AWD Braking Discussion: 80 Rear Proportioning Valve Adj and Service

[SUMOTOY]

80 Rear Proportioning Valve

Thats the first thing I've read from you that I completely understand. Now, how do I check to make sure my proportioning valve is actually proportioning?
Buck

Go here first (from the tech page on this forum):
http://www.safari4x4.com.au/80scool/tech/brake.html

After reading this, you also need to make sure that the sleeves and bushings are not bound (just about every toyota rear I've seen is after a couple years). You change the proportioning curve if the pivots are stuck. You have fixed proportioning if the pintle valve in the unit itself is stuck.

As we proceed understand my disclaimer:
Toyota considers brake proportioning valve to be a non service item. Replace with new only. If in doubt, buy new.

To service, remove the arm pivot point at the axle. Lube the pivot pin with grease, this allows angle 'A' (in the tech article above) to remain constant throughout the range of travel. Before installing the pivot, make sure the rear prop valve pintle pin moves freely. Remove the boot on the valve and move the arm up and down. If the pin doesn't move, you can either remove the assembly (Mr. T says not serviceable, replace) and work the pin with WD40 or PB blaster. I normally soak the pin for a couple days, and it usually will work itself free. The problem doing this is that if it's got too much rust/crud, it can compromise the pintle seal. Hence the reason for R&R, not service. It's not cheap, and I've had good luck reviving many toyota and similar rear prop valves. My disclaimer: If in doubt, buy a new prop valve. What works for me, won't necessarily work for you.

The FSM gives the Adustment spec of 90mm for length 'A' in the tech article above. That is for a brand new truck, bone stock. If you have access to brake pressure equipment, you can really do a lot of work with the rear valve to get the most out of the rear braking for your given setup. As Jack pointed out in another thread, you always want front brake proportioning to exceed the rear. Rear brakes locking before the fronts under heavy braking loads, is a major no-no in terms of vehicle control. With test equipment, you want to have less brake pressure rear at ALL rear prop travel range. I won't give my personal targets, as I will only recommend factory procedure here, anything else needs a detailed knowledge of your specific truck and it's mod deviation from stock.

I had good luck deleting the Prop valve alltogether on my 4R and plumbed in a Tilton 7 position valve. In the 4R system the overrun (brake pressure not thru the valve) went into the right front brake line. I haven't looked beyond lubing and unsticking my rear prop valve on my truck, so I don't have further information on this mod specific to the 80.

On a lifted truck, you will need to compensate for lift with a lift bracket attached to the rear axle. Most lift kits supply these IME. Edit: If not, you *need* to know that your proportioning valve is always going to yield less braking force regardless of load than what Mr. T engineered (read: you are reducing total brake capacity, and adding to front brake wear/load.)

This Device affects Rear Axle Brake Capacity in a non locked CDL (off) driving mode. Prior to ABS activation it dictates the proportion of the Rear Axle Brake Force, for a given truck weight distribution compared to Front Axle Brake Force. Rear proportioning is usually presented as a % of front axle brake force. As a rule 70-80% is the maximum R:F braking force.

This Device affects Total Brake Capacity in CDL (on) mode driving in all brake force applications.

HTH

ST

 

[Romer]

Good info Scott, thanks

Even I understood it.

 

[Doc]

On a lifted truck, you will need to compensate for lift with a lift bracket attached to the rear axle. Most lift kits supply these IME.

No kits supply them to my knowledge. However, you can achieve the desired amount of correction (for an OME kit) by lowering the rear prop. valve about .25".

There are not enough threads on the rear axle arm thingy to get enough adjustment there.

 

[alia176]

So, I wonder how much to lower the valve for a 4" or a 6" lift?

 

[SUMOTOY]

So, I wonder how much to lower the valve for a 4" or a 6" lift?

In my 4R, the NWOR 2in spring kit came with a longer axle pivot plate (relocate the rod attachment point at the axle leave the prop valve alone). You can lower the prop valve (decrease leverage angle) or relocate the pivot mount on the axle side (decrease leverage angle). I have *always* worked with the alxe/control arm side on all vehicles where I was changing rear proportioning.

I'm very surprised these spring kits don't include this piece. In raising COG, it's pretty key to make sure the brake proportioning valve operation is addressed/retained back to stock levels.

ST

 

[Romer]

They do not come with the OME kits and I don't think I have heard of them coming with any other kits.

 

[Doc]

So, I wonder how much to lower the valve for a 4" or a 6" lift?

IIRC the 'slot' is about .5" long, so by just a pure wild guess, I'd say lower the valve as far as it'll go.

 

[SUMOTOY]

IIRC the 'slot' is about .5" long, so by just a pure wild guess, I'd say lower the valve as far as it'll go.

It might be interesting to run some comparison values of rear brake pressures with a lifted truck. There is no way you can lift a truck 6, 4 or even 3 inches from stock and have anything close to the stock the prop valve settings without modifying the location of the valve or the location of the arm in relation to the axle.

ST

 

[landtank]

I made an extension piece out of 1/2" tubing bought at Home Depot. The inner Diamter allowed for tapping for 12mm bolt so it screwed onto the existing piece and was adjustable.

Low dollar and completely functionable

 

[landtank]

It might be interesting to run some comparison values of rear brake pressures with a lifted truck. There is no way you can lift a truck 6, 4 or even 3 inches from stock and have anything close to the stock the prop valve settings without modifying the location of the valve or the location of the arm in relation to the axle.

ST

If you read through the FSM it has you add a certain amount of weight when adjusting the valve.

At the stock spring rate this will change the valve by a predetermined amount.

Now with springs of larger capacity, they will respond at a slower rate with the same amount of weight.

I see this as an obstacle in getting back to the factory setting for all conditions from empty to fully loaded and getting the designed proportioning in the braking system for that range.

I chose to focus on a weighted situation which left me a little strong when unloaded. Not sure if was the right choice but the truck behaves as good as I can remember in an avoidance situation.

 

[Bearcat80]

IIRC the 'slot' is about .5" long, so by just a pure wild guess, I'd say lower the valve as far as it'll go.

In my experience that was too much. It was fine until it snowed and then the extra force to the rears caused the ABS to kick in way too soon. I adjusted the valve until the ABS acted more appropriately. Since the valve is the hinge point of a radius, it takes very little movement to compensate for the lift at the other end of the radius. Mine is about .25" lower than stock. Your results may vary, I would recommend the procedure in the FSM. I actually found that panic stops in a snowy parking lot are a good way to fine tune this process.

 

[landtank]

In my experience that was too much. It was fine until it snowed and then the extra force to the rears caused the ABS to kick in way too soon. I adjusted the valve until the ABS acted more appropriately. Since the valve is the hinge point of a radius, it takes very little movement to compensate for the lift at the other end of the radius. Mine is about .25" lower than stock. Your results may vary, I would recommend the procedure in the FSM. I actually found that panic stops in a snowy parking lot are a good way to fine tune this process.

I don't see the ABS coming on in a snowy condition as bad, it's doing it's job and you have good braking and control. However setup as you are if you needed to panic stop while heavily loaded the braking would not be there as the valve was set low to prevent the ABS from coming on in snow.

I don't see my ABS as being overly active in winter on my truck but I run a very aggressive tire also.

 

[SUMOTOY]

In my experience that was too much. It was fine until it snowed and then the extra force to the rears caused the ABS to kick in way too soon. I adjusted the valve until the ABS acted more appropriately. Since the valve is the hinge point of a radius, it takes very little movement to compensate for the lift at the other end of the radius. Mine is about .25" lower than stock. Your results may vary, I would recommend the procedure in the FSM. I actually found that panic stops in a snowy parking lot are a good way to fine tune this process.

I agree that's not such a bad way to fine tune. As long as it's always the rear that is getting the ABS activation first. Turn the ABS off alltogether should yield front skid first always as well.

ST

 

[SUMOTOY]

If you read through the FSM it has you add a certain amount of weight when adjusting the valve.

At the stock spring rate this will change the valve by a predetermined amount.

Now with springs of larger capacity, they will respond at a slower rate with the same amount of weight.

I see this as an obstacle in getting back to the factory setting for all conditions from empty to fully loaded and getting the designed proportioning in the braking system for that range.

I chose to focus on a weighted situation which left me a little strong when unloaded. Not sure if was the right choice but the truck behaves as good as I can remember in an avoidance situation.

I see a lot of factors that could affect rear proportioning. The normal procedure for max proportioing valve settings from the factory is to load both the front and rear axles to maximum GVWR and measure weight distribution under maximum braking force. That is the setting for Max rear prop. These are how most SAE papers present "max proportioning" and maxium Weight transfer for a given chassis.

ST

ST

 

[landtank]

The normal procedure for max proportioing valve settings from the factory is to load both the front and rear axles to maximum GVWR and measure weight distribution under maximum braking force.

Not according to my TOYOTA FSM. It states that you want to load the rear axle only so that the total weight (including the vehicle) is 2,932 lbs on that axle.

 

[KliersLC]

I don't see the ABS coming on in a snowy condition as bad, it's doing it's job and you have good braking and control. However setup as you are if you needed to panic stop while heavily loaded the braking would not be there as the valve was set low to prevent the ABS from coming on in snow.

I don't see my ABS as being overly active in winter on my truck but I run a very aggressive tire also.

The only problem is that I have extensively tested straight line stopping distances one snow and ice hammering the pedal and letting the abs do its thing, and then feathering the pedal to keep the abs from coming on. Feathering the pedal reasulted in far shorter stops on ice and snow.

That being said, the abs works well for a panic--jam the brakes and you still maintain control, but on snow and ice, after the initial brake jam, feather the pedal and you will stop much sooner.


Maybe this is a hijack, and I am no expert, but the ABS system on the 80 seems pretty primitive to me.

My 92 camry had much faster pulses and you could hammer the brakes going 70 on sheet ice, and it would stop in much better than the 80. Granted, it always stops better than the 80, but there was no sise to side sliding like the 80 has. I was very impressed by the abs on the camry, but not so impressed by the abs on the 80.....

 

[landtank]

The only problem is that I have extensively tested straight line stopping distances one snow and ice hammering the pedal and letting the abs do its thing, and then feathering the pedal to keep the abs from coming on. Feathering the pedal reasulted in far shorter stops on ice and snow.

Maybe so but adjusting the proportioning valve only effects the rear brakes. What you have done is lessen your braking ability, I don't see how that can help in any situation.

The LC has 4 wheel sensors but the two rear wheels are paired together and controlled as one.

The Camry has 4 wheel sensors and all 4 wheels are controlled individually. Add to that less mass and the correct tire size and I'm not surprised it works better.

 

[SUMOTOY]

Not according to my TOYOTA FSM. It states that you want to load the rear axle only so that the total weight (including the vehicle) is 2,932 lbs on that axle.

That's only a test for proper operation. I speak to how Mr. T chose the size of the prop valve and the range of the prop valve settings. That's usually done dynamically, so that under max GVWR and max braking, a target rear braking force is not exceeded.

ST

 

[SUMOTOY]

...
Maybe this is a hijack, and I am no expert, but the ABS system on the 80 seems pretty primitive to me.

My 92 camry had much faster pulses and you could hammer the brakes going 70 on sheet ice, and it would stop in much better than the 80. Granted, it always stops better than the 80, but there was no sise to side sliding like the 80 has. I was very impressed by the abs on the camry, but not so impressed by the abs on the 80.....

Actually a very good point and not a hijack at all. Dan might be able to help here by running the part number between a few chassis. IME with the Audis, the same ABS computer was used in just about every audi quattro chassis with 4 sensor>3 channel ABS. So, the wheelbase, massive weight and hardware differences weren't part of ABS programming.

With 4 channel 4 wheel ABS/traction control systems in the very late 90's, the ABS/TC system computers got more chassis specific.

The same algorithum on different hardware can yield different performance. Certainly the early 80 ABS is primitive compared to the latest FJ system, and most likely feels primative due to the fact that as COG rises, "impending lockup only" ABS computers don't do very well compared to a lower COG.

Maybe so but adjusting the proportioning valve only effects the rear brakes. What you have done is lessen your braking ability, I don't see how that can help in any situation.

That's not true. Adjusting the prop valve affects total brake capacity, and can put brake force distribution closer to the factory targets (in relation to Ideal).

The LC has 4 wheel sensors but the two rear wheels are paired together and controlled as one.
The Camry has 4 wheel sensors and all 4 wheels are controlled individually. Add to that less mass and the correct tire size and I'm not surprised it works better.

Rick, as far as I know there is no ABS in operation that controls 4 wheels individually. There are some traction control systems that do that. 4 sensors and 4 wheel ABS doesn't mean you have an independent rear circuit, and I'd bet the camry doesn't. As a rule n ABS programming, Select Low Principle applies to the rear circuit, which means that under ABS braking activation the pressure to each rear brake is identical to the rear brake with the least amount of traction (the greatest amount of lockup).

4wheel/4channel ABS is normally used in AWD/RWD vehicles as a traction control device under acceleration. It's very rare (I haven't cked 2006 models tho) to find any vehicle with independent rear Braking ABS activation. Less mass isn't really a function of ABS in and of itself, Drive an X5 sport, 20in wheels, heavier than the LC, it will stop shorter than the camry.

ST





ST

 

[landtank]

Rick, as far as I know there is no ABS in operation that controls 4 wheels individually. There are some traction control systems that do that. 4 sensors and 4 wheel ABS doesn't mean you have an independent rear circuit, and I'd bet the camry doesn't. As a rule n ABS programming, Select Low Principle applies to the rear circuit, which means that under ABS braking activation the pressure to each rear brake is identical to the rear brake with the least amount of traction (the greatest amount of lockup).

According to my 1992 Camry manual these cars which are front wheel drive only have a discrete 4 wheel ABS system.

Contained in the actuator assembly is 4 solenoids, 1 for each wheel. to control lockup.