: 93-96 FW Combination Valve tag?



N0DIH
01-25-06, 12:59 AM
Does anyone have the original little tag on the brake combination valve?

I am pretty sure mine is JY, I'll double check. AC Delco.co. shows P/N for All is 172-2104, except J55 is 172-2113. What is the coded difference? Yet mine has a different code than what they list.

ACDelco.com shows the Impala SS, Caprice and Buick Roadmaster has LJ. AC Delco P/N 172-2104. That is a drum and a disc brake car. Interesting that they share combination valves.

Everyone always preaches that the B/D body cars have 95% front braking and 5%. Still trying to figure out how that was ever determined. After my bad experience with brakes on my FW, I find that very hard to believe. There is definately a lot of power in those rear brakes.

Why am I wondering? Being I see a difference in the tags from car to car, I am wondering if there is a Towing Package difference, which might account for the braking power I feel in this car.

Katshot
01-25-06, 05:45 AM
I'll take a look on my car. I think the 95/5 thing is just a ratio pulled out of the air. I think the actual ratio is closer to 70/30 as I recall but I'm not sure.

Katshot
01-25-06, 08:09 AM
Mine's an "LJ"

N0DIH
01-25-06, 10:38 AM
Katshot, is yours a FE1 or FE2 car originally?

Katshot
01-25-06, 11:43 AM
It's FE1.

BCs71
01-25-06, 12:49 PM
I think the misnomer of the 95/5 braking thing comes from the Impala SS and Caprice 9C1 vehicles with rear disc brakes. There were given the same exact combi proportioning valve as the rear drum cars (all other B/D bodies) for which the combi valve was designed for....

The combi valve was meant for rear drum cars, so therefore the rear disc cars with the same combi valve have a tendency to nosedive under hard braking since the rear discs are getting much less braking duty than they were intended for.

In the Impala SS world the guys with rear discs will modify the combi valve with a new bolt in the front that brings the proportions back to 70/30 like it should be.

N0DIH
01-25-06, 01:04 PM
Mine doesn't nosedive much, and under hard braking it doesn't take much to kick in the ABS on the back, but enough that if there was significant weight there like a trailer, that it would really do well.

I'll have my parts guy or Dal check on the PN's, I am curious. It almost feels like the valve I have might be best on the Impala SS.

I know the 1979-1981 Firebirds (yes, a base bird could) w/4 wheel disc had a different master cyl AND combi valve. As well as a different position on the brake pedal lever to allow for more leverage. They did a lot of changes compared to the std brakes (11" front, 9"x2" rear). I always wanted to go discs on it, I might do the Impala brakes on it, that would be a good combo.

Katshot
01-25-06, 01:20 PM
So you're saying that drum brakes are more efficient at converting pedal pressure into stopping power? Interesting since everyone seems to think just the opposite, hence the disc conversions?
Also, don't forget that the "combi" valve is just that. It combines both proportioning and metering. So if you only seem to notice the problem under heavy braking, the proportioning section of the valve could be the problem. It's really a delicate balancing act between the master cylinder and combi valve so playing with them can throw the whole thing out of whack.

ocjmakaveli
01-25-06, 02:13 PM
The deal as I read it was some impala ss guy went on a brake dyno with a stock braking system all 4 wheel disc and he found out it was 90-10 basically the rear discs weren't being used much and they were lasting way too long.

So he tried the stealth bolt mod and then the metering mod.Which are both great with 4 wheel disc brakes you really feel a big difference after.

What I remember reading is drums don't need much brake fluid to work because of the small size of the wheel cylinders compared to a caliper so 90-10 works well for drums but with discs its lacking.

Drums are efficient but upto a certain point and the maintenance of drums is an issue for people especially me if I can knock a few things of maintenance off my lists then I will go for it.

There's a reason why cars went from early 4 wheel drums to late 4 wheel discs....practicality.

When my drums were properly adjusted the brakes did feel great but after a few weeks they would need adjusting which was annoying to me. With the rear "dummy" disc brakes they just require good maintenance when you replace them otherwise they are fine all year round.

I asked once if the bolt mods could be done on disc-drums stock setup but i was told it would most likly cause "constant and frequent rear lock-ups because of the extra fluid being thrown into the rear".

DaveSmed
01-25-06, 02:21 PM
Metering mod? Did they still have metering valves on the 4 wheel disk cars? Correct me if i'm wrong but isn't the metering valve needed only for disk/drum setups?

Katshot
01-25-06, 02:53 PM
Metering mod? Did they still have metering valves on the 4 wheel disk cars? Correct me if i'm wrong but isn't the metering valve needed only for disk/drum setups?

Matter of fact, you are correct Dave. The metering valve is only there to delay disc brake actuation until AFTER the rear shoes have been actuated. On 4-wheel disc setups, metering is not needed.

N0DIH
01-25-06, 03:59 PM
Actually, yes, I am saying that. The drums brakes are a self energizing design, where discs are not. The more pedal you put into a drum brake, the more the primary (short) shoe gets forced into the drum, pushing around the adjuster to the rear shoe (long, aka, the secondary shoe) which also has the exact opposite force on it from the wheel cyl, and with the turning motion of the drum, creates an ever increasing force to slow the car. It really is a killer design when you look into it. The design of it actually multiplies force. Ever notice a manual brake disc car? Rare, if ever made (Corvette maybe?). Find a manual brake drum car? Yes, very common at one time. And still popular with drag racing.

The proportioning valve #1 task is to delay the rear brakes from working until the front get with the program, as the drums WILL lock up if energized too early. The initial force spike when you nail the brakes will lock them suckers up fast. There used to be a residual pressure valve in the master cyl, but I have heard new systems have designed it out. It helped keep some preload in the rear lines.

It does keep it in balance though, so as more and more force is on the pedal, the proportioning is proper. IIRC, the fronts are pass through, and the rears are cushioned and the pressure on the rear is then allowed to go to 100%.

Yes, an out of whack system (like mine was, which is why I had some much problems) is a pain to drive in, especially in the snow!

Discs are superior to drums for fade, not necessarily for power, size for size, as the width makes a huge difference. There is a LOT of surface area on drums still, and a lot of friction material making drums have little effort for the force needed to stop you compared to discs. Which is why the 79-81 F Body's had a different lever postion on the brake pedal, to increase the forces with the lever, as in the drum brake position, it wasn't enough.

How many semi's see discs? None that I have. Cost isn't a big issue compared to power. 10 wheels with 18"X8" drums is awesome! Or a Cadillac Limo with the 11.5"x2.75". Or the 1976 Olds with 12"x2", or the rarer 11"x4" J55 brakes. Powerful the force is....

You can read up on that part in your favorite GM FSM, it explains it in good detail, or here: http://auto.howstuffworks.com/drum-brake1.htm (for drums)

http://auto.howstuffworks.com/master-brake2.htm (for the proportioning valve)


So you're saying that drum brakes are more efficient at converting pedal pressure into stopping power? Interesting since everyone seems to think just the opposite, hence the disc conversions?
Also, don't forget that the "combi" valve is just that. It combines both proportioning and metering. So if you only seem to notice the problem under heavy braking, the proportioning section of the valve could be the problem. It's really a delicate balancing act between the master cylinder and combi valve so playing with them can throw the whole thing out of whack.

Katshot
01-25-06, 05:15 PM
Actually, yes, I am saying that. The drums brakes are a self energizing design, where discs are not. The more pedal you put into a drum brake, the more the primary (short) shoe gets forced into the drum, pushing around the adjuster to the rear shoe (long, aka, the secondary shoe) which also has the exact opposite force on it from the wheel cyl, and with the turning motion of the drum, creates an ever increasing force to slow the car. It really is a killer design when you look into it. The design of it actually multiplies force. Ever notice a manual brake disc car? Rare, if ever made (Corvette maybe?). Find a manual brake drum car? Yes, very common at one time. And still popular with drag racing.

The proportioning valve #1 task is to delay the rear brakes from working until the front get with the program, as the drums WILL lock up if energized too early. The initial force spike when you nail the brakes will lock them suckers up fast. There used to be a residual pressure valve in the master cyl, but I have heard new systems have designed it out. It helped keep some preload in the rear lines.

It does keep it in balance though, so as more and more force is on the pedal, the proportioning is proper. IIRC, the fronts are pass through, and the rears are cushioned and the pressure on the rear is then allowed to go to 100%.

Yes, an out of whack system (like mine was, which is why I had some much problems) is a pain to drive in, especially in the snow!

Discs are superior to drums for fade, not necessarily for power, size for size, as the width makes a huge difference. There is a LOT of surface area on drums still, and a lot of friction material making drums have little effort for the force needed to stop you compared to discs. Which is why the 79-81 F Body's had a different lever postion on the brake pedal, to increase the forces with the lever, as in the drum brake position, it wasn't enough.

How many semi's see discs? None that I have. Cost isn't a big issue compared to power. 10 wheels with 18"X8" drums is awesome! Or a Cadillac Limo with the 11.5"x2.75". Or the 1976 Olds with 12"x2", or the rarer 11"x4" J55 brakes. Powerful the force is....

You can read up on that part in your favorite GM FSM, it explains it in good detail, or here: http://auto.howstuffworks.com/drum-brake1.htm (for drums)

http://auto.howstuffworks.com/master-brake2.htm (for the proportioning valve)

Ahh, I think you're missing a couple points. First of all, in a drum brake, the hydraulic pressure doesn't move only the primary shoe, it moves BOTH. There ARE some older applications that used one-sided wheel cylinders but I haven't seen any in years. The Proportioning valve does NOT do what you said, that's the function of the Metering valve. The proportioning valve is for maintaining proper front/rear bias during heavy braking. The big pluses for disc brakes are, fewer parts, greater performance in various conditions, better heat disipation, faster actuation, greater stopping force, and of course ease of packaging. And finally, most new over the road trucks and busses actually DO use disc brakes nowadays. When I was still in the business, they were finally becoming wide spread in the industry but were still rather expensive.

N0DIH
01-25-06, 06:16 PM
The wedge action (aka, servo action as GM likes to call it) is killer for power application. I agree with the plus's, but not all of them. Often the plus of one doesn't always make up for the minus of the other.

Fewer parts? Yes. Stays in constant adjustment, yes. Faster application? Maybe, that would be hard to prove, drums are quick, real quick to release being it has springs to return the shoes, and they are excellent for drag racing due to the quick relelase off the line. Greater overall performance? probably, but not braking force for 1 application. Repeated, yes, discs are better. Better heat diss, yes. Definately. But then again, Herb Adams has some fixes for that to make drums perform on par in every catagory to discs, in racing too. Greater stopping force? Size for size, I would like to see some real world tests on that. I wouldn't be forking out dough on either side quite eagerly. A properly setup drum is one good brake.

Basically you can get tons more power in a drum brake of same diameter than a disc, and if you need more, just make it wider. No, you can't come along as say a 11in disc is better than an 11in drum. I'll pull out the Olds 11"x4" and we will have fun comparing. There will be no comparisson. The drums will kill the discs for power.

They don't compare well. You can pack a lot more power in a compact space with drums than discs, always have, always will. The heat rejection of discs is a key strong point that drums don't do well at stock, but can be modded very easily to. I'll see if I can dig up the mods on it. They are questionable on the street to go all out though.

I did have it backwards on the front vs rear starting first, now that I think about it, you can feel the rear start to stop you just before the front start to kick in.

I noted how the drum brake works, following it CCW, read through it again.

"The more pedal you put into a drum brake, the more the primary (short) shoe gets forced into the drum, pushing around the adjuster to the rear shoe (long, aka, the secondary shoe) which also has the exact opposite force on it from the wheel cyl, and with the turning motion of the drum, creates an ever increasing force to slow the car." Maybe I should have worded it a little better I guess.

I think I saw a picture of a dual wheel cyl drum brake that doesn't have the duo servo action, yuk.


Ahh, I think you're missing a couple points. First of all, in a drum brake, the hydraulic pressure doesn't move only the primary shoe, it moves BOTH. There ARE some older applications that used one-sided wheel cylinders but I haven't seen any in years. The Proportioning valve does NOT do what you said, that's the function of the Metering valve. The proportioning valve is for maintaining proper front/rear bias during heavy braking. The big pluses for disc brakes are, fewer parts, greater performance in various conditions, better heat disipation, faster actuation, greater stopping force, and of course ease of packaging. And finally, most new over the road trucks and busses actually DO use disc brakes nowadays. When I was still in the business, they were finally becoming wide spread in the industry but were still rather expensive.

Katshot
01-26-06, 08:37 AM
Disc brakes ARE faster to apply. That's why there is the need for a metering valve. To DELAY the activation of the disc brakes until the drum brakes are in action. Releasing? I can see how the drum brakes "could" release faster but, that's NOT why races like to use them. The reason is the most basic of reasons. less drag. Drum brakes are NOT in constant contact with the drum, whereas disc brakes ARE in constan contact with the rotor/disc. This is the main reason why you need a metering valve on disc/drum combination vehicles. It takes time for the shoes to move out of their resting place and contact the drum. As for which "...can pack a lot more power in a compact space...", I think you have it backwards. IMO, a disc brake setup can provide greater torque in the same size package. Just the physics of the 2 applications don't add up. Drum brakes are only applying friction to one surface and therefore cannot apply as much ultimate force as the "clamping force" applied by a disc brake setup. Plus, if you look at the hydraulic forces alone, the surface area of the caliper piston is FAR greater than that of your typical wheel cylinder.

N0DIH
01-27-06, 12:22 AM
I can see discs maybe being faster to apply initially (if allowed to), but I think what I mean is the time between is slight, and the power application of drums comes on faster.

With discs, it is a 1:1 force application for braking power, for drums, it is like 4:1 (guess), as in more force is generated than applied due to the force of the drum moving forward multiplying it.

I am not 100% convinced that the discs need to be delayed. If the front brakes do 70%+ of the braking force, why delay it? Kick it in ASAP. I guess to have them effectively start braking at the same time.

Sorry, I got my license when drums in the front were long gone, but was there combination valves (well, separate valves then) on the 4 wheel drum cars?

Ok, on this link http://www.inlinetube.com/pro_valves.htm we have them saying the hold off REAR application to get the fronts to apply first.

On this link http://www.mpbrakes.com/mpfaqvalving.htm it says that the rear drums apply slower than front discs. Contrary to the other link.

This link seems to agree with your position, http://hotrod.com/featuredvehicles/46118/.

I guess 2 out of 3 agree with you on the need for the delay.

DaveSmed
01-27-06, 12:52 AM
That first website contradicts itself if you scroll down to the "68-69 Hold off valve" heading. I have always heard that metering valves are to delay front brake activation to allow the rears to contact, so that in a panic stop, the front brakes don't get %100 of the load and possibly lock up in a panic situation. Thus why disk/drum cars are the only vehicles that need such a valve. As far as 4 wheel drum cars, mine is an oddball in that I see NO valves at all (though I haven't looked all that hard either). My friends 67 C10 does have a proportioning valve/pressure differental switch though on his 4 wheel drum setup though. Also you mentioned a residual pressure check valve earlier? That is to help keep the rubber cups expanded in certain applications to prevent air from being drawn into the system. You usually see cup expanders though, those metal domes on the springs inside the wheel cylinder. they do the same thing. One thing I haven't seen mentioned yet as an advantage for disk brakes is weight. Big drums are heavy unsprung weight, that turns into more rotating inerta for the brakes to stop. Another advantage is dIsk brakes play a lot nicer with ABS, that self energizing "wedging" action is difficult to control as quickly and accurately as a simple brake caliper.