Power Brake Boosters - disk vs drum how to tell them apart?

Discussion in 'The whoa and the sway.' started by schwemf, Apr 30, 2017.

  1. schwemf

    schwemf Mike Schweitzer

    1970 Skylark/GS power brake boosters differ between drum and disk brake equipped vehicles. Does anyone know how to tell them apart?
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    If there ever was a difference, and I have my doubts about that, there is none in the aftermarket. There is probably a difference in the push rod length, and definitely in the master cylinders.
     
  3. BrianTrick

    BrianTrick Brian Trick

    There was NO difference in the booster itself.
     
  4. schwemf

    schwemf Mike Schweitzer

    According to the Hollander Interchange Manual, not only are the boosters different between drum and disk brake equiped Buick Specials, but they're Buick specific and year specific as well:
    -Disk brake equipped Buick Special '69-70 only
    -Disk brake equipped Buick Special '71-72 only

    Now the master cylinders became more generic (interchanging with many more models including Chevrolets) starting in 1971. But in '69-70 they too only interchange with disc brake equipped Buick Special '69-70; Grand Prix '69; Olds F85 '69; Tempest '69
     
  5. rogbo

    rogbo Gold Level Contributor

  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Right, the rods are 2 different lengths for the different master cylinders. The difference is in the depth of the master cylinder depression that the rod contacts. The booster though is the same.
     
  7. schwemf

    schwemf Mike Schweitzer

    Booster Dewey says that, for our 1970 Skylarks, a slightly weaker spring was used within the disk brake booster to allow greater pressure to operate the front disk brakes . Checking a 1969-era parts manual challenges this claim, however, as the only two springs within the booster are not called out to be different for drum vs disk brakes. These two springs are the air valve spring within the power piston and the power piston return spring.

    The 'bigger' Buicks (Le Sabre, Wildcat, Electra, and Riviera) have completely different boosters for drum vs. disk, and different push rods.
     
  8. BrianTrick

    BrianTrick Brian Trick

    The piston bore in the master cylinder is the real difference. The drum/drum MC have a 1 1/4" bore for less pressure/more volume. The disc/drum MC have a 1 1/8" bore for less volume,more pressure. Then the valves proportion that. The 70-older cars used the same valve down on the frame,for disc and drum,and they added the second valve next to the MC,for disc.In 71,they figured-out how to put it all into one valve,down on the frame.
    Yes,there are different push rods for the master cylinders,for both power and non-power. Some have a small dimple for the rod,while others have an actual hole bored into the piston. You have to match these to get everything to work right. The clevis rod from the back of the booster connects to the lower hole on the pedal,and the non-power clevis rod from the MC connects to the upper hole on the pedal. You will also notice two separate location holes for the stoplight switch on the pedal bracket.One is for power,one is for non-power.
     
  9. schwemf

    schwemf Mike Schweitzer

    Thank you so much Brian for taking the time for your detailed explanation. This is the info that I needed to know, as the 70 GS that I'm currently restoring has a cobbled-together disk brake conversion by a previous owner that I'm trying to sort out.
     
  10. BrianTrick

    BrianTrick Brian Trick

    I have a customer's car here now,that I have to uncobble as well. They converted to discs,and changed the booster to a smaller aftermarket style,but nothing else. Still had the drum prop valve and drum/drum MC. He couldn't figure out why the brakes were so bad. Night and day difference now,with the correct MC and valve.
     

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