1966 proportioning valve

[jordan grant]

Did a 1966 Corvette have a brake system proportioning valve?My car has power brakes(possibly not original,how would I check this?)and has no valve anywhere in the brake line system.Jordan

[David D.]

Re: NO *NM* *NM*

[Duke W.]

Re: 1966 proportioning valve

The base brake system does not have a proportioning valve. It has a
"switch" that lights the "brake" light on the IP if there is a difference in front and rear brake pressure, which would indicate a hydraulic failure.

The J-56 HD brake system DOES have a proportioning valve, and it is adjustable.

Duke

[John H.]

Re: 1966 proportioning valve

A J50 power brake '66 has no valves of any type in the system - the distribution block with the connection for the differential pressure warning light for manual and J50 cars didn't start until 1967.

[jordan grant]

Re: 1966 proportioning valve

Is it correct that any "proportioning" was done in the master cylinder?(providing it is an original)and NOT aftermarket.My car has an aftermarket mc and the brakes on one of the rear wheel(s)tend to lock up when applied quickly.This,needless to say is a little unsettling!!!,forget it,if the roads are at all wet!!.Thoughts??.Jordan

[Mark #28455]

Re: 1966 proportioning valve

Are your brakes well bled? My car was acting similarly until I bled an air bubble from the front brakes.

[Doug Flaten]

Re: 1966 proportioning valve

I have wondered the same thing at times. I have a '65 Lincoln with a single reservoir and a disc / drum setup that uses a proportioning valve. The valve has been a source of leaks in the past and is no longer available. I have thought about replacing with a dual reservoir master cylinder and wondered if I still would need a proportioning valve.

[Duke W.]

Re: 1966 proportioning valve

On the C2 base disk brake system "proportioning" is controlled by the front/rear caliper piston sizes, and the result is fixed 65/35 distribution of brake torque.

Disk/drum systems often have proportioning valves to limit rear line pressure because duo-servo drum torque characteristics are non-linear relative line pressure compared to the essentially linear characteristics of a disk brake.

Front/rear proportioning for maximum performance braking is a function of
tire adhesion, static weight distribution, and vehicle center of gravity.
For a given vehicle and brake system, best proportioning for maximum performnce
stops may not be best for more moderate stops typical of normal braking or for best braking in wet conditions.

There are a lot of engineering and physics issues here, so unless you are quite familiar with them, know the basic brake system design calculations, and know all the specifications for the installed hardware and available alternative hardware, you are probably better off to repair the OE system and not attempt to re-engineer it while wearing a blindfold.

Single wheel lockup problems can be caused by contaminated brake pads or other problems on that specific corner that locks, including a dragging parking brake.

Duke

[jordan grant]

Re: 1966 proportioning valve

That pretty much answers any subsequent questions I might have had Duke!!VERY helpful!!.There is a problem with the emergency brake cable at the right rear,as you hinted,this just may be the problem.

So,If I understand it correctlly,fluid pressure leaving the master is equal to both lines,feeding different size pistons in the front and rear pistons.This would apply to OEM and aftermarket master cylinders?.Jordan

[Terry M.]

Re: 1966 proportioning valve

Generally on pre-ABS brake systems there are three devices which control brake function:
Metering Valve
Proportioning Valve
Differential Pressure Switch

As an over-simplification: Metering valve allows the slow build of pressure to the disk portion of a disk/drum system.
Proportioning valve controls front to rear brake line pressures.
Differential pressure switch turns on a light when there is a difference in line pressure - generally front to rear difference although there have been a few diagonal systems.

Not all disk/drum systems have all three valves as separate components. Some are combined in a combination valve, and in some cases some of these valves are built into master cylinders. In some cases not all three valves are used.

For the 1965 to 1982 Corvette dual disk system no metering valve is used, and the proportioning valve was used on only some of the J56 HD brake systems, I believe. I expect the differential pressure switch is used only with the dual master cylinders of 1967 and newer.

[Duke W.]

Re: 1966 proportioning valve

The Corvette M/C has equal bore and piston sizes for both front and rear m/C pistons, but some tandem M/Cs have different piston sizes.

Pedal force = line pressure x piston area.

From the above relatioship if you increase M/C piston size you need more pedal force to create the same line pressure, but pedal travel will be less, and vice versa.

This relationship can also be applied at the caliper, so for a given line pressure, a larger piston provides more pad clamping force.

Duke

[Duke W.]

Re: 1966 proportioning valve

Just to add to the confusion, a lot of GM disk/drum systems have a device that combines both differential pressure detection and rear line proportioning in a single (typically) cast iron housing that is called a "combination valve".

Duke

[Michael W.]

Re: 1966 proportioning valve

Terry,

I think a proportioning valve became standard in the late '70's (1976?) to compensate for changes in the front/rear weight balance. The differential pressure switch function was retained.

Mike

[Terry M.]

Re: 1966 proportioning valve

Mike,
That is why I stated the "I believe" at the end of that sentence about proportioning valves. I was painting the whole post with a broad brush, but I was confident those with more knowledge would be more specific.

[John H.]

Re: 1966 proportioning valve

My '69 Z/28 is an example of having all three separate devices in one system. It has the distribution block with the usual differential pressure warning switch, a metering valve between there and the line to the front disc brakes that blocks any pressure to the front discs until the line pressure exceeds 40 psi (so the rear drum shoes can expand against the drums before the front calipers see any pressure), and there's a proportioning valve down on the subframe in the line to the rear drums that proportions and limits rear drum system pressure under max braking to avoid rear brake lockup. Later cars combine all these functions in one device - the "combination valve".