What is this? (LSPV Valve)

[Grit dog]

So couple of conflicting (it seems) experiences here. One would think under "light" use, the LPSV provides a somewhat reduced pressure from the master cyl (or is there a normal proportioning valve up front as well?). When the truck is loaded, the valve is rotated providing more/up to full pressure from the master cyl.

One person's experience is that the valve can keep the rear brakes from working and eliminating it puts it back to "normal."
Another's experience is that "normal" is with it working and when it sends full pressure, the brakes have too much pressure in the rear and lock up easily.

Leads me to believe that if the LPSV is eliminated that a different proportioning valve is needed up front to reduce rear brake pressure compared to front pressure? No?

Plenty to do and not messing with it in the near future unless I have to, but sounds like it will be going in the recycle bin when I have the opportunity!

It would be cool to get it functional if the truck was towing or hauling heavy, but I don't see that happening. Got a 1000 ft-lb coal roller for those jobs.

Thx for the input guys!

 

[Blue Ox]

There is a proportioning valve in the front combination valve as well.

I suspect one of the differences has to do with trucks deviating from as-built condition. My truck is pretty much stock. Craig85 said he has a 4" lift. Grit dog's truck appears to be lifted as well. I'm going to assume that along with the lifts come much larger tires. Larger tires apply more leverage to the brakes, so should be less prone to locking from higher braking force.

A friend who had the same kind of truck in fairly stock form always swore the valves were junk, but also had a permanently mounted industrial compressor as well a lot of shop tools and heavy truck parts onboard all the time. You know, the truck was loaded. So the valve was probably disengaged anyway.

I would think that if the valve is eliminated you'd need to have an additional proportioning valve if you're not in one of the above described conditions. Then you'd have to be able to tune it if you wanted to load the truck. The beauty of the height sensing valve is that it's function is automatic. On one occasion I had the truck loaded to near exactly it's maximum GVW of 8,600 lbs and braking was completely normal. Didn't have to think about it.

Rode great too.

 

[Keith Seymore]

Wow; a lot of conversation here. You guys are pretty much circling in on a correct understanding of the valve. Basically it was a primitive Rube Goldberg mechanical rear wheel anti-lock brake system, tripped on or off based on vehicle load and the resulting trim height change.

The only thing I would mention is that it was added to help pass a very specific portion of the MVSS Federal brake test. It was not unheard of for us to do everything we could to skew the results towards a "pass", including requesting a particularly talented driver to run that portion, or adding "cheater" hardware.

I'd pull it off if it was on my truck.

K

Disclaimer: I am not a lawyer nor do I play one on TV. Offer not available in Delaware. Void where prohibited by law.

 

[Grit dog]

Yes good conversation, and sounds like another ineffective 80s engineering idea.
So toss it in the pile with catalytic converters, AIR systems, and early overdrive auto transmissions.
Fortunately my 80s truck is almost completely de 80ified. Once this little gem is gone, she’ll be just like a 70s model but with delay wipers and cruise control!

 

[Blue Ox]

Yeah,... but no.

I had two trucks where this ineffective engineering idea was inoperative, and I wouldn't want to drive either of them that way. Good on you if yours works without it, but both of mine were unsafe to operate without it.

 

[Italianwagon]

Good info here. Literally was just crawling under my truck and saw the disconnected valve and was like what the heck is that.

 

[Grit dog]

So had a moment to play with the LSPV. (It’s not connected to the linkage on the axle.)
Because after I talked about how good the brakes were and then spun the wheel on the valve, they didn’t seem as aggressive.
I turned the valve to a different orientation and bam! Brakes are back! I can lock up the rears on dry pavement again, but I have to hit the brakes fairly hard.
With the bigger tires Aired at about 30psi it’s acceptable performance.
I can see for sure though if you had skinny tires at 80psi, the rears would lock up too easily.
On the upside, it’s one project I don’t need to do to get more braking power.

 

[Grit dog]

Valve must’ve rotated itself back to half power again.
Ran a few errands. Brakes were aggressive and then they aren’t again.
I need to test how much of the rotation is “full power” and I can attach the linkage again to keep it there I think.

I understand this ain’t for everyone’s setup, but anyone who has this thing still connected should mess with it a bit.
It seriously feels like 4 wheel disc with full pressure to the rear brakes. Probably be even better once I have tires that aren’t 10 years old and hard, sun baked tread.

 

[Circular Pastry]

I understand this ain’t for everyone’s setup, but anyone who has this thing still connected should mess with it a bit.
It seriously feels like 4 wheel disc with full pressure to the rear brakes. Probably be even better once I have tires that aren’t 10 years old and hard, sun baked tread.

I just discovered this on my G30 van, well my buddy who is helping out with the stuff over my head or outside of my number of hours in the day did, I think I want to keep it, how has yours been working out?

 

[Grit dog]

I just discovered this on my G30 van, well my buddy who is helping out with the stuff over my head or outside of my number of hours in the day did, I think I want to keep it, how has yours been working out?

I haven’t been driving it since last fall. But it works although I don’t have it hooked to the suspension and haven’t figured out how to clock it to actually work.
When I turn it to full braking, eventually it rotates away from that point and back to reduced braking.
I’d toss it in the garbage except my brake lines are perfect and haven’t found or fabricated a fitting to simply swap it out without messing with the factory brake lines.

 

[Bextreme04]

My 1980 K25 doesn't have the proportioning valve and was a factory heavy duty truck. It had the 350/TH400/205, high gross weight heavy duty suspension, 14BFF rear with 13x2.5"drums and no LSPV. It stops on a dime... no locking up issues at all.

 

[Blue Ox]

I haven’t been driving it since last fall. But it works although I don’t have it hooked to the suspension and haven’t figured out how to clock it to actually work.
When I turn it to full braking, eventually it rotates away from that point and back to reduced braking.
I’d toss it in the garbage except my brake lines are perfect and haven’t found or fabricated a fitting to simply swap it out without messing with the factory brake lines.

Don't toss it! I need spares.

 

[Circular Pastry]

I haven’t been driving it since last fall. But it works although I don’t have it hooked to the suspension and haven’t figured out how to clock it to actually work.
When I turn it to full braking, eventually it rotates away from that point and back to reduced braking.
I’d toss it in the garbage except my brake lines are perfect and haven’t found or fabricated a fitting to simply swap it out without messing with the factory brake lines.

I'll be sure to share after he shows me, but when discussing it with my buddy he said there's a part that you loosen, then make your adjustment, and tighten to hold the adjustment. Is that how yours is? I'll get a pic when we get under it today, I have to know what I'm dealing with before the big Pikes Peak, Yellowstone, Rushmore vacay we're getting her ready for. He said rn it's set perfectly though.

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[Grit dog]

I'll be sure to share after he shows me, but when discussing it with my buddy he said there's a part that you loosen, then make your adjustment, and tighten to hold the adjustment. Is that how yours is? I'll get a pic when we get under it today, I have to know what I'm dealing with before the big Pikes Peak, Yellowstone, Rushmore vacay we're getting her ready for. He said rn it's set perfectly though.

Sent from my Pixel 4 using Tapatalk

Yes, there is a bracket attached to the axle and the valve that rotates the valve as the axle to frame height changes (IE loaded down = rotate valve to full, empty ride height reverts to partial braking)
You can change the position of the valve in increments (splined shaft) and re-hook up to the bracket.
Personally, with a van, and presumably decent weight on the rear axle especially on vacation, I'd figure out how to lock it in the full on position. If your springs aren't sagged with the load or you have something keeping the back end of the truck at unloaded height, the valve won't go to on, even if it's "adjusted perfectly."

The difference in braking power is very noticeable and would be very welcome in mountain driving. I'd want full brakes and not let some half baked 40 year old technology potentially render the rear brakes only partially effective when coming down any grade in the Rockies...
The beauty of these trucks is the braking power is awesome if the big drums in back are doing their job 100%. If they're not, IMO, it's mediocre.

 

[Blue Ox]

The factory method for setting the valve was a plastic gauge that engaged the flat on the shaft and a hole on the valve body. Then the linkage was attached to the spline with the truck at unloaded height. The plastic gauge would break off when the suspension moved. Of course, it's obsolete, and I've never seen more than the piece that was still on the shaft. So no reverse engineering. Now I measure the angle of the flat vs the linkage with a t-bevel or something similar if I have to disassemble or replace one.