function of proportional valve

crabtruck · 08-17-2007, 03:54 PM

Saw this comment and wanted to hear what you all think of it...

"the rears have to come on before the front.
the proportional valve holds the fronts off for an instant so the rears grab almost the same time"

mrjohnwayne · 08-17-2007, 04:49 PM

hmmm... You know I've never delved into the workings of a proportioning valve, However I do not believe that statement to be true. I would think that they get pressure at the same time. The only function of that valve is to limit the amount of pressure going to the rear etc...

crabtruck · 08-17-2007, 08:00 PM

Guess I should have said up front that I hold a dim view of the statement and
because of the way it was delivered to an unsuspecting person asking for help.
I rarely if ever get truly angry but it tends to happen when I see the ignorant fleecing the the innocent.

If it's offered as an opinion or a possability then ok, I do a lot of guessing myself and I got no beef.
But mushy pedal caused by late timing in the valve as a common fact ?!? And you want back brakes biting early ?!?

I did "clip" that quote but the rest was more of the same thinking. I'm gonna go eat, helps me forget everything LOL
Probably be mine turn to say something stupid next.

scottie · 08-17-2007, 08:23 PM

that is a 100% true statement

75K30 · 08-17-2007, 10:30 PM

Yes guys it is a fairly true statement. However,,,,,, The entire braking theory is different for different types of brakes. In the case of the above, the statement "rear brakes are applied first" is true for a disc / drum
set-up. This is actually done by the "Metering valve" and not the prop. valve. This is all part of the combination valve which is a three part valve.
1.metering valve
2. pressure differential switch
3. prop valve.

The metering valve has to apply pressure to the rear drums first because the drums shoes are further away from the drum itself than the pad is to the rotor. Since it has to travel further, it must get pressure first.

PDS is just an alert to let you know that one of the two pressure systems has lost pressure. If the front to rear differential (not axle, difference in pressure from front brakes to rear) is too great, it will signal a brake indicator.

Prop Valve.
This just limits the amount of the pressure applied to the rear brakes regardless of type. Rear brakes for obvious reasons need less pressure, so the prop valve simply regulates pressure.

that is just a brief overview as I remember it. Has been a while, but I believe these to be very true statements. Matter of fact, I would bet a cold beer on it!!!!!!

crabtruck · 08-17-2007, 10:48 PM

ok scottie

crabtruck · 08-18-2007, 01:49 AM

In fairness, I did leave out that this was for a 94 K1500, same truck I've had for 13.5yrs. It was not about my truck nor in this forum. Combo's do vary a little true. And there are 4 valves in this one. So here's my take based on what chevy spec'd for it.

Properly adjusted shoes are very close to drum when released. The bores on the rear are tiny compare to front calipers. However required operational force is very different because of the return springs on rear. So it's not a case of volume and travel, it's a case of pressure. The metering valve takes care of this by withholding pressure to the fronts until enough is available to operate the backs at the same time. Any car/truck could need a proportioning valve because of the way braking acts on it to lighten the rear. This truck and many others has more need with disc/drum setup and about 5000# on front vs an ounce on back. Specifically it acts to compensate for the metering valve's advantage when it limits the rear preasure or the rears would be perpetually locking up with light or no load. Light braking in the rear from the start (no delay) works to keep truck from pinning the nose and spinning the rear around, similiar to the idea of brakes on a trailer and how you fight whip. Simply put, the metering valve makes sure they work at the same time, and the proportioning valve makes sure the rears don't over do it.

That would mean both lines of the statement are flat wrong.
Wonder how much of that comes from folks calling the combo "the proportioning" valve?

75, you crack me up, you say it's fairly true, then correct the bejeebers out of it. LOL
Were you a poli-sci major? Do you owe me a beer or not? LOL

Scottie, you figured I was due up for saying something stupid next?

Now, has anybody seen the fourth valve?

Never mind.

75K30 · 08-18-2007, 09:45 AM

Just due to the way a shoe has to travel, It pivots one end, and does not move the entire shoe like the pad can move.
Some manually adjusted prop valve can and will allow the front to rear bias. This is typically used for racing applications, and the Japanes have been using load sensing variable adjustments for years now. It was nothing more than a link tied to the rear axle to determine ride heigth and limit braking bias to the rear while empty, and allowed more when loaded.
Lifted toyota trucks suffered from poor brakes as a result of this. That linkage was so far away, the truck thought it was way beyond empty and the rear brakes rarely worked at all.
Should also mention "residual pressure". There is always pressure left in the line, and disk brakes require a different residual pressure than drums do. In a drum / disc set up, the drum brakes must react first, (still has to trave futher) so the resdidual pressures must be maintained at different rates to properly activate the brakes. (this is where rear drums are actually activated first).

Jessie · 03-07-2008, 07:41 PM

I had to replace one one time on 1978 ford ltd. With no prior experience with a proportioning valve the guy at the ford place here in short terms told me that the valve equalizes pressure between the front and rear so as to keep you from being ejected thru the windshield upon applying brakes. He must have known what he was talking about because the exact problem at that time was premature ejection thru the windshield at only the slightest touch of the brake.

08-17-2007, 03:54 PM	#1
crabtruck Senior Member Join Date: Jul 2007 Posts: 1,464	function of proportional valve Saw this comment and wanted to hear what you all think of it... "the rears have to come on before the front. the proportional valve holds the fronts off for an instant so the rears grab almost the same time"

08-17-2007, 04:49 PM	#2
mrjohnwayne Moderator Join Date: Apr 2005 Location: Texas Posts: 3,104	hmmm... You know I've never delved into the workings of a proportioning valve, However I do not believe that statement to be true. I would think that they get pressure at the same time. The only function of that valve is to limit the amount of pressure going to the rear etc... __________________ John Wayne 2000 F-350 7.3L Powerstroke 4x4

08-17-2007, 08:23 PM	#4
scottie Senior Member Join Date: Jan 2006 Location: South Florida (Palm Beach), originally from south of BOSTON Posts: 644	that is a 100% true statement __________________ im a straight shooter with upper management written all over me

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08-17-2007, 08:00 PM	#3
crabtruck Senior Member Join Date: Jul 2007 Posts: 1,464	Guess I should have said up front that I hold a dim view of the statement and because of the way it was delivered to an unsuspecting person asking for help. I rarely if ever get truly angry but it tends to happen when I see the ignorant fleecing the the innocent. If it's offered as an opinion or a possability then ok, I do a lot of guessing myself and I got no beef. But mushy pedal caused by late timing in the valve as a common fact ?!? And you want back brakes biting early ?!? I did "clip" that quote but the rest was more of the same thinking. I'm gonna go eat, helps me forget everything LOL Probably be mine turn to say something stupid next.

08-17-2007, 10:30 PM	#5
75K30 Senior Member Join Date: Jul 2006 Posts: 2,518	Yes guys it is a fairly true statement. However,,,,,, The entire braking theory is different for different types of brakes. In the case of the above, the statement "rear brakes are applied first" is true for a disc / drum set-up. This is actually done by the "Metering valve" and not the prop. valve. This is all part of the combination valve which is a three part valve. 1.metering valve 2. pressure differential switch 3. prop valve. The metering valve has to apply pressure to the rear drums first because the drums shoes are further away from the drum itself than the pad is to the rotor. Since it has to travel further, it must get pressure first. PDS is just an alert to let you know that one of the two pressure systems has lost pressure. If the front to rear differential (not axle, difference in pressure from front brakes to rear) is too great, it will signal a brake indicator. Prop Valve. This just limits the amount of the pressure applied to the rear brakes regardless of type. Rear brakes for obvious reasons need less pressure, so the prop valve simply regulates pressure. that is just a brief overview as I remember it. Has been a while, but I believe these to be very true statements. Matter of fact, I would bet a cold beer on it!!!!!!

08-17-2007, 10:48 PM	#6
crabtruck Senior Member Join Date: Jul 2007 Posts: 1,464	ok scottie

08-18-2007, 01:49 AM	#7
crabtruck Senior Member Join Date: Jul 2007 Posts: 1,464	In fairness, I did leave out that this was for a 94 K1500, same truck I've had for 13.5yrs. It was not about my truck nor in this forum. Combo's do vary a little true. And there are 4 valves in this one. So here's my take based on what chevy spec'd for it. Properly adjusted shoes are very close to drum when released. The bores on the rear are tiny compare to front calipers. However required operational force is very different because of the return springs on rear. So it's not a case of volume and travel, it's a case of pressure. The metering valve takes care of this by withholding pressure to the fronts until enough is available to operate the backs at the same time. Any car/truck could need a proportioning valve because of the way braking acts on it to lighten the rear. This truck and many others has more need with disc/drum setup and about 5000# on front vs an ounce on back. Specifically it acts to compensate for the metering valve's advantage when it limits the rear preasure or the rears would be perpetually locking up with light or no load. Light braking in the rear from the start (no delay) works to keep truck from pinning the nose and spinning the rear around, similiar to the idea of brakes on a trailer and how you fight whip. Simply put, the metering valve makes sure they work at the same time, and the proportioning valve makes sure the rears don't over do it. That would mean both lines of the statement are flat wrong. Wonder how much of that comes from folks calling the combo "the proportioning" valve? 75, you crack me up, you say it's fairly true, then correct the bejeebers out of it. LOL Were you a poli-sci major? Do you owe me a beer or not? LOL Scottie, you figured I was due up for saying something stupid next? Now, has anybody seen the fourth valve? Never mind.

08-18-2007, 09:45 AM	#8
75K30 Senior Member Join Date: Jul 2006 Posts: 2,518	Just due to the way a shoe has to travel, It pivots one end, and does not move the entire shoe like the pad can move. Some manually adjusted prop valve can and will allow the front to rear bias. This is typically used for racing applications, and the Japanes have been using load sensing variable adjustments for years now. It was nothing more than a link tied to the rear axle to determine ride heigth and limit braking bias to the rear while empty, and allowed more when loaded. Lifted toyota trucks suffered from poor brakes as a result of this. That linkage was so far away, the truck thought it was way beyond empty and the rear brakes rarely worked at all. Should also mention "residual pressure". There is always pressure left in the line, and disk brakes require a different residual pressure than drums do. In a drum / disc set up, the drum brakes must react first, (still has to trave futher) so the resdidual pressures must be maintained at different rates to properly activate the brakes. (this is where rear drums are actually activated first).

03-07-2008, 07:41 PM	#9
Jessie Senior Member Join Date: Feb 2008 Posts: 93	I had to replace one one time on 1978 ford ltd. With no prior experience with a proportioning valve the guy at the ford place here in short terms told me that the valve equalizes pressure between the front and rear so as to keep you from being ejected thru the windshield upon applying brakes. He must have known what he was talking about because the exact problem at that time was premature ejection thru the windshield at only the slightest touch of the brake.