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 Post Posted: Mon Apr 10, 2017 4:26 pm 
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Joined: Fri Apr 07, 2017 5:57 am
Posts: 4
New to the board here.
Hoping someone can help me that has some experience with the larger bore (7/8") S-10 rear brake cylinders installed in a G Body (Buick Grand National). I have heard that these can be a little touchy (dicey) in damp / wet conditions as they tend to lock up very easily. My car will predominantly be driven on the street.
Since the brake bias will have changed....wondering if upgrading the front brake callipers to the oversized
2-3/4" bore GM Metric Brake Callipers would correct this issue. They claim to have +20% more clamping force over the standard 2-1/2" GM Metric Brake Callipers.

2nd question: I believe the factory Metric Brake Callipers are Low Drag. Not sure if the oversized Metric Callipers (2-3/4") are Low Drag or Non Low Drag.
Since I have converted the car to vacuum brakes and have a stepped master cylinder installed would it make any difference in brake performance?

Thanks in advance.

Dave


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 Post Posted: Wed Apr 12, 2017 1:51 pm 
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Joined: Tue Jun 08, 2004 1:06 pm
Posts: 226
Location: Houston, TX
I ran the 7/8" bore S10 wheel cylinders on my 1980 Elco with the same 14" wheels and tires front and rear. I run manual brakes also. When I was first testing the manual brake system with the stock front calipers, 7/8" bore wheel cylinders, and the stock 24mm bore master cylinder, the rear would easily lock up before the front brakes on dry pavement with manual brakes using Wagner Thermoquiet Shoes with an EE heat rating. This easy lockup will most likely be more prevalent on wet pavement.

In my opinion, 7/8" bore wheel cylinders are overkill. In my opinion, I would only use them on a drag car and only up size to a 7/8" bore wheel cylinder if I was having trouble holding the car on the line when foot braking the car. Drum brakes are pretty efficient at braking and are more touchy than rear disc brakes even with the stock size wheel cylinders.

From my experience, ALL re-manufactured calipers today are NON low drag (normal) calipers. ALL part numbers are the same for the calipers from 1978-2004 for g-body, third gen f-body, and 1st gen (square body) and 2nd gen S10 pickups (round body with single piston front calipers) which all use the same "metric" caliper designed for the g-body. LOW drag calipers will only work with STEP bore master cylinders, but NON low drag (normal) calipers can work with both STRAIT bore and STEP bore master cylinders.

From my experience, ALL new calipers today are NON low drag (normal) calipers. If you go with a 2.75" piston front caliper it will be NON low drag (normal). NOT all big bore, 2.75" calipers have an actual 2.75" piston, so make sure, after you get them, to measure the diameter of the piston. Wilwood does have an actual 2.75" piston, cast iron caliper, and is lighter weight than ANY replacement cast iron caliper and seems to be designed with more rigidity. I have these on the front of my El Camino with the 7/8" bore wheel cylinders, but I have NOT test drove the car yet as I am finalizing an turbo SBC engine swap (ignition issues).

Any time you increase the size piston size of a caliper or wheel cylinder, the clamping forces will go up and braking performance will increase if using the same master cylinder. Since you have a step bore master cylinder, it should easily work with larger 2.75" calipers and larger 7/8" bore wheel cylinders. The pressure bore of the STEP bore master cylinder is 24mm and MAY make the front brake more touchy and very easy to lock up. The remedy to this is a larger pressure bore, STEP bore master cylinder (i.e. B-body?), but the larger pressure bore MAY reduce overall braking performance because it supplies less line pressure to the calipers.

Smaller bore master cylinder = more line pressure, and less volume with the same pedal stroke, than a larger bore master cylinder.
Larger bore master cylinder = more volume, with the same pedal stroke, but less line pressure.
Smaller bore calipers and wheel cylinders = less clamping force with the same size master cylinder.
Larger bore calipers and wheel cylinders = MORE clamping force with the same size master cylinder.

Up sizing to a larger bore caliper and keeping the same size bore master cylinder will create a slightly longer pedal travel with the step bore master cylinder, but the calipers will have less "feel" and transition to lockup abruptly.

_________________
David Schultz
MalibuDave
www.manualbrakes.com
https://www.facebook.com/manualbrakescom-576443512373024/


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 Post Posted: Wed Apr 12, 2017 5:19 pm 
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Joined: Fri Apr 07, 2017 5:57 am
Posts: 4
Thanks for your explanation Dave. After reading your reply, think I'm gonna leave things stock (both front & rear).

dave


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