Rear disks (more proportioning valve talk)

[dyeager535]

So after years of hearing about this mod, I FINALLY stumbled upon it. (and in retrospect, I need to mention that maybe this is truck-worthy for our rear disks, which is why I posted it)

kdrolts post is pretty good and detailed. [FONT=Verdana, Arial]J.Wutz HeavyChevy mentions [/FONT]donor years of 9C1:
http://impalassforum.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=23;t=000081#000007

Here's a guy offering the "stealth bolt":
http://www.clearimageautomotive.com/Brakes.htm

Note that the first link mentions multiple changes, not just the bolt.

Link to the original proportioning valve mod page:
http://web.archive.org/web/20010419234304/www.b-body.net/Technical/Brakes/ComboValveMod/index.html
none of the pics work for me though.

Discuss. (lol)

 

[PsRumors]

Is this applicable to our applications?

 

[83ZZ502_Jimmy]

Wow, having had an Impala SS, I never made any corellation with this mod to our trucks... I assume it could work..

I always knew of it, but never made the jump...

John

 

[dyeager535]

I *think* it would. However, they mention specific years for the donor pieces, so obviously not all proportioning valves are made the same. (well, use the same parts anyways)

Some working pictures of the Impala proportioning stuff would help, if you could take a truck one apart and compare.

 

[bp71k5]

This actually sounds backwards. Here's the quote,

Because of functional differences between drum and disk brakes, cars with rear drums use a device which delays the onset of braking to the drums as well as reducing the pressure to the rear calipers.

On a stock truck, the "delay" is actually on the front discs to allow the rear pads to have enough time to start grabbing.

Also, if you want to swap discs on the rear, you generally need to "reduce" the pressure going to the rear discs, not increase it. Otherwise the rears lockup before the front.

 

[dyeager535]

Is there actually delay to the fronts? That would cause some extremely poor braking behavior, especially if the weight bias of the trucks is worse than a car. (no idea, anyone?) Locking up, coming loose in the rear, all would be a result of brake apply in the rear first. This would be extremely obvious on ice/snow as the rear WOULD come around instantly, and I've never experienced that. (It's happened, but not because of the brakes)

I dunno about the last caliper comment, since it starts off talking disk/drum, that may just be a typo. I'm not the author, can't interpret the intent there.

 

[smalltruckbigcid]

Is there actually delay to the fronts? That would cause some extremely poor braking behavior, especially if the weight bias of the trucks is worse than a car. (no idea, anyone?) Locking up, coming loose in the rear, all would be a result of brake apply in the rear first. This would be extremely obvious on ice/snow as the rear WOULD come around instantly, and I've never experienced that. (It's happened to me, but I wasn't using the brakes)

I dunno about the last caliper comment, since it starts off talking disk/drum, that may just be a typo. I'm not the author, can't interpret the intent there.

On the weight bias I *think* you would have to figure that based on a percentage. Then divide that by conact area on the ground to get a weight per square inch. Something like my truck has 10lbs of ground pressure in the front and the car has 9lbs. And then repeat for the rear. I don't know if that makes it too complicated or adds an extra step.

I think most trucks are about a 65-35 front to rear empty. Most cars are on the line of 55-45 or close to that in a 60-40 configuration. Because rear axle weight can change on a truck by the load many of them have a weight sensing prop valve above the rear axle. Thats something many of us toss...
George

 

[bp71k5]

Is there actually delay to the fronts?

I think I said that wrong.

Since the rear brake pistons have much more travel to overcome before they start slowing the truck, there is a pressure valve on the front end of the combo valve that only allows pressure to the front lines when there is some built up on the rear. This allows the front and rear to start slowing down closer together. Technically, even in this setup the front calipers start slowing the truck before the rear.

When you swap in rear discs, you don't need the "delay" on the front anymore so that is what a new combo prop valve does. It reduces the pressure to the rear and gets rid of the valve for the front.

Here's a ck5 link to a tech article that explains it. (Look towards the end)
http://coloradok5.com/atrondiskbrakes.shtml

 

[Blue85]

Also, if you want to swap discs on the rear, you generally need to "reduce" the pressure going to the rear discs, not increase it. Otherwise the rears lockup before the front.

The pressure you need depends on the rear calipers you select. In general, a drum brake needs less pressure than a disc because it is "positive gain" - as the shoes contact the drum they are pulled further out by the friction. The opposite is true for discs (hence no springs on disc brakes). But sometimes we choose large brakes out back in the conversion. Some 4W disc cars have almost no proportioning because this is taken care of by piston sizing.

But back onto the topic, I have been meaning to pick up some residual pressure valves to see the effect on pedal travel. I have been aware of this "bolt mod" for years, but have never heard confirmation that it works on C/K trucks.

 

[dyeager535]

Actually, the tech article mentioned above shows the C/K valve disassembled, if the parts look the same, no reason I can think of that the mods wouldn't work.

Except of course that the Impala's probably have a specific disk/disk master, but if you put that P-Van master that was found to fit on our trucks, plus this mod to the valve, I'd think you'd be set.

As mentioned in the tech article, it seems with the truck disk swaps 5 people will put disk brakes on their trucks, and there will be 5 different results. Too many variations in these trucks (master cylinders, weight, etc) for there to be a one size fits all answer I suppose.

 

[ntsqd]

I'm going to have to dig into the RW-ABS on my '91. Tempting to dump it and the combo valve when I get around to doing the rear discs.

I've been aware of the "delay valve" for years. When GM first put front discs on cars they had a separate delay valve. MISF removed that valve from his '69 Gran Prix (disc/drum) with no apparent change in braking. What I hadn't realized until just recently was that GM had moved the delay function into the combo valve. I had erroneously thot that p-valves had replaced the delay valve.

Addressing the weight bias thing: There is the static weight bias and then there is the dynamic weight bias. Measuring the static bias only will tell you part of the story. Unfortunately the dynamic bias changes based on how fast you're decelerating. If you knew the front spring rate, how fast you're decelerating, and how much the front suspension compressed; then you could calculate the dynamic bias for that singular deceleration rate. I'd be surprised if folks outside of an OE even bother with this.

Also, having worked in racing brake design for a while I got curious about what kind of "Horsepower" the brakes developed. That 400 HP number mentioned in the first link is an easy target on a pass car. Can get to 3/4 of that with one caliper and rotor. Trucks are harder due to their large tires reducing the rotor RPM.