Dave


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Posted by Paul (in NY) on Monday, August 14, 2006 at 11:08AM :

In Reply to: Pls Recommend Brake Mods for Disks posted by Dave Speed on Monday, August 14, 2006 at 8:47AM :

I use a Bendix 11584 Dual Circuit Master Cylinder. The NAPA number is MC 39017. The Application is 1979 Chevy C-10 1 Ton Pickup with JD-8 Option. Decoded;
The conventional MC’s (JD-7 option) have a bore of 1.250”. I found on my Bumblebee that even though the brakes worked GREAT, I had a low pedal when pushed hard. I found that the above MC was available, same mounting. This MC has a Bore of 1.3125. This gives approx 15-20% more reserve which means you have a nice high pedal when applied hard. The JD-8 option was used by Chevy with Hydroboost booster. It does not know the difference when used on a Vacuum Booster as in the Bumblebee. The Same Chevy C-10, 1 Ton Pickup MC with the JD-7 option was for Vacuum Booster but had a Bore of 1.250”

If you are using disks on all 4, you need to consider a proportion valve. This is available from Summit Racing for $49.00. It goes in the line to the rear brakes. When you are stopping easy or hard, weight is transferred to the front axle, the rear gets lighter. That’s why the nose goes down when stopping. The harder the stop, the more weight transfers to the front. We know and accept that maximum braking is while the wheel is still rotating, just before the point of lockup. A locked up, skidding tire is not efficient braking. As weight is transferred forward, the rear wheels will lock up quicker since weight is removed. When using disks in the front and drum’s in the rear, this is not normally a problem. This is due to the inefficiencies of drum brakes. They don’t have the stopping power that disk provide, thus they will not lock up as easy and quickly.

When using Disks on all 4, without proportion valve, the rears will lock up quickly as weight is transferred forward, since disks are so much more efficient. Now we have the front brakes doing most of the work stopping while the rear disks are in lockup skidding. The proportion valve allows us to reduce the braking on the rear, so that during hard stopping (max weight transfer to the front) the rear disks have less power applied and are not locked but rather just at the point of lockup, still rotating.

Adjustment of a proportion valve requires driving about 30 mph on an isolated road. You nail the brakes hard, if the rear locks up before the front, adjust the proportion valve to reduce brakes to the rear. Keep doing this until you are adjusted so that all 4 will lock up at the same time. Keep in mind, lockup is NOT what you want for hard braking, but even, equal lockup between front and rear assures you have the adjustment made properly. Now you can apply hard pedal (emergency stop) and keep all 4 wheels just rotating (max brake), right at the point of lockup, and not have skidding wheels.

My Bumblebee has Helitool Suspended pedals. The MC mentioned above bolts right up to these suspended pedals. I have never investigated mounting this MC in the stock location, using the stock brake setup. But with some thought and a little fab, anything can be made to fit/work. Shown in the picture is the Master Cylinder mentioned. Look below the two pressure switches and you will see the Proportion valve sitting verticle with its adjusting knob.

Paul






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