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Disc Brake, MC, Booster Problems

Discussion in 'The Garage' started by crawford, Sep 30, 2004.

  1. crawford

    crawford 1/2 ton status

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    I recently did a disc brake converion to my 14b and it didn't have the braking performance that I expected. So I decided to make a few changes to improve performance. The first thing I did was disabled the metering and proportioning valves in the combination valve. I also replaced my old master cylinder with one from a 78 Eldorado. I installed a proportioning valve too. I'm still having issues with my brakes though. The pedal is pretty hard, but I have very little stopping power. I have to really stand on it to get it to stop. The booster is my biggest suspicion. The vacuum seems strange to me in particular. On the mc there is a vent that goes from the bottom rear of the mc to the opening on the booster. See the pics below. Should I be getting vacuum out of that vent? Did my booster coincidentally fail while I was changing all this stuff? Also is the rear port on this new mc the rear brakes and the front port the front brakes? Any ideas would be appreciated. MC, PV, Booster Vent Closeup
     
  2. Muddytazz

    Muddytazz 1 ton status

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    I would take a guess and say your booster is going out.
     
  3. BowtieRed

    BowtieRed 1/2 ton status Author

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    that's be my guess too, pull the hose to the booster with the engine off, RIGHT AFTER you cut it off it should go "schwft!" if it doesn't turn the engine on, pull the hose again, see if you still have vacuum at the boost eng of the hose. if you do the booster is bad, if not then your hose is clogged, or you have a LARGE vacuum leak, or your check valve is bad. /forums/images/graemlins/thumb.gif
     
  4. BowtieRed

    BowtieRed 1/2 ton status Author

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    one more thing, remember disc brakes require far more effort to operate than drums, so you may want to find a larger booster and MC, like off of a gas superduty (99+)
     
  5. 85mudblazin

    85mudblazin 1/2 ton status

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    have you upgraded the MC?? the 3/4 ton and 1 ton ones have a bigger piston in them which gives more fluid flow which equals more stopping power. you may also want to try a adjustable proportioning thing, that way you could adjust braking from front to rear.
     
  6. smprider112

    smprider112 1/2 ton status

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    ya I've always heard you need to either use a 3/4 ton booster...or a hydroboost like Diesels have /forums/images/graemlins/thumb.gif
     
  7. crawford

    crawford 1/2 ton status

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    I have 16" of vacuum at the booster and the check valve is working properly. My biggest question now is whether or not I should be getting vacuum through that small vent on the master cylinder? Is there a cross section drawing of a booster on the net?
     
  8. diesel4me

    diesel4me 1 ton status Premium Member

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    The diaphram has croaked in your booster if you have vacuum at the hole in the lower rear part of the master---My 79 C10 had the same symptoms as yours when the booster went--I tried blocking the hole by pulling a rubber hose through it,it stopped the vacuum leak and made the engine run better again,but I was standing off the seat to stop the damm thing--almost crashed it in stop and go traffic,it felt like if you hit the brakes hard and fast,the pedal would not go down for a second,then you had to push with both feet hard to stop it.I had a hard time finding the right booster for mine,the hollander book at the boneyard said only a 79 or 80 C10 would fit--they had no good ones that year--I went home and dug out the booster I took off a 73 K10 I junked 8 years ago--it looked the same to me,its on my truck and it works fine--I cant tell what the difference would have been-- /forums/images/graemlins/dunno.gif /forums/images/graemlins/dunno.gif /forums/images/graemlins/screwy.gif /forums/images/graemlins/thinking.gif
    P.S. Make sure your master cylinder isnt leaking out of the back end into the booster(you have to unbolt it to check)--the vacuum may have sucked some fluid out of the rear piston seals,possibly damaging them.I had a recent rebuilt one,and it wasnt leaking,so I re-installed it.When shopping for a new booster,at autozone it was about 80 bucks for a rebuilt booster only,around 115 bucks for both the booster and a master together. /forums/images/graemlins/crazy.gif
     
  9. grundy99

    grundy99 1/2 ton status

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    Does anyone know if you can just replace the 1/2 ton MC with a 3/4 ton one, maybe a MC that was set up with the 13in rear brakes. I looked up the part numbers for the boosters and the K-10 was the exact same part number as the k-20. I used an 83 for reference beings how that is the year of my truck. /forums/images/graemlins/thinking.gif
     
  10. balzackks84

    balzackks84 1/2 ton status GMOTM Winner

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    The front and rear lines are switched. I think you have to go to a k-30 to see the changes in the mc because the 12 bolt was the 3/4 ton rear end (although there was a 14s/f available it still used the 1/2 mc) Im still running the stock 84 1/2 mc with an inline \proportioning valve all the way open with no complaints.

    Remember your still driving a fullsize p/u with 36" + tires. Its not going to stop like an eldorado.
     
  11. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    If the peddle is firm, up high and just hard to push it sounds like booster. Like was pointed out "the larger the bore the harder it will be to push. I didn't have any problems with the 1/2 ton. My one ocncern with the 78 Elderodo master is they the elderado had Anti lock rear breaks with a double booster set up. It may have to much rear circuit that hr second system in the Caddie dealt with.

    I would start looking at maybe going to hydro boost if the stock booster is not enough or is bad. Just find a 1 ton or Diesle gm in the junk yard and it should have what you need to do the conversion.
     

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