still, weak in the rear.

    -Then I cleared the lines going from the MC to the prop, and the lines going from the prop to the rears. All clean. At this point it had to be something within the prop.

    -I took the prop to the bench, and fluid ran through both sides easy. (Not easy enough on the rear as I found out later.)

    -I then took off the brake sensor fitting from the top of the PV. I saw the groove on the switch piston that controls chamber pressure between the fronts and rears. (BELOW)

    [​IMG]

    I noticed that the piston was pushed toward the rear chamber, limiting flow to the rears. I thought I had it figured out. I simply moved the piston back to center with a pick, and put the switch terminal back on. (Its the white plastic one.)

    -I get the brake system back to together and go to bleed the rears (RR,LR,RF,LF) thinking Im in the clear. Nope... still no pressure. Check the brake light... not on. WTF? Later I found out my brake light was a POS.

    -Taking the PV out again, I look at the piston position. Again, its back to the rear chamber side, limiting flow.

    -After some thread rumaging and a little research I was lucky to get a tip from MudFrog here on CK5. He had the exact same problem, and said "gutting" the prop would solve my problem. This involves eliminating the mechanism that limits fluid to the rear brakes... and mine was doing it constantly.

    -Looking at the PV the same way you would under the truck with your head on the front axle, you see a larger bolt on the right side for the rear outlet port. For gutting the PV, this needs to be taken off.

    -From there you take out the stop plate assembly (refer to the diagram above) and the spring behind it. From there you see a white pin-type thing. With a needle nose grab that and pull it out. Connected to that is the switch piston itself, and will come out. So now, there's a clear path between the front and rear chambers. The rear cant be limited by the piston, and so the fronts and rears are now part of the same pressurized system.

    -I put the switch terminal back on, hooked it all up, and proceeded to bleed. Fluid was flying out of the rear like a gyser. Perfect. Bled for about 20 minutes, and two quarts of brake fluid later, I have an air free, high pressure system that stops great. Hope this helps! I cant wait to drive my truck, thats for sure.
     
  11. imiceman44

    imiceman44 1 ton status

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    Well what you did is a band aid fix.
    I had the same exact problem, but I didn't do that.
    I am still trying to study everything I see here to figure out why a stock system suddenly doesn't work anymore.
    I think maybe a different spring should fix the problem.
    From the look of the diagram, the spring pushes the valve towards the rear, maybe a softer spring would be better?
    Did anyone think of that?
    Even for a Disc/Disc conversion, this might work.
    Just a softer spring, or if the bias should be the oppsite, a stronger spring?
    Just a thought here.
     
  12. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Well there is two floating valves. I just dissabled mine for a Disc Disc conversion. To disable the PV you break the part in the diagram marked "Valve stem" off. THe valve stem also floats and it had a small piston on the end that is spring loaded. With no pressure the valve stem blocks fluid to the rear so you cannot vacume bleed the rear circuit because it just pulls the valve closed.
    When you press the brake peddle it pressurizes the first chamber and causes the valve stem to pull off the seat. The big spring acts on a piston that as pressure builds in the rear circuit it will move back on the valve stem and again block the pressure going to the rears so it can never exceed it's maximum. For more rear pressure you use a stronger spring for less you use a softer. No spring I would worry that it could seal the system with fluid trapped in it and keep the rears activated. SO I don't like the idea of taking the spring out at all.
    For Disc Disc snap off the valve stem and then it just becomes a distribution block. To kill the metering valve is easy. just take the metering valve out and remove the rubber seal on the end (marked Metering valve seal in diagram) and reinstall and then it's a distribution block.
    I just did this to mine with a Disc Disc conversion and it helped. I do have a Adjustable PV in the rear circuit. With 35's I can stop nearly as well as my wifes bone stock 79 on 31's. Peddle has a little more travel then stock but that's an issue with volume on the rear circuit from the Master and not a PV problem.
    Now the problem that was happend with the switch piston would also cause fluid to be blocked to the rear circuit. It's the fail safe if the rears loose pressure it keeps pressure before the PV to keep the peddle up on the MC. One of the seals on the stitch piston has proabaly failed on his PV and why it kept poping and closing off the fluid to the rear circuit.
     
  13. dammit32

    dammit32 1/2 ton status

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    Boy from the diagram that looks pretty complex to me...I wish there was an easier way for a disc/disc system /forums/images/graemlins/confused.gif
     
  14. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Yeah, there is a lot more to that thing then you would think.
    KILL it for Disc Disc or get a Disc Disc version from www.inlinetube.com I tried to keep it in there for thew conversion and it gave me nothing but trouble. I truned it into a Distribution block by the above method and put a Adjustable PV in the rear circuit and that gave me good results. The loss of pressure feature still works. with the Adjustable i can back off the pressure in the rears to the point I can trigger the light /forums/images/graemlins/grin.gif
     
  15. dammit32

    dammit32 1/2 ton status

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    So you think I should replace the PV with an adjustable and call it done?
     
  16. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Yep. It's the only consistant results. The peddle will be low but you will have good brakes. I'm working on the low peddle thing. Just got sidetracked by Thanksgiving, a death and my wifes DD tearing up.
     

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