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Can't Get Rear Brakes To Bleed! Master Cycler Q?

Discussion in 'The Garage' started by ncgamedog, Jan 4, 2007.

  1. ncgamedog

    ncgamedog 1/2 ton status

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    i have a 78' k5 60/14bff. did the rear disk swap along w/ new 3/4 ton master, and i cannot get real pressure to the rear. i bench bled the cyclinder then got nothing. thought maybe a line was cloged (evan though as 1/2 ton had perfect brakes) so i started at the back and worked forward disconnecting the lines at every joint and blew them out. no clogs! thought maybe the prop valve was triped so i bypassed it and all i can get is a small trickle no pressure. the master i bought is for a 78' 3/4 ton. i evan tried the old 1/2 ton same thing. do i need a master from a later truck w/ factory disks? or what ideas do ya'll have? i need help please! thanks - justin
     
  2. YZEATER

    YZEATER 1/2 ton status

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    search for posts under my name. what part # master cylinder did you buy? i used 1-TON MASTER CYLINDER- Part # 101534 - ADVANCE AUTO it has a 1 5/16" bore and bypassed the prop valve and have the same setup as you.
     
  3. Leper

    Leper 1/2 ton status

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    There have been 3 posts just like this in the last couple of months. Searching the older ones will provide more results.
     
  4. Leper

    Leper 1/2 ton status

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  5. bp71k5

    bp71k5 3/4 ton status Premium Member

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    Also remember a disc brake prop valve is restricting ~40-50% of the pressure to the rear brakes, you won't see as much fluid force there as you do in the front which isn't restricted. I had the same issue. It just takes a real long time to bleed. (many hours) Or if you have an adjustable prop valve, you could open it up all the way, bleed the brakes and then close it up to where you need it. ( I haven't tried it but it sounds like it would work ok)
     
  6. YZEATER

    YZEATER 1/2 ton status

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    how does this fit in when he said he bypassed it??
     
  7. bp71k5

    bp71k5 3/4 ton status Premium Member

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    Good point. Let me try to redeem myself with better advice. :doah:

    How about this... The other problem I had was the MC pushrod was too long (I used the old one) and it would not allow the MC piston to return all the way back when you take your foot off the brake pedal. It wasn't able to pick up any new fluid for the rear brakes while I was pumping them. You might check the brake pedal freeplay and make sure it's adjusted right for the new MC.

    It's easy to check if this is happening:
    1. Fill MC with fluid while it's bolted in the truck (and remove the MC cover)
    2. disconnect brake lines from MC (keep a bucket handy to catch it)

    a. If lots of fluid then runs out both holes, it's ok.

    b. If fluid doesn't run out or only from one hole, unbolt the MC from the booster and then if fluid runs out of both holes, the pushrod is too long or the brake pedal is not adjusted right.

    c. If fluid still doesn't run out, then the MC is not bled yet.

    Even if that doesn't help in this case, it might help others with similar problem.
     
  8. HarryH3

    HarryH3 1 ton status Author

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    What calipers are you using out back? The free play has to be taken up if you're using the Caddy calipers with e-brake, or you'll never get a solid pedal...
     
  9. bp71k5

    bp71k5 3/4 ton status Premium Member

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    That's another good point. Once I hooked up the parking brake for the caddy calipers, it was noticably better brake feel.
     
  10. ncgamedog

    ncgamedog 1/2 ton status

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    I am using 3/4 front calipers. thanks for the help guys i dont have the search feature yet. tell you what, i read through the posts linked in the thread and it still seems there is no real solution. i mean, there are people out there w/ the same setup using a stock master w/ acceptable results:(. i thought all 1tons of that vintage where hydroboost? i will check the adjustment of the pushrod b4 i start spending money, but i just dont fell like thats it, even though it seems the most plausable. what is a p30 van? like a conversion van or service? what about the posibilty of using a late model master from a silverado w/ f/r disks? they are somthing like 33mm (close to 1 5/16"). I just dont want to get caught up into spending hundreds of dollars on useless parts. thanks agian- justin
     
  11. YZEATER

    YZEATER 1/2 ton status

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    the master cylinder i listed was less then 20 bucks and solved my problems.
     

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