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Pre 79 brake proportioning valve


moalaska

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Is there any type of brake proportioning valve on a 78 chassis?  I did the axle upgrade a my mini mirage, I didn't see the proportioning valve in the usual place where it is in the later models.  Install new rear brakes and brake hardware at the same time and tighten on the star wheels as much as I can without them dragging too much. My toy home has a tendency to lock up the front brakes before the rears, this isn't a problem in the summer but in Winter climates and snow and ice, the only way to get the rear brakes to get close to lock up is to really hammer down the brake pedal.  If my front brakes are locked up I am just sliding and have no steering control. I have driven Vans and large vehicles that handle great in the snow because the brake proportioning is correct. Is there some valve somewhere else where I could adjust the brake bias? Or is the only thing I can do is try to come up with some sort of aftermarket solution.  This thing would handle great in winter if more of braking power could be directed towards rear so that front and rear would"lock up" at same time.  I'm sure the break proportioning was designed correctly before someone slammed 3000 pounds of camper onto a light truck.

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https://photos.app.goo.gl/ehPgQDm3zLU75d92A 

https://photos.app.goo.gl/An2Dyw1FsgWamCoBA

 

I'm not sure if these links work, I took photos of my brake MC.  It's a 15/16 brake booster, so it's larger.  Rear brake line goes into a valve body under brake booster, then to rear.  Is this valve some sort of factory proportioning/limiting valve?  If so could I put an adjustable one in it's place?  I wonder if this master cylinder is stock.  It has separate reservoirs for rear and front section.  And divorced separate clutch MC with dedicated reservoir.

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The proportioning valve you need to look for should be near the rear passenger side. Just follow the brake lines right near the wheel. I know they were in the 1979 truck cause Toyota still sells the part. I also found a reference to them being on a 78 when I Googled it.

https://toyotaminis.com/forum/threads/load-sensing-brake-proportioning-valves.19724/

There is a picture here but you have to join the group to enlarge. If you want the most rear braking you should remove it completely. Not needed on a camper cause your load is never going to be light. 

Linda S

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On 11/13/2019 at 10:59 PM, moalaska said:

Is there any type of brake proportioning valve on a 78 chassis?  I did the axle upgrade a my mini mirage, I didn't see the proportioning valve in the usual place where it is in the later models.  Install new rear brakes and brake hardware at the same time and tighten on the star wheels as much as I can without them dragging too much. My toy home has a tendency to lock up the front brakes before the rears, this isn't a problem in the summer but in Winter climates and snow and ice, the only way to get the rear brakes to get close to lock up is to really hammer down the brake pedal.  If my front brakes are locked up I am just sliding and have no steering control. I have driven Vans and large vehicles that handle great in the snow because the brake proportioning is correct. Is there some valve somewhere else where I could adjust the brake bias? Or is the only thing I can do is try to come up with some sort of aftermarket solution.  This thing would handle great in winter if more of braking power could be directed towards rear so that front and rear would"lock up" at same time.  I'm sure the break proportioning was designed correctly before someone slammed 3000 pounds of camper onto a light truck.

The brake proportioning valve on the 1978 is attached directly below the tandem master cylinder(see photo). They are factory preset for a disc/drum combination. They rarely go bad. When they do it’s usually the rears that lockup first not the front.

Before assigning blame to this valve, are you sure you have no residual air in the rear brake lines? Pay particular attention to the fabric brake line from the chassis to the rear axle. Is it new, or are you reusing the old line? A dry rotted line can leak and reduce pressure.

I installed a gm 14 bolt ff axle with much larger rear brake shoes and my brakes work fine.

I’m not sure what the toyotaminis link is about. It doesn’t show up in the FSM that I have, nor did it come on my vehicle.

These valves are still available on eBay. Good luck. Fred.

8043E2F5-9230-4514-869C-51489612FBA9.jpeg

Edited by fred heath
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The bottom line in a toy home it is a useless device in the pickup the proportion valve was there to keep the rear brakes from locking up with little or no weight in the bed there is no such a thing as a toy home being too light in the backend. I totally removed mine it was rusted up any way,  there is not a shot of locking up the rear brakes. It was controlled by frame height vers diff height. 

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Ok so after doing some research and confirming with your replies, my truck has no LSPV.  It does have the standard proportioning valve under the brake master.  I have bled brakes, as well as new brakes and hardware, and new soft line in rear.  I believe the issue is the factory valve is set up for a truck, not a motorhome with an extra couple thousand pounds on the rear.  The the front tires note break friction well before the rear.  The brake bias is now incorrect for the load of the vehicle.  Plus with my particular home, the wheelbase is stock, there is no frame extension between front and rear axle, they added frame after rear axle so much more weight behind the rear axle, think teeter totter.  I could get an aftermarket adjustable valve and put it in place of the factory one under the master cylinder, or probably remove it all together.  

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31 minutes ago, moalaska said:

Ok so after doing some research and confirming with your replies, my truck has no LSPV.  It does have the standard proportioning valve under the brake master.  I have bled brakes, as well as new brakes and hardware, and new soft line in rear.  I believe the issue is the factory valve is set up for a truck, not a motorhome with an extra couple thousand pounds on the rear.  The the front tires note break friction well before the rear.  The brake bias is now incorrect for the load of the vehicle.  Plus with my particular home, the wheelbase is stock, there is no frame extension between front and rear axle, they added frame after rear axle so much more weight behind the rear axle, think teeter totter.  I could get an aftermarket adjustable valve and put it in place of the factory one under the master cylinder, or probably remove it all together.  

If your chassis ID starts with “RN28” it is the standard 1 ton cab & chassis used for motor homes and commercial applications. If you have front disc brakes it would be a mistake to remove this valve. The valve is designed specifically for this type of vehicle. If it’s installed, it’s supposed to be there. Remove at your own risk.

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42 minutes ago, moalaska said:

I could get an aftermarket adjustable valve and put it in place of the factory one under the master cylinder, or probably remove it all together. 

Is there a relatively simple way to bypass the valve and find out how it does if you go equal and full force to the front and rears?  It might be as simple as installing a barrel connector instead of the valve?  Then you can find out what kind of grab you get in front and back but still be able to easily return to stock if it give unexpected results.

I still have the LSPV but it's wired to go 'full to back'.  I get decent balance between front and back which I monitor by checking brake/hub temps after using the brakes hard.  I get nice even temps all around.

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48 minutes ago, fred heath said:

If your chassis ID starts with “RN28” it is the standard 1 ton cab & chassis used for motor homes and commercial applications. If you have front disc brakes it would be a mistake to remove this valve. The valve is designed specifically for this type of vehicle. If it’s installed, it’s supposed to be there. Remove at your own risk.

I didn't think Toyota sold cab and chassis trucks in 1978 and before, I thought the motorhome manufacturers just took standard trucks and built on them.  I know they had specific chassis for commercial and motorhome applications in later years, but I didn't think that far back.

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40 minutes ago, AtlantaCamper said:

Is there a relatively simple way to bypass the valve and find out how it does if you go equal and full force to the front and rears?  It might be as simple as installing a barrel connector instead of the valve?  Then you can find out what kind of grab you get in front and back but still be able to easily return to stock if it give unexpected results.

I still have the LSPV but it's wired to go 'full to back'.  I get decent balance between front and back which I monitor by checking brake/hub temps after using the brakes hard.  I get nice even temps all around.

The lsvp and regular proportioning valve are two different items.  And in could pull the but off bottom and pull spring and plunger out.  It wouldn't hurt anyway to clean it out.  

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10 minutes ago, moalaska said:

I didn't think Toyota sold cab and chassis trucks in 1978 and before, I thought the motorhome manufacturers just took standard trucks and built on them.  I know they had specific chassis for commercial and motorhome applications in later years, but I didn't think that far back.

Mine is a 1978. They used the same c&c for 1976&1977. If it starts RN28, it’s the c&c not a regular 1/2 ton pickup.

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32 minutes ago, Derek up North said:

I'm not yet convinced that an RN28 didn't also include the Hilux pickup.

image.png.5450b3b41edee2437adf60508b72b28d.png++

 

 

Actually Derek, you’re correct. RN 28 was used for the long bed version. The GO82 rear axle was used on the 1 ton. The RN 28 combined with GO82 was the 1 ton c&c. I’ve never seen a motor home coach installed on anything but the 1 ton chassis.

If you look at the ID plate under the hood you’ll see that RN28 and GO82 are permanently embossed. I don’t believe you could get the RN28 without the GO82 axle.

3112264A-8D4F-40BE-B08B-48558B996EAB.jpeg

Edited by fred heath
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11 minutes ago, Derek up North said:

I would install a manually adjustable proportioning valve and set it so the front brakes lock up a bit before the back brakes. I certainly don't want my back brakes locking before the front brakes.

That's what I'm thinking, as is my fronts lock up way early.  And as far as chassis codes goes, I've found many references online as to rn28, and rn28l being regular pickups as well as for early sunraders and motorhomes.  I'm not convinced either that Toyota had a dedicated cab and chassis at this time other then selling trucks to manufactures with no bed, or maybe the manufactures simply removed the bed.  On my home, they simply reused the bed taillights and installed them on the camper horizontally.  

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1 minute ago, Derek up North said:

I'm not sure what a GO82 axle is or if it's any different/stronger than any other axle of the day or if it's specific. I don't recall Toyota offering a '1-Ton' (SRW) truck until the '80s.

image.png.f3645664b88ba7a1953669941b5f0816.png

And again, I've also found references on Toyota mini truck forums with owners of trucks having the same combination, g082 and rn28.  I don't think it was specifically a cab and chassis, just the ideal option combination for an overloaded motorhome.  Ive read somewhere before doing my axle swap that they technically rated that axle at 1 ton, although it really shouldn't be with the single wheel bearing design.

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1 minute ago, Derek up North said:

I'm not sure what a GO82 axle is or if it's any different/stronger than any other axle of the day or if it's specific. I don't recall Toyota offering a '1-Ton' (SRW) truck until the '80s.

image.png.f3645664b88ba7a1953669941b5f0816.png

It has the 8” r&p vs the 7.5” on the half ton. I’m don’t really care either way. This is what I’ve been able to obtain from Toyota and other sources during my axle swap. Nothing else to add 🙂

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1 hour ago, moalaska said:

... as far as chassis codes goes, I've found many references online as to rn28, and rn28l being regular pickups as well as for early sunraders and motorhomes.  I'm not convinced either that Toyota had a dedicated cab and chassis at this time ...

I came across this (with no year referenced):-

image.png.a4b7948ddf90674c74a2bcec1cee7d16.png

It does reference a 'Heavy Duty Cab/Chassis' as well as the different pickups. I'm not enough of an early truck expert to ID the year though the cargo tie-downs look just like this truck's.

image.png.fbf1f3a74ac6544ee6224b8557b0cd66.png

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13 minutes ago, moalaska said:

...  they technically rated that axle at 1 ton, although it really shouldn't be with the single wheel bearing design.

Calling something '1-Ton' can get kind of confusing. There never seems to be any spec that's 2000lb, 2200lb (Long Ton) or 2205lb (Metric Tonne).

This '1-Ton' has a payload of 2655lb.

1988 1 Ton Zoom.jpg

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44 minutes ago, Derek up North said:

I'm not sure what a GO82 axle is or if it's any different/stronger than any other axle of the day or if it's specific. I don't recall Toyota offering a '1-Ton' (SRW) truck until the '80s.

image.png.f3645664b88ba7a1953669941b5f0816.png

It has the 8” r&p vs the 7.5” on the half ton. I don’t really care either way. This is what I’ve been able to obtain from Toyota and other sources during my axle swap. Attached is the weight rating from my owners manual. The 1/2 ton came standard with the 7.5” r&p. The 3/4 ton (or 1 ton as commonly referred) came equipped with the 8” r&p and was designated GO82. I’ve added all I’ve got. Have fun 🙂

B05C35CE-0B2D-421B-80C7-F7FAC4E3BF29.jpeg

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Toyota did indeed sell a truck cab and chassis in 1978 with a GVWR of 4600lbs. 500lbs more than the regular trucks. Have you crawled under there and really looked? If you don't have LSPV near the rear passenger wheel someone must have already removed it. Here are the specs for that year

1978_01.jpg

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I think you will find it almost impossible to lock up the rear brakes except the very lightest toy home and then it's doubtful. A empty pickup yes you maybe able to. With a motorhome the weight shift is not there it basically stops flat it does not nose dive like an empty truck or for that matter a car where the majority of the stopping is on the front wheels. A single bearing diff is not a 1 ton axle somewhere along the line Toyota made a "Stout" It had a 1 ton rear with a single wheel I have only seen one.

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1 hour ago, linda s said:

Toyota did indeed sell a truck cab and chassis in 1978 with a GVWR of 4600lbs. 500lbs more than the regular trucks.

I wonder what the differences are that let them increase the GVWR by 500lb and why they didn't do the same for at least the long bed trucks?  ¯\_(ツ)_/¯

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28 minutes ago, Maineah said:

I think you will find it almost impossible to lock up the rear brakes except the very lightest toy home and then it's doubtful. A empty pickup yes you maybe able to. With a motorhome the weight shift is not there it basically stops flat it does not nose dive like an empty truck or for that matter a car where the majority of the stopping is on the front wheels. A single bearing diff is not a 1 ton axle somewhere along the line Toyota made a "Stout" It had a 1 ton rear with a single wheel I have only seen one.

Well I live in Alaska, very easy to lock up brakes in winter, this is why proper brake bias is important.  If I'm at say 1/4 braking, on ice my fronts will lock up, and I lose steering control, and braking effectiveness.  It takes mashing the pedal to the floor to get the rears to be effective.  Most people wouldn't notice the difference unless you are driving in Arctic conditions.  My front and rear braking should be near equal on ice, or close to it.  On pavement The extra traction will indeed cause a weight shift and more braking force being transferred to the front.

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2 hours ago, linda s said:

Toyota did indeed sell a truck cab and chassis in 1978 with a GVWR of 4600lbs. 500lbs more than the regular trucks. Have you crawled under there and really looked? If you don't have LSPV near the rear passenger wheel someone must have already removed it. Here are the specs for that year

Well this is where the LSPV would be on newer toys.  As you can see, there is just one bracket for the brake line, no holes for bolts or other brackets for a LSPV, it's obvious I never had one to begin with and one was never removed.  So I am still trying to get a proper brake bias for driving in winter.  Something to learn everyday even for those on here that have been on the forum for a while.

IMG_20191115_161004_2.jpg

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Ok so I took my proportioning valve apart, removed the guts, and my rear brake performance is the same.  So I think I can either add a adjustable proportioning valve to the front, replace the brake master cylinder, or replace the rear brake Pistons.  I think the master. Cylinder is fine, and I know the rear Pistons work but I guess they can be sticky.  Might have to come up with a custom solution to make my brakes function better.

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Well if what you have doesn't work the way you want, this is a simple answer...https://www.wilwood.com/MasterCylinders/MasterCylinderValves

There is also a metric version that requires a 10mm bubble flair. There are SAE to metric adapters for the original valve

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11 hours ago, Derek up North said:

I wonder what the differences are that let them increase the GVWR by 500lb and why they didn't do the same for at least the long bed trucks?  ¯\_(ツ)_/¯

Easy. The pickup bed weighs about 500 lbs. Also, if you think about it there were other commercial applications besides motor homes. U Haul etc. Why buy a complete truck just to discard the bed.🙂

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53 minutes ago, Derek up North said:

The GVWR has nothing to do with the weight of the pickup bed.

image.png.292ead62593ea4117fc8d3e1e5585671.png

My bad. The only difference I can see is the c&c uses 7.50-14 6 ply tires and the pickup uses 7.0-14 6 ply tires. My owners manual is for a conventional pickup. The different tire size may allow the extra 500 lbs. At this point in time does it really matter?

4ACE83ED-1ECA-4D7B-B0DF-2718019CEC2D.jpeg

AF04A475-D800-49DE-921E-C9CF6DA81644.jpeg

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18 minutes ago, Derek up North said:

Academic curiousity.

No arguing that bigger tires were fitted but if that was the only difference, why wouldn't Toyota spend a few bucks more and up the GVWR on all their trucks by 500lb?

Probably nobody left alive to answer that question 😂.

At least now we’ve established they did sell a separate c&c labeled a special body vehicle.

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14 hours ago, moalaska said:

Well I live in Alaska, very easy to lock up brakes in winter, this is why proper brake bias is important.  If I'm at say 1/4 braking, on ice my fronts will lock up, and I lose steering control, and braking effectiveness.  It takes mashing the pedal to the floor to get the rears to be effective.  Most people wouldn't notice the difference unless you are driving in Arctic conditions.  My front and rear braking should be near equal on ice, or close to it.  On pavement The extra traction will indeed cause a weight shift and more braking force being transferred to the front.

This is the problem the fronts now because of the added rear weight are actually doing less because of the greatly reduced weight shift. In a normal application front brakes have more sweep area than the rears this is because they do the majority of the breaking not so in an overloaded truck even on dry pavement so bottom line you need more rear braking on a loaded truck.  Years ago it was not unusual for larger dump trucks to have no front brakes at all! By the way we have snow on the ground and ice on the roads right now it's probably because I live in Maine!

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