Brake line block question
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From: Yorktown Virginia
Brake line block question
Hello, I have a 1969 Indy Pace Car RS/SS, I have added disc brakes on the rear. Do I still need to install the block that is under the drivers door mounted on the sub frame? Any help would be appreciated.
Rob F..
Rob F..
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Yes you need to install a proportioning valve, The real question is do you want to install the original one. I do not have a direct answer but If I explain how the system work it may help you decide how far you want to go.
There are two things you need to think about when you change from drums to disk.
1.) Constant off pedal pressure. A drum brake system have pressure in the brake line all the time. It is just enough pressure to over come the springs and keep the shoes touching the the drums. This is bad for rotor brakes and the pressure is high enough to slightly apply the brakes and can cause the brakes to over heating while at highway speeds. I believe in 1969 the check valve that held this pressure was in the master cylinder.
2.) The proportioning Valve (I think that what you mean by block?) controls how much pressure is sent to the back wheels in a ratio to the pressure sent to the front wheel. A very simple example would be if you apply the brakes on your stock system you may get 500 PSI to the front rotors and 250 PSI to the rear wheels. The reason for this is because the back brakes do a lot less stopping then the front brakes and its critical that they do not lock up under hard braking which could send the car into an uncontrolled spin. Much like a factory carburetor the proportioning valve is tuned at the factory for a specific car. Riding height, engine weight can all go into the tuning of the proportioning valve not to mention the size and type of brakes. If the rear brakes you installed did not come with an application (meaning your camaro) specific proportioning valve you should either find the right one or buy an adjustable one and tweak it on the road adding as much pressure as possible but avoiding lock up in an emergency stop.
Yes there are many people that do not understand this and just add brake parts and do not tune them and they will tell you they have never had an issue. Yes you will get down the road and still stop at the stop sign and as long as you plan on driving the car to local shows early Sunday morning it will probably be fine, But if you are planning on driving it to the point you will notice those rear disks your system needs tuned just like a carb after a cam change.
If you think the the cars are bad you should work on the tow capable trucks. They have variable displacement proportioning valves. As you add load to the back of the truck a lever changes the pressure level going to the back of the truck. This was a huge pain on standard pickup to get right. It is also why the first ABS system where on the back wheels of pickups long before they found there way into every car. While newer car still have proportioning valve they a much less a factor in a car with ABS.
There are two things you need to think about when you change from drums to disk.
1.) Constant off pedal pressure. A drum brake system have pressure in the brake line all the time. It is just enough pressure to over come the springs and keep the shoes touching the the drums. This is bad for rotor brakes and the pressure is high enough to slightly apply the brakes and can cause the brakes to over heating while at highway speeds. I believe in 1969 the check valve that held this pressure was in the master cylinder.
2.) The proportioning Valve (I think that what you mean by block?) controls how much pressure is sent to the back wheels in a ratio to the pressure sent to the front wheel. A very simple example would be if you apply the brakes on your stock system you may get 500 PSI to the front rotors and 250 PSI to the rear wheels. The reason for this is because the back brakes do a lot less stopping then the front brakes and its critical that they do not lock up under hard braking which could send the car into an uncontrolled spin. Much like a factory carburetor the proportioning valve is tuned at the factory for a specific car. Riding height, engine weight can all go into the tuning of the proportioning valve not to mention the size and type of brakes. If the rear brakes you installed did not come with an application (meaning your camaro) specific proportioning valve you should either find the right one or buy an adjustable one and tweak it on the road adding as much pressure as possible but avoiding lock up in an emergency stop.
Yes there are many people that do not understand this and just add brake parts and do not tune them and they will tell you they have never had an issue. Yes you will get down the road and still stop at the stop sign and as long as you plan on driving the car to local shows early Sunday morning it will probably be fine, But if you are planning on driving it to the point you will notice those rear disks your system needs tuned just like a carb after a cam change.
If you think the the cars are bad you should work on the tow capable trucks. They have variable displacement proportioning valves. As you add load to the back of the truck a lever changes the pressure level going to the back of the truck. This was a huge pain on standard pickup to get right. It is also why the first ABS system where on the back wheels of pickups long before they found there way into every car. While newer car still have proportioning valve they a much less a factor in a car with ABS.
Last edited by Gorn; May 15, 2022 at 01:22 PM.
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