Proportion valve

[Joseph richardson]

I did do some searching for this here, but the refernced part numbers only came up with 2 results, and neither applied to my question that I could tell.

In the process of updating my brakes all around, I ran into this TSB that applies to the 8600# GVW Burbs with RPO code C6P. Here is the link to the TSB.

TSB #99-05-24-001A applies to several pickups, burbs, and vans from 1992-2000.

This doesn't apply to me, I have the 7600# GVWR (RPO C6A), but I do have a question as it relates to the proportion valve part #12548265. Supposedly, in the bigger Suburban with the 14 bolt rear, this prop valve helps in fixing the high front brake wear/truck pulling when braking issue by changing the proportions from 85F/15R to 80F/20R.

This topic is discussed, I think pretty well over at The Truck Stop.

Like every suburban owner out there knows, they are scary to stop. I haven't been able to find the proportions information for my stock prop valve. Would anyone know what the prop valve proportions are on my Burb? I have the JB6 brakes

Assuming mine is 85F/15R proportions, would it be possible to install this valve to have the slightly bigger PSI going to the rear? My truck does pull a bit when breaking, and the rotors that are about a year old are pretty much toast and need replacing. Seems 12-18 months is pretty quick to wear down new rotors.

I am doing all of the normal stuff, new pads/shoes, rotors/drums, flex brake lines (cant go to SS atm, so going to new rubber lines), new wheel cylinders (i think I went to the 1 1/16" cylinders), rears got all new hardware, just installed the NBS MC and new power booster last night. Debating on doing the calipers now or waiting until I do the front brakes again later, although I do have the new rotors.

 

[Ken K]

With your description of the newer proportioning valve in values of percentage of brake application. In theory, this would be (Old) 85% stopping done by the front & 15% by rear. Going to the updated valve should make the desired change that will result in 80% stopping by the front brakes & 20% by the rear. Applying "Less Hold Off" before applying the rear brakes, should transfer more stopping power to the rear and less to the fronts. It makes sense and I don't think there would be any downside. You still have the original proportioning valve to change back if the desire effects are not satisfactory.
The rear brakes, being shoes, have several service issues to make sure are correct and frequently over looked. First, they wear into the baking plate, creating grooves the shoes could catch on during application movement. This issue is corrected with an angle grinder lightly dressing these pads to made smooth. There are 3 on primary and 3 on secondary side and usually have small dimples to hold lubricant. This is #2 overlooked issue. Using a high temp brake lube (ACDelco / Bendix / other) containing PTFE and applied onto clean dry pads with small 7/16 drop (10m) should be enough. I apply a little at the top anchor where the shoes pivot, but just for noise.
Using the larger wheel cylinder will assist the amount of force by hydraulic advantage, resulting in more applied force. My only concern would be the amount of fluid required to apply, but I think you should be ok. Otherwise, the pedal travel will increase a little.
Interesting issue. I am just picky on brakes being done right, but in this case, you're looking for improvement and less wear on the front pads and rotors.
I would guess there is no ABS, so the change is straight forward. Using a ceramic % compound in the front pads might help, but I'm sure metallic is the first listing. Remember, pads like "Thermo-Quite" are softer, thus less noise. Using pads that are certified friction using dual dynamic dyno electronically calculated, like those tested as if on a real vehicle weight transfer as tested by "Greenling Labs" in MI. These are found in the ACDelco line as D3EA on the box.
Hold-off pressure is in the proportioning valve, but also the internal valve moves to the low pressure side if a failure occurs and turns on the light. Residual line pressure, is 10 psi rear and 2 psi front disc. (Front disc may have no residual on many models) The device that controls this, is a "Duck-Billed" shaped rubber insert at the master cylinder, located behind the inverted flare seat. Service manuals show inserting a screw into the seat, then pulling out to remove. No one does this in shop anymore and doubt parts are even sold. But just saying this is where it's hiding. The 10 psi keeps the wheel cylinder cups inflated to keep moisture / air from entering and reduces pedal travel.

You're research into this newer proportioning valve with the specs of 80F/20R maybe the best answer for your'e problem.

I maybe in the the "TMI" zone with a long post, but like I said, I am picky about brakes and more knowledge than you need is better than too little.

Hope this helps, but never tackled this exact issue in 30+ in the shop.
ASE Master Tech since 1978 / retired.

 

[Joseph richardson]

I do have ABS system. I am aware that there is a big chance that if I do this I will need access to a scan tool to bleed the ABS module of air. I have an acquaintance with the tool who usually will do this things for me at a pretty good discount. He helped me set timing after I had to replace the intake manifold gasket last weekend.

I'm honestly kind of surprised that this part number has only come up 2 other times here, with a lot of people trying to improve their rear brakes. In theory, this might go a long way towards that goal and be an actual GM part # going in.