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The brakes on my 65 have been converted to the 67 dual master cylinder system. A proportioning valve is used. However, I read in one of the forum posts that the distribution block (proportioning valve) isn't needed with this conversion. Is this true? if so, why?
The car doesn't need a proportioning valve because the front and back calipers have different diameter pistons to give the correct braking force front to back. The brakes work correctly with a single port master cylinder for that reason.
It is not a proportioning valve ............it is a pressure differential switch to let you know if front or rear brake systems fail. It only turns on a BRAKE FAILURE light in the cabin.
Now the L88 race cars and others with Option J56 did have a special brake proportioning valve installed for racing applications. But that is it. A total of only a few hundred had this special option for 1967.........and it was only for the BB cars.
It is not a proportioning valve ............it is a pressure differential switch to let you know if front or rear brake systems fail. It only turns on a BRAKE FAILURE light in the cabin.
Now the L88 race cars and others with Option J56 did have a special brake proportioning valve installed for racing applications. But that is it. A total of only a few hundred had this special option for 1967.........and it was only for the BB cars.
Larry
How do you know what he has? If someone modified the brakes, there is no reason that they couldn't have added an actual proportioning valve. I certainly did when I converted to front disks on my '64, and it is something that almost any car can benefit from if properly dialed in. I recognize that it isn't a necessity with disks all around, but that doesn't mean that it wasn't done.
How do you know what he has? If someone modified the brakes, there is no reason that they couldn't have added an actual proportioning valve. I certainly did when I converted to front disks on my '64, and it is something that almost any car can benefit from if properly dialed in. I recognize that it isn't a necessity with disks all around, but that doesn't mean that it wasn't done.
A pic from the OP would tell.
1. OP has 1965 with all disk brakes............not disk/drum combination (63/64) where a proportioning valve should be used. He would have no need to install a true proportioning valve when converting to a dual MC.
2. OP calls distribution block a proportioning valve in his third sentence.
I found this related description a while back when I was reading up on the difference between the two and the fairly common occurrence of members and guests using one term, when they actually mean another term. I got a chance to see a few real proportioning valves up at Corvette Carlisle in 2017 when they displayed a gathering of L88 corvettes. There were a few L88 "tribute" cars there as well. God knows where they were able to dig up those proportioning valves to go along with the J56 brakes. Very cool to see real proportioning valves instead of just pictures of ones. Sigh . . . I guess I really need to get out more. Anyway, here is the blurb. Enjoy.
Proportioning Valve - A proportioning valve restricts the amount of pressure to the rear of a brake system under heavier braking conditions to prevent the rear brakes from locking up too soon. Proportioning valves were usually only found on vehicles with front disc brakes.
Distribution Block - This block distributes the fluid to different lines in the system and includes a failure warning switch that turns on a warning light in the event of a half system failure. It does not proportion or meter the brake fluid coming from the master cylinder.
I found this related description a while back when I was reading up on the difference between the two and the fairly common occurrence of members and guests using one term, when they actually mean another term. I got a chance to see a few real proportioning valves up at Corvette Carlisle in 2017 when they displayed a gathering of L88 corvettes. There were a few L88 "tribute" cars there as well. God knows where they were able to dig up those proportioning valves to go along with the J56 brakes. Very cool to see real proportioning valves instead of just pictures of ones. Sigh . . . I guess I really need to get out more. Anyway, here is the blurb. Enjoy.
Proportioning Valve - A proportioning valve restricts the amount of pressure to the rear of a brake system under heavier braking conditions to prevent the rear brakes from locking up too soon. Proportioning valves were usually only found on vehicles with front disc brakes.
Distribution Block - This block distributes the fluid to different lines in the system and includes a failure warning switch that turns on a warning light in the event of a half system failure. It does not proportion or meter the brake fluid coming from the master cylinder.
C.J.
Here is a '67 J56 Proportioning Valve and the dual pin calipers. For education purposes.
Most disk/drum GM cars of the era have a device called a "combination valve". This consisted of both a rear brake proportioning valve and a differential pressure switch in the same housing with a single wire connection for the switch.
On a 1g C2/3 stop, weight transfer yields a distribution of about 65/35 percent F/R, and GM established this as the front/rear proportioning based on piston area, which means that 65 percent of total piston area is at the front and 35 percent at the rear.
Since J-56 was designed for racing, an adjustable prop. valve was included to alter rear proportioning based on tire adhesion and track conditions such as dry or wet.
The above referenced combination valves used on disk drum systems are fixed and not adjustable. Rear brake line pressure has to be limited because of the self-energizing nature of typical drum systems. Disk brake torque is essentially linear with line pressure, but drum brake torque "curves up" with increasing line pressure, so rear line pressure needs to be limited to prevent premature rear brake lockup on a hard stop.
I adjust mine by doing panic stops. You want the fronts to lock up just before the rears. Start with full pressure to rear and do stops, letting the brakes cool as necessary in between, and keep restricting the flow to the rears until you achieve this point.
This is easy in my '64 because I use the Tilton system and have it mounted under the dashboard behind the parking brake so can do it on the fly.
I adjust mine by doing panic stops. You want the fronts to lock up just before the rears. Start with full pressure to rear and do stops, letting the brakes cool as necessary in between, and keep restricting the flow to the rears until you achieve this point.
This is easy in my '64 because I use the Tilton system and have it mounted under the dashboard behind the parking brake so can do it on the fly.
I would agree. If the rears lock first, the rear of the car will want to swap places with the front of the car. Ask me how I found this out.
Jim L and a few other racers may also want to add comments...........but toddalin explains it well.
I found this related description a while back when I was reading up on the difference between the two and the fairly common occurrence of members and guests using one term, when they actually mean another term. I got a chance to see a few real proportioning valves up at Corvette Carlisle in 2017 when they displayed a gathering of L88 corvettes. There were a few L88 "tribute" cars there as well. God knows where they were able to dig up those proportioning valves to go along with the J56 brakes. Very cool to see real proportioning valves instead of just pictures of ones. Sigh . . . I guess I really need to get out more. Anyway, here is the blurb. Enjoy.
Proportioning Valve - A proportioning valve restricts the amount of pressure to the rear of a brake system under heavier braking conditions to prevent the rear brakes from locking up too soon. Proportioning valves were usually only found on vehicles with front disc brakes.
Distribution Block - This block distributes the fluid to different lines in the system and includes a failure warning switch that turns on a warning light in the event of a half system failure. It does not proportion or meter the brake fluid coming from the master cylinder.
C.J.
This was good information. My 64 was upgraded to front disc only, there was an adjustable valve attached to the frame in the rear brake line. I have since added rear disc brakes. Since the distribution block does not meter the fluid it seems this setup is going to work for 4 wheel disc as well. Have not driven it yet. Maybe next week. The picture included is my master cylinder and distribution block. The shop that did the work were real hamburgers and the M/C has 2 chambers, front larger than rear. I don't know what it is.
Adjustable Prop valves are a good thing even on factory systems. I had the Vette on pictured on my '78 Trans Am autcrossing as factory system rear brakes would lock up on hard braking-weight transfer off the rear to front especially with softer/factory front springs. It eventually leaked and no rebuild kits so had to use a Mopar Direct connection.
Factory engineers design their system for the average driver and parameters they(engineers) feel are important.
Change spring rates,shocks, tire size you have undone the factory engineers specs!
And all "newer" cars with 4 wheel discs use a factory prop/distribution valve.Just like some engineers thought drum brakes need a residual pressure valve in the line or MC and others don't.
I plan to take that distribution valve off and replace it with a real proportioning valve, adjustable. It seems that the front brakes simply pass through the block. The switch tells you if you had a failure. Since the 64 has no light for that switch, it is useless. Looking at Willwoods proportioning valve, it appears the front brake line is also simply a pass through and the valve only controls the rear brakes. Given that, seems only the stand alone proportional valve is all that is needed.
Thank you, D.
This was good information. My 64 was upgraded to front disc only, there was an adjustable valve attached to the frame in the rear brake line. I have since added rear disc brakes. Since the distribution block does not meter the fluid it seems this setup is going to work for 4 wheel disc as well. Have not driven it yet. Maybe next week. The picture included is my master cylinder and distribution block. The shop that did the work were real hamburgers and the M/C has 2 chambers, front larger than rear. I don't know what it is.
That MC is made for a disc/drum car. Disc brakes require more fluid to operate than drum brakes.
Also, since someone asked.here is the procedure for adjusting the J56 proportioning valve available on 1966 and 1967 C2 Corvettes.
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As earlier stated. 
Proportioning Valve on C2
