AD Manual Brake Conversion on a 1980 El Camino

manualbrakes.com

Not-quite-so-new-guy
Feb 11, 2016
24
3
Since being around brakes a lot since I started manualbrakes.com, sometimes I get a reoccurring question when people are doing the conversion.

What ports do my lines bolt up to?

The ports on all G-body master cylinders have a ½-20 inverted flare port (port closest to the firewall) for the front brakes and a 9/16-18 inverted flare port (port located toward the front of the master cylinder) for the rear brakes. The manualbrakes.com KIT comes with adapters to mate the stock GM line fittings to the MOPAR style master cylinder’s 3/8-24 inverted flare outlets.

For most GM applicatiosn, remember:
Rear port of master cylinder goes to the front brakes.
Front port of master cylinder goes to the rear brakes.
 
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manualbrakes.com

Not-quite-so-new-guy
Feb 11, 2016
24
3
Master Cylinder Bleeding Procedures.

NOTE: DO NOT bench bleed a master cylinder on the car. On a g-body the master cylinder sits at an angle and it WILL NOT get all the air out of the master cylinder. Use a vise to hold the master cylinder level to the ground to bleed the master cylinder of all its air.

I like to use plugs to bleed the master cylinder of air instead of the procedure that uses hoses to recirculate the fluid from the master cylinder ports back up to the reservoir. Why?

When using plugs to close off the ports of the master cylinder, this procedure will let you know if all the air is out of the master cylinder AND if the master cylinder is bad. You don't want to find out your master cylinder is bad after you have it installed and are trying to bleed the rest of the system. You most likely will not get all the air out of the system when your master cylinder is bad. New or rebuilt, it is always good to make sure your master cylinder is in good working order before bolting it onto the car. It will one less thing you have to trouble shoot if you run into other issues when you are trying to trouble shoot braking issues.


Steps to bleeding a master cylinder:

1. Mount the master cylinder in a vise with the bore of the master cylinder level with the ground. Do not use the top of the reservoir as a guide because is may not be level with the bore of the master cylinder. It may be at an angle versus the bore of the master cylinder.
2. Use the appropriate size solid plugs to plug the outlets of the master cylinder so no fluid can escape the ports.
3. Fill the master cylinder with the appropriate amount of brake fluid.
4. Use a rod to SLOWLY cycle the master cylinder piston in its bore. DO NOT use a flat head or phillips heat screwdriver because they have sharp edges and could harm the bore of the master cylinder. I usually use a nut driver that is used for ¼” drive sockets as a rod because the end does not have any sharp edges and there is a handle to hold onto.
5. After cycling the master cylinder piston SLOWLY a few times, the piston should become rock solid and only move about 1/16 of an inch or less down the bore.
6. After the piston becomes rock solid, push in on the master cylinder piston and hold for 45 seconds. If the piston slowly moves down the bore of the master cylinder, you have a bad master cylinder. If the piston says rock solid and does not move, you master cylinder is good.
7. Mount to your car and bleed the rest of your system starting with the brakes furthest away (passenger rear) from the master cylinder and working your way to the closest (drivers front) brake.
 
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manualbrakes.com

Not-quite-so-new-guy
Feb 11, 2016
24
3
This is from The Brake Man at tbmbrakes.com from his Facebook page. It may make you think a little differently about aluminum calipers and multiple piston calipers.

12 RULES YOUR BRAKES LIVE BY:

1. Pad area (volume) has no effect on braking torque. Pad area (volume) effects life and heat management.
2. Caliper clamping force is a function of piston area (on one side of the rotor) times line pressure.
3. The hotter the brake pad gets, the faster it will wear.
4. Deflection anywhere in the brake system will result in a proportional reduction in clamping force.
5. Piston count has nothing to do with clamping force, piston area does.
6. Caliper deflection that exceeds piston O-ring retraction (around .020”) will result in brake drag.
7. Given the same design, aluminum calipers will always deflect more than steel, cast iron, or steel reinforced calipers.
8. If your pads are tapered, your calipers are deflecting.
9. All brake pads perform best in a temperature range. Too cold is just as bad as too hot.
10. The smallest, lightest rotor that will dissipate the necessary heat is the best rotor for the application.
11. A smaller bore master cylinder increases line pressure.
12. Larger diameter caliper piston(s) increase clamping force.
 
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malibudave

Greasemonkey
Supporting Member
Thread starter
Mar 12, 2010
187
43
Houston, TX
I pulled this information from the CPP website. I include this information on this thread because, during a manual brake conversion, the differential valve in the proportioning/combination valve may be triggered and cause only one side of the system to work resulting in poor braking performance.

The differential valve is built into most GM prop valves. It is for safety. If one side, front or rear, of the brake system looses pressure, the differential valve is triggered blocking off the low pressure side of the brake system so that the master cylinder can still provide pressure to the other side of the brake system. This ensures that there is some for of braking as a way to stop the vehicle. If a differential valve was not part of the braking system, and there is a loss of pressure in one side of the system, the master cylinder would not be able to build pressure. This would result in NO brakes.

Combination/Proportioning Valve Test

Use a test light by attaching a clip to a positive contact on the vehicle and touch the point of the tester to the electrical connection of the combination valve. If the the light does NOT come on, the valve system is operating correctly and no further testing is required.

If the light does come on, this indicates that the pressure differential valve is stuck in the front or rear position.

Bleed the brake system to determine if the front or rear lines are blocked off. Set up one front wheel and one rear wheel for bleeding at the same time. Crack both bleeder screws and gently pump the pedal a few times.

The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal pressed. An unblocked line will squirt fluid out the bleeder.
The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.

Slowly press the pedal with steady pressure a number of times until the light goes out; this will center the differential valve. You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out, close the bleeder screw. (See fig. below)


 
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rafael martinez jr

Not-quite-so-new-guy
Dec 13, 2017
47
18
new jersey
here is my brake setup
 

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malibudave

Greasemonkey
Supporting Member
Thread starter
Mar 12, 2010
187
43
Houston, TX
What size master cylinder do you need with your manual brake setup?

You want to choose the smallest bore master cylinder that will give supply the entire brake system the volume it needs on the first initial brake pedal stroke. Smaller bore master cylinders generate higher line pressures, but supply less volume. Higher line pressures mean less effort by the driver to get the car stopped. Going to a larger bore master cylinder does reduce line pressures, but usually you are increasing piston sizes which allow for larger clamping forces on the rotor.

From the factory, g-bodies with manual brakes came with a 7/8” (0.875”) bore master cylinder. This is a “size” smaller than the 24mm (0.945”) power booster master cylinder.

A 7/8” bore master cylinder will work with MOST front disc / rear drum setups. Usually any aftermarket front brake systems will need roughly the same volume of fluid that the stock. The stock metric caliper, that came stock on G-bodies, S10s, 3rd Gen F-bodies, came with a single 2.5” diameter piston. A single 2.5” diameter piston caliper will need roughly the same fluid as a 1.75” dual piston or four piston caliper.

You will usually need to upsize your master cylinder when you have or convert to rear disc brakes. If using a factory, stock, rear brake system, usually you will need to upsize to a 24mm (0.945”) master cylinder to supply the volume of extra fluid that a caliper needs over a wheel cylinder. Usually a stock rear caliper will have piston with a diameter of 48mm (1.89”). Usually any caliper with single 2.0” diameter piston or less will need a 24mm or 15/16” bore master cylinder.

For aftermarket rear disc systems, you need to know what caliper you have and what piston size you have in your rear disc KIT and then choose the bore size of your master cylinder. The cheapest and popular rear disc system uses the same metric caliper that is on the front of the car. Since the rear caliper requires the same volume as the front, you will have to go with a 1.0” bore master cylinder because of the increased volume needed for the rear calipers. Some aftermarket rear system may also include a 1.75” diameter dual piston or four piston caliper. This will also require a 1.0” bore master cylinder. Some of the aftermarket calipers will have piston sizes in the 1.375” range. The 1.375” piston sizes are close in volume to most of the stock rear disc setups, so a 24mm bore master cylinder is best.

The Exceptions

The only exception I have found is when Strange or Aerospace brakes systems are used. If using Strange or Aerospace brakes, contact the manufacturer to see what they suggest for master cylinder size, but they will most likely suggest a 1.03” bore master cylinder with their front brake setup and rear drum setup. For their front and rear systems, they usually suggest a 1-1/8” (1.125”) bore master cylinder.

Let me know if you have questions about this information.
 

malibudave

Greasemonkey
Supporting Member
Thread starter
Mar 12, 2010
187
43
Houston, TX
In a manual brake system, the master cylinder size you choose should be dictated by the total volume of your brake system (calipers and wheel cylinder) and what type of seat you sit in to drive.

What difference does a seat make? When you are putting your effort into pressing the brake pedal to stop the car, the seat cushion does not supply a lot of support. Some of your effort that you apply to the brake pedal is also pushing yourself back into the cushion. The effort you use to press the brake pedal is less effective because some of that effort is wasted as your body sinks into the seat cushion. The more effort you apply to the pedal will also sink you deeper into the seat cushion. If you have a racing seat (i.e. Kirkey), it has little to no cushion as it is made of aluminum. A hard back seat like a Kirkey will give you more leverage and allow most of the effort you apply to the pedal as your body is supported by a hard surface and not a soft seat cushion.

If you have a racing type seat, you can get away with a larger bore master cylinder. Some people use 1-1/8” in diameter and have no issues when using a racing seat.

A larger bore master cylinder will require more effort to stop the car for a given front and rear brake setup. A smaller bore will require less effort to stop the car for the same front and rear brake setup as the larger bore master cylinder.
 
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