Part # is 339780
Description is "DISTR ASM"

List is $79.69 and I paid $59.77.

Main body is identical to original. Switch connector is also an exact match. The rear piece (part leaking on my old one) that screws into the main body is the same except for about an 1/8" band machined around the diameter. Thought that might be worth mentioning for the purists.

 

OK, sorry for the slow response but I tried to dig up some of our old K/H Proportioning valve usage charts. Farthest back I could find was 78’. Anyway, here’s what I pieced together: (1) Any brake valve that is mounted horizontally with 2 bolts through the side of the valve to a bracket welded to the frame below the master cylinder is a combination Differential AND Proportioning valve. For those that need to know, the proportioning valve has a 500 PSI knee point with a .41 slope. The valve will have the ID code “RR” stamped on the side of the body just below the switch. I believe this is correct for 72’-82’ MY (for sure 78’-82’).
(2). Any valve that is mounted vertically and is staked to a bracket, which is bolted to the top of the frame (with one bolt) below the alternator, is just a differential valve NO proportioning. Believed to be true for 68’-71’.
Just to clarify, the “differential” valve is plumbed across the two halves of the master cylinder and senses a lack of pressure on one side or the other and turns the RED brake light on the dash in the event of pressure loss. The proportioning valve reduces rear brake pressure above a predetermined pressure point to avoid premature rear wheel lock up. Note that if you remove the prop valve you may experience rear wheel lockup during high decel stops. If the rears lock before the fronts, the vehicle will spin. For 68’-71’ I believe GM may have tried to control rear lockup with either different lining material front-to-rear or smaller rear caliper piston diameter.
Hope this helps more than confuses.

 

OK, sorry for the slow response but I tried to dig up some of our old K/H Proportioning valve usage charts. Farthest back I could find was 78’. Anyway, here’s what I pieced together: (1) Any brake valve that is mounted horizontally with 2 bolts through the side of the valve to a bracket welded to the frame below the master cylinder is a combination Differential AND Proportioning valve. For those that need to know, the proportioning valve has a 500 PSI knee point with a .41 slope. The valve will have the ID code “RR” stamped on the side of the body just below the switch. I believe this is correct for 75’-82’ MY (Revised 5/15/05). (2) Any valve that is mounted vertically and is staked to a bracket, which is bolted to the top of the frame (with one bolt) below the alternator, is just a differential valve NO proportioning. Believed to be true for 68’-74’ (revised 5/15/05).
Just to clarify, the “differential” valve is plumbed across the two halves of the master cylinder and senses a lack of pressure on one side or the other and turns the RED brake light on the dash in the event of pressure loss. The proportioning valve reduces rear brake pressure above a predetermined pressure point to avoid premature rear wheel lock up. Note that if you remove the prop valve you may experience rear wheel lockup during high decel stops. If the rears lock before the fronts, the vehicle will spin. For 68’-74’ I believe GM may have tried to control rear lockup with either different lining material front-to-rear or smaller rear caliper piston diameter. Also, Uncle Sam got deeper involved in stopping distance requirements and the prop valve was required to keep the car in a 10' wide lane during a panic stop. (Note, this post has been updated based on input from others)
Hope this helps more than confuses.

 

I have the same problem on mine
how does this oring look?can i replace only the oring?
must i replace the Brake proportioning valve?

 

Great info!

The differential valve ONLY config was present in '73 and '74 cars also, not sure after that.

 

After giving it some more thought, I'm pretty sure that the internal o'rings can not be changed. That is because there is a pressed in bushing that guides the proportioning spool. Once it's pressed in, there's no getting it out. Since this was not a Kelsey Hayes valve I'm not 100% sure so I'll do some more research to confirm it.

 

Thanks for the legwork Duke!

My '69 just had the switch and I know in stock form the rears locked up before the fronts.

BTW, No one has ever recommended any different brake pads for the rear than the front...

 

To anyone who has removed the device. How did you do it? I am thinking about using two brass blocks but looking for ideas/pics of how others did this.

 

A very easy way to do it is make yourself an adaptor. Start with a nice piece of brass, I can provide it. Drill and tap 3 holes with NPT pipe thread, again I have the taps. Mount this brass block in the stock location and using a pipe to brake line flare adaptors connect the stock lines back up. I have these adaptors in the shop also.
For the back a simple inline adaptor works. Flare to flare union.

 

I've never noted any difference between rear calipers over the years either. The original pistons had phenolic heat pads screwed to the pistons, but I believe the rear calipers are interchangeable & dimensionally the same from 65 up.

 

I'm not an expert on these.... but I did disassemble a few & replace the o-ring(s).

 

ahhh yes. thanks. I guess I could also make a block about the same size and shape as the 'light switch' and drill for both the front and rear's in the same block(but not connected of course)..would make a nice clean install and no reforming of the existing brake lines.

 

I have my original block in the shop. Maybe it could be dismantled and plug the passages that are not needed. A brass plub slid into the passages and silver brazed into place would be easy.
You can have it.