Proportioing Valves

[Ronno6]

Does a proportioning valve really "care" what size wheel cylinders,calipers, shoes or rotors are utilized in the brake system?

i.e. 1" bore w/ 11x2 shoes; 15/16" bore w/11 3/16 x 2 3/4" shoes; 1 3/16" bore w/any shoe; 1" rotor; 1.25" rotor, etc.....

Should one prop valve be expected to prevent rear drum lockup with any combination??

I have seen prop valves listed as "all GM disc/drum systems,"
then there is Inline Tube company that lists different prop valves for almose every different year/make/model of GM vehicle.

Are there differences?

[VileZambonie]

These trucks came with combination valves which incorporate a proportioning valve. They are incapable of caring about anything. It is strictly based on pressure. read up on the combination valve in the tech section.

[VileZambonie]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=16967.0

[Henry]

Hi Ronno6:
Yes, I believe it does. The "proportioning valve" you mention is probably really a "combo valve" in that it has three functions:
1. Metering: prevents front disc braking until the shoes of the rear drum brake contacts the drum.
2. Proportioning: during hard braking, the proportioner reduces the rear brake pressure and delays the rear wheel skid due to the weight of the truck being transferred to the front wheels.
3. Failure warning switch: when either the front or rear brake circuit fails due to low pressure, the switch is activated which turns on your dash warning light.

In my mind, you cannot have the combo valve work correctly and perform the functions of metering and proportioning with any vehicle much less different sizes of brake parts.
Furthermore, my 76 service manual has a statement "The input-output characteristics of the valve (i.e. split points), vary with vehicle usage. It also mentions that the combo valve is non-servicable if it fails.

Assuming the parts vendors do not have charts showing the usage of the different proportioning valves, I suggest you look in the service manual (or supplement) for your model year truck...there should be a couple of charts on the brake system which show what system you have (JB1, JB3, etc) and correlate that to the front and rear brake sizes and what your power brake unit is. There should also be a chart that correlates your truck model to your ton rating, wheelbase, GVW rating, merchandizing option, and finally  the brake system. Once you have established what brake system you are supposed to have (JB-whatever) you should be able to go to a Chevy parts book and find out what is the correct part number for the combo valve. If your truck is a "Frankentruck" with different braking components than was originally designed, maybe you can read through the charts and determine what your current truck brake configuration is closest to and use that.

Seems like there should be a company out there like SSBC (Stainless Steel Brake Co) that can talk to you about this stuff and you can see if my thoughts are correct or not.

Regards,
Henry

[Ronno6]

These trucks came with combination valves which incorporate a proportioning valve. They are incapable of caring about anything. It is strictly based on pressure. read up on the combination valve in the tech section.

I have seen them called by either name.
OK. We'll call them combo valves for short.
The combo valve cutaways that I have seen show at least 2 springs.
See fig 50-42A
https://www.pakwheels.com/blog/understanding-brake-bias-safe-driving/

Springs can vary wildly in rate, depending on wire material and diameter of the wire and the spring. Hence, the possibilities are endless as to which springs can be in which combo valve.

My point is: JB3 brakes use a 1" wheel cyl with 2" wide shoes.
The JB5 uses a 15/16" cyl with 2 3/4" wide shoes.
(Both use 2.94" bore calipers.)

I have read about people using larger bore cylinders in these systems without problems.

I would have to think that the 15/16" cyl would require more pressure to apply the wider 2 3/4" shoe than the 1" cyl would to apply the 2" shoe. Maybe that is not the case.
Would the same combo valve work the same in either system?

ps- I have the pdf of the Chevy Truck Parts Manual. I have a hard time finding ANYTHING in that book except 67 pages of speedometer gears......

[Ronno6]

I have found OEM part #'s for the combo valves:

JB3: 18008087 which eventually is superseded by 25515632 and is available for $75.00 or so
JB5: 18008085  which eventually is superseded by 25515635 which I have not found yet.

Those parts manuals are fun to navigate............

[VileZambonie]

The pressure control portion of the valve limits the pressure the same regardless of what you put on for shoes or wheel cylinders. The replacement valves are standard and designed to be one size fits all.

The important thing is to have the correct brake balance so if you are outfitting your truck with JB5, use the corresponding parts and of course properly installed and adjusted. Have you identified what you have? Did you measure your shoes, what size rotors do you have, which booster do you have, is the spid label in tact?

[Ronno6]

I have become intimately familiar with what components are on my C10:
Front" JB5  1.25" rotors w/matching calipers, spindles,bearings and hoses.
Rear: JB3 11" x 2" shoes and drums, 1" bore wheel cylinders, though I tried 15/16" JB5 cylinders in a futile
effort to reduce rear force.
Booster: JB5 dual diaphragm
Master cylinder: same for both systems.

SPID label long gone.

I have a set of backing plates on the way that I hope are correct for JB5 rears. I intend to convert if they are correct.

[Ronno6]

Most all adjustable prop valves I have seen are 1/8" FPT.
Some are 3/8 x 24 inverted flare.
Both are designed for use with 3/16" brake line.
One must use adapters to fit to 1/4" brake lines.
Why are there NO valves for non-adapted 1/4" line usage?

The backing plates were not correct.
Looks like an adjustable prop valve is in my future.....

[Fast68Chevy]

1/4" rear line is factory. until you get to rear hose  then its back to 3/16"

factory combo valve is for disc/drum.  there were diffeetn factory oem configs though  sometimes two front lines and sometimes just one.  early ones were cast iron like on my '75  and later ones were steel, 

[Ronno6]

How can I tell a disc/drum CV from a disc/disc unit?

How would a disc/disc CV perform in a disc/drum system?

[VileZambonie]

disc/disc uses a proportioning valve. disc/drum uses a combination valve which includes a metering valve. You would not use a standard prop valve in a disc/drum system.

[Ronno6]

disc/disc uses a proportioning valve. disc/drum uses a combination valve which includes a metering valve. You would not use a standard prop valve in a disc/drum system.

I understand that.
Are they visually different?
How would a disc/disc valve operate in a disc/drum system, if installed in error??