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Technical Brake problem. Help!

Discussion in 'The Hokey Ass Message Board' started by Hot Rod Brando, Dec 21, 2014.

  1. Hi all, working on my dad's '48 ford coupe. He passed away a few years ago and had a brake problem that he never fixed. I'm visiting and trying to fix it. It has GM brakes, front and rear (early 80's GM calipers). Everything is brand new. Booster, master, lines etc. Getting no pedal. Isolate master, firm pedal. Block front lines, firm pedal. Block rear lines, no pedal. So it's in the front. I get no line pressure (gauge in bleeder hole). But if I take the caliper off the mount and clamp the pad to the caliper you get pedal. So is it possible the rotors (which are new) are too narrow?? I am stumped beyond stumped. I've reverse bled everything. I just can't get any good pressure out of the fronts. It's like there isn't enough pedal travel to get front brakes. (They just barely grab when the pedal is on the floor. With the car on and booster working you can't feel any pressure at all. Master is mounted in its original spot on the firewall. Even though the pads are making contact with the rotor it's like there is a bunch of "slack" somewhere. I just went and looked at another set of pads and they are the same thickness. Has anyone ever had to shim the pads to get good pedal? Sorry for the ramble but I'm trying to cover everything I have done. Recap: all new components front to back, master works fine (isolated) rear works fine isolated. Fronts no workie.

    Brandon
     
  2. Baumi
    Joined: Jan 28, 2003
    Posts: 3,043

    Baumi
    ALLIANCE MEMBER

    Maybe you need a bigger size master cylinder, like a 1" bore vs a 7/8" ... But even with the wrong size, the pistons in the calipers should not go back all the way but just so much the rotors can turn freely after appying the brakes. Otherwise, as the brake pads wear off the pedal travel would increase even more if the pistons went all the way back in the bores... maybe there´s a failure in how the calipers were assembled. If I remember correctly the O-rings on the pistons are wedge shaped to avoid that the pistons slide back too easily...
    Excuse my not so good English, I hope you understand what I´m trying to say...
     
  3. Grahamsc
    Joined: May 13, 2014
    Posts: 466

    Grahamsc
    Member
    from Colorado

    Are your caliper mounting pins free ?
    With the piston pushed back in the caliper , can you move the caliper freely?
     
  4. Sounds like your MC pushrod is to short.
     

  5. Grahamsc
    Joined: May 13, 2014
    Posts: 466

    Grahamsc
    Member
    from Colorado

    Chopped 51 may be onto it
    Disc calipers take more volume than drum( look at the size of the piston).
    Or a mc not capable of the volume
     
  6. I was thinking volume also, as it sounds like you covered the other bases. Maybe check pressure with one of the calipers out of the circuit; if you can get pressure then, it would indicate inadequate volume.

    I was just was messing around with the brakes on my '47, '70s GM midsize calipers, Nova rear drums, and a 1" Mustang master. Pedal was a lot lower than I liked. Checked and found that on a comparable OEM GM set-up a 1-1/8" master was used. Switched up to a 1-1/16" master; made a world of difference in pedal height.
     
  7. SimonSez
    Joined: Jul 1, 2001
    Posts: 1,637

    SimonSez
    Member

    If you have early 80's GM calipers, could they be the low-drag versions?

    They are designed to pull the pads further back from the rotor so they don't rub, and they were originally setup with a different master cylinder.

    More details here ...

    http://www.remaninc.com/pdfs/lowdragcalipers.pdf
     
  8. Baumi
    Joined: Jan 28, 2003
    Posts: 3,043

    Baumi
    ALLIANCE MEMBER

    very good info , thanks Simon!
     
  9. unkledaddy
    Joined: Jul 21, 2006
    Posts: 2,865

    unkledaddy
    Member

    They made the GM Metrics in low-drag. I can't open the link that SimonSez posted, but the only way to tell is to disassemble the calipers, remove the seal and determine if the bevel is a 15 or 30 degree angle. You need a step-bore master cylinder with LOW drag calipers.

    If you want to purchase (NOT low-drag) metric calipers here's links to the ones I'm using;
    http://www.southwestspeed.com/?sec=view_part&id=9963
    http://www.southwestspeed.com/?sec=view_part&id=9964
    They are excellent castings and NOT low-drag. I bought new ones from AFCO that looked exactly like the rebuilds
    I returned to Advance. Advance's were a mix of REGULAR and low-drag., and the AFCO one's looked so bad to be
    new that I just returned them.

    But, I had the exact same problem you described and reverse bleeding did nothing but suck air into the right front.
    We rebled them the old-fashioned way and nothing but air bubbles came out, and when we finished the brakes were
    great. I think the reverse bleeding may have loosened up a trapped air bubble.
     
    Last edited: Dec 22, 2014
  10. Bruskie
    Joined: Apr 9, 2014
    Posts: 50

    Bruskie
    Member

    what is the position of the bleeders r they high or low on the calipers i've seen people mount them on the wrong side before & have the same problem
     
  11. BJR
    Joined: Mar 11, 2005
    Posts: 9,818

    BJR
    Member

    Is the correct port on the master plumbed to the disc calipers? If not that may be your problem.
     
  12. 1948plymouth
    Joined: Feb 22, 2011
    Posts: 109

    1948plymouth
    Member
    from Minnesota

    Couple of good suggestions. I've seen both happen before.


    Posted using the Full Custom H.A.M.B. App!
     
  13. Ok, thanks for all the suggestions! So, everything was bought as a kit. (mustang II, etc etc). The booster and master came with the set up from places like pirate jacks and MP brakes. For some reason, my Dad bought new front calipers from NAPA after the car was together. The calipers are metric. They COULD be low drag. I don't mind swapping them to see if that does the trick. The other thing I noticed is that the piston is already out about 1/8 to 1/4" with brand new pads. I also noticed that Speedway sells caliper shims for these calipers when using narrower rotors. I do know it's not the master cylinder because as soon as I put pressure against the brake pad and then push in the brake pedal, it is firm. My totally uneducated guess at this point is that the piston is too far out to be of any good and that's why the pedal is going down to the floor. Kind of the same thing when your pads are almost worn out. My further guess is that Dad turned in the "shims" to the NAPA guy with the cores. Gonna get a set of the non low drag calipers and some shims to just try and see if one or both do the trick. I'll keep y'all posted.
    And to answer some of the other suggestions:
    • Master ports are correct (4 wheel disc system and master, came as one unit already plumbed)
    • Bleeders at 12 oclock (well, really damn close)
    • Reverse bleeding was only done today, after, no kidding, at least 20-30 times trying to bleed them the old way. Mom now has one really big thigh...
    • Master is an 1 1/8
    • tried a longer pushrod but it bottomed out without making and more pressure.

    Brandon
     
  14. unkledaddy
    Joined: Jul 21, 2006
    Posts: 2,865

    unkledaddy
    Member

  15. stimpy
    Joined: Apr 16, 2006
    Posts: 3,546

    stimpy

    another thing to look at is in the 80's they started to remove the residual pressure valves in the masters because of antilock brakes as they moved the valve to the a/l brake unit or combination valves . and the pistons seal/boots have enough pull back to open the space at the rotor ( specially if low drags ) . so you have to pump them up . you should have 2 pounds of pressure at the master on the front system . measure the rotors to make sure they are the proper thickness and most front rotors are 1.00 + " and vented . non vented are ussually the rear or .81 .
     
  16. bonzo-1
    Joined: Oct 13, 2010
    Posts: 342

    bonzo-1
    Member

    Try pulling the calipers off with the line still attached and pump the brakes a few times. Check to see if the pistons are moving out. Your pistons may be bouncing. C clamp the pistons back in and give another try When you push the pistons make sure the line is at the highest point. This should push any air (if any) back to the master.
     
  17. junk
    Joined: Mar 15, 2008
    Posts: 200

    junk
    Member

    My first thought was using the small reservoir on the master for the front discs. So not allowing enough volume of fluid into the caliper.

    My second though is do you have residual pressure valves in the brake lines? I was thinking the 48 master is under the floor and I wonder if you need residual pressure valves.

    Good luck. Brake pressure issues can be very maddening.
     
  18. unkledaddy
    Joined: Jul 21, 2006
    Posts: 2,865

    unkledaddy
    Member

     
  19. Hackerbilt
    Joined: Aug 13, 2001
    Posts: 6,254

    Hackerbilt
    Member

    How far out the pistons are don't make them ineffective. They will work correctly right until you blow the piston out the hole! LoL
    My bet is on low drag calipers. Seen it before and the symptoms were the same. Easy and cheap to just replace them with some new non-low drag calipers and then you are sure.
     
  20. junk
    Joined: Mar 15, 2008
    Posts: 200

    junk
    Member

    Yeah, missed the firewall mounted master.

    Definitely sounds like the low drag caliper reading the explanation. Next question is what about the factory brake system let the low drag calipers work Ok?
     
  21. unkledaddy
    Joined: Jul 21, 2006
    Posts: 2,865

    unkledaddy
    Member

    OEM came with a step-bore master cylinder if the vehicle had LOW drag calipers.
     

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