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Canadian Poncho Homepage -> Canadian Poncho -> 65-70 Full Size Chassis Tech Forum -> B-body disc brake question
Post Info TOPIC: B-body disc brake question
gparis7


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B-body disc brake question
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This question is related to my 68 Caprice but it applies to all 65-70 Canadian B-bodies so here goes. When I bought the Caprice, it had drums on the front, although the car had a disk brake pedal pad. When I investigated I found that the drums and spindles had junkyard markings on them. I figure the original four piston calipers were leaking and the owner got frustrated and had drums installed.

I wanted it original (well nearly) both for appearance and safety. I had a set of front disc spindles and single piston calipers from a 69, which I installed along with nearly all new parts as well as a functional factory prop valve. The brakes work great, with one issue. The power boost on the disc system ( I used the same booster as with the drums) provides much less boost than it did for the drum system.

Does anyone know the reason for this? Does it have to do with the pressure specs of the disc vs the drum master cylinder? Or is it related to any valving in the booster? I did experiment with adjusting the length of the push rod and it seemed to make some difference, but I still have a relatively small vacuum boost. (I also have the same issue on my 69 Parisienne after the drum to stock disc conversion).

I would appreciate any insights as to what might be the issue, and how it can be resolved.
Thanks.

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StriperSS


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Did you use the same master cylinder? Disc brakes systems have a larger volume reservoir than drums, so you may notice your pedal going farther to the floor. It's possible on a full on panic stop, to empty a drum brake reservoir with a disc brake system. That would not be a good thing.  (understatement)

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gparis7


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I used a correct factory disc brake master cylinder with correct prop valve. I kept the booster that was on the car as I assumed it was original and therefore was part of the original disc brake package (albeit a four piston caliper system).

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cdnpont


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Disc brake calipers need significantly more hydraulic pressure acting on the piston than drum brake wheel cylinders. I don't think a drum spec booster will generate near enough assist at the master.

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gparis7


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On my 69 Parisienne I definitely have a drum brake booster. On the 68 Caprice I'm not sure, but maybe I'll swap it for a known disc booster on that car and see what is the result.

-- Edited by gparis7 on Thursday 8th of April 2010 08:51:23 PM

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rabbit64cs


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Check the holes on your brake pedal arm. There will be 2 holes that the pushrod can go in , this will change the mechanical advantage required to engage the brakes.

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rabbit64cs


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any resolve??

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gparis7


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My drum brake pedals I have two holes. All of the disc brake pedals including the one on this car have only the one hole. Adjustment of the rod length makes some difference but the assist is still less than I would like. I may swap in a known disc brake booster and see what happens.

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rabbit64cs


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good to know about the 2 holes VS 1 hole, somthing to look for in the future...

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