Wrong size booster??

Discussion in 'The whoa and the sway.' started by synchro66, Jun 27, 2010.

  1. synchro66

    synchro66 Well-Known Member

    I have just replaced all wheel cylinders in my NON power assist Drum/Drum 71 Skylark. While I was at it, I replaced the Master cylinder and added an 11" Delco Moraine booster. I bench bled the m/cyl and wheel cylinders but the brakes are very touchy/grabby and almost put you thru the windshield with only light application, even at about 5mph. My question is, ...Is the booster too large for the Drum/Drum application or do i need to adjust the pedal ratio, or could it be something else??
    Hope somebody can help
    Cheers
    Bruce
     
  2. Golden Oldie 65

    Golden Oldie 65 Well-Known Member

    There should be two holes in the upper brake pedal, one is for manual brakes and the other for power brakes. The lower one is the one you want to use for power brakes. On the other hand, touchy drum brakes are somewhat typical of these cars. I remember when I first bought mine which had the original power drum brakes on it. When I hopped in the car the guy told me to be careful because the brakes were very touchy. I was mentioning that my dad had a `66 T-Bird once that had brakes like that when I touched the pedal and did the same thing as you did, darn near went through the windshield. I swear the guy laughed for 5 minutes straight :laugh:
     
  3. FJM568

    FJM568 Well-Known Member

    Make sure you are using a power drum brake master cylinder with the booster and not the original non-power drum brake booster. There is a difference. Along with what Bill mentioned about the holes in the brake pedal.
     
  4. synchro66

    synchro66 Well-Known Member

    Thanks guys....I checked the pedal and it is on the lower of the two holes, so I guess the ratio is right. Yes the M/C is the corvette style dual m/c which I assume is for a power setup.
    What about the 11" booster? ....Is anyone running this with a drum/drum combination??? without any pedal issues? my next door neighbour is a motor mechanic and he seems to think that this could be the source of the problem ....any thoughts???
     
  5. FJM568

    FJM568 Well-Known Member

    You need to make sure that new master cylinder you have on there now is for the power DRUM/DRUM setup and not a power DISC/DRUM master cylinder.

    There is a non-power DRUM/DRUM master cylinder.
    There is a power DRUM/DRUM master cylinder.
    There is a power DISC/DRUM master cylinder.

    There is even a non-power DISC/DRUM master cylinder.

    You want the one in RED above.

    The 69 Olds Cutlass I have has the power drum brake setup and has the 11" booster. I don't think the booster is your problem, but I could be wrong. Let us know what you find out.

    I'm sure power drum brakes were available on your 71 Skylark. Not knowing if you can get parts for your Skylark at your local parts store in Australia, just tell them you need a power drum brake master cylinder and booster for a 71.

    Another thought just occured to me. You may also need to add a proportioning valve to your setup. Not positive on that, though. Someone else may have to chime in on that one to be sure...
     
  6. Golden Oldie 65

    Golden Oldie 65 Well-Known Member

    I don't think the non-power master cylinder will even work on the power booster. Every manual brake master cylinder I've seen has the deep pocket on the piston and all the power brake master cylinders have a shallow pocket. The actuator rod on a power booster would not reach far enough to push the manual brake piston.

    No proportioning valve is needed on drum/drum setups.
     
  7. Golden Oldie 65

    Golden Oldie 65 Well-Known Member

    This might be a shot in the dark but I am the curious type so I'd probably try it if it were mine. You probably have a good vacuum source for the power booster, but how about restricting the vacuum supply? Maybe use a fitting with a smaller hole just to see what happens? I've never tried this but what can it hurt?
     
  8. FJM568

    FJM568 Well-Known Member

    That is a good idea.

    I just remembered that the 69 Cutlass I have(was my grandfather's) has always had "touchy" brakes even from when the car was brand new. After I got the car, I had to replace the master cylinder and the "touchiness" went away. They feel like "regular" brakes now. Also brake shoes make a big difference. Use a good organic material brake shoe, and not the semi-metallics or metallics.

    I once owned a 91 Mazda 4x4 pickup. I put some semi-metallic shoes on the back ABS brakes, and that is all it did was lock them up with the slightest touch. After asking around and talking to a few mechanic friends, one of them kind of chuckled and told me to put an organic pad on there. Never locked up again after that...

    Just some more ideas for you...
     
  9. synchro66

    synchro66 Well-Known Member

    Thanks Jim, I have a feeling I may have the disc/drum m/cylinder...I wasnt aware that there were any differences other than bore size...I think mine is a 1" I will call MBM brakes over there tomorrow morning and check if I have the wrong one. I usually buy most of my stuff from the US anyway...It only takes about 4-5 days to get here so no probs
    Thanks
    Bruce
     

Share This Page