What is this? (LSPV Valve)

[Craig 85]

RPO code C6P is for 8600 GVW.

 

[WFO]

RPO code C6P is for 8600 GVW.

Plus, through 1986 they were set up to use leaded gas. Had the larger hole in the filler for the larger leaded fuel nozzle, and no catalytic converters.

Also, in Texas at the time, the seat belt law said any truck over a 3/4 ton didn't require seat belt use. 8600 GVW made it to where seat belt laws didn't apply, until they changed the law in the 90s.

 

[Blue Ox]

Right. It got the truck past the light duty classification. On the diesel it got you the VIN J engine which had more horsepower and no EGR. That and obviously heavier springs and the JB7 brake setup with larger calipers, pads and the 13" drums in the rear. They also spec'd heavy duty shocks although most shock manufacturers don't recognize that anymore.

When I mentioned the C6P thing before I only meant to differentiate the LSPV within the 20s, not to exclude the 30s.

 

[skysurfer]

IIRC, it was just 84 to 87 20 and 30 series trucks that got it.

They were still being installed through the '89 model year.

I can't imagine you could just eliminate that valve. When I did the C6P conversion on the Suburban the rears would lock violently until I added that valve. It was not safe to drive without it. Absent that I would think some kind of proportioning valve would be needed.

The TSB GM issued on the lspv instructed mechanics to just bypass the valve. I'm guessing whoever did the op's truck had the same issue as you and added their own adjustable valve.

The tsb can be found in post number 5 but don't click on it if you're at work, I'm getting a nsfw popup along with the tsb.
http://www.gmsquarebody.com/threads/the-brakes-breaks.5984/

 

[Jerry87v20]

So on my title under GVW 10001.... is that odd? It's an 87 Silverado V20

 

[Blue Ox]

They were still being installed through the '89 model year.



The TSB GM issued on the lspv instructed mechanics to just bypass the valve. I'm guessing whoever did the op's truck had the same issue as you and added their own adjustable valve.

The tsb can be found in post number 5 but don't click on it if you're at work, I'm getting a nsfw popup along with the tsb.
http://www.gmsquarebody.com/threads/the-brakes-breaks.5984/

I've seen the TSB. Based on my experience I can't believe they said to just eliminate it. Yet, there it is.

And I tried a number of alternatives. Smaller wheel cylinders, more clearance on the adjuster. Any stop that wasn't gently feathered,... that "self-energizing" property of drum brakes just took over.

 

[HotRodPC]

So on my title under GVW 10001.... is that odd? It's an 87 Silverado V20

That probably has to do with your state. Even if your truck is an 8600 GVWR truck, it is still under 10,001. Probabaly has to do with laws. < 10,000 and > 10,001.

 

[Wes P]

Posting in all LSPV-related threads for reference:

LSPV delete on '85 C20 increased braking performance.

Basic procedure:
- Remove valve, post-LSPV hard line, hose, axle bracket
- Measure, measure again, cut, drill, tap, weld stop nut on bracket (or add a screw)
- Release axle hard line from clips
- Install bracket, brake hose (180 flip), zip ties
- Bleed brakes

Parts: 2 zip ties.
Materials: brake fluid
Time: 4 hours, leisurely, without experience, being careful.
Note: Be gentle with hard lines. Plug the hard line after disconnecting. Don't empty the master cylinder.

You must be registered for see images attach

 

[Blue Ox]

I finally got the LSPV (HSPV according to GM) valve swapped from my Suburban parts donor so I can stop a little aggressively without locking the rears. Hoping to see what makes these tick, and hoping I can salvage my original valve for future repairs I disassembled the "non serviceable" and tragically now obsolete valve.

Here are the components laid out:

You must be registered for see images attach

I have a rough idea of how it works. Basically the same as a conventional proportioning valve except with a step or split cam that allows it to function as a proportioning valve or not based on ride height. The only thing I can't quite get my head around is how the valve actually blocks the pressure increase to the brakes. There's no obvious seat or port blocking that I can see.

Anyway, if you're pondering the mysteries of these valves, that's what they look like inside.

 

[Grit dog]

^ Interesting. My truck still has the LSPV in line. Linkage disconnected, although someone went through the trouble of making lift brackets for the linkage....
It's been about as many years as the truck is old since I'v worked on these 80's trucks, so question?
This valve spins freely, basically. there's a little resistance at one point in the rotation. Brakes seem to work fine. Gotta go try to lock up rear brakes, but I believe they are working suitable based on how well the truck stops. It stops like new basically, not like something with compromised rear drum brakes.

The question is does anyone know how much this theoretically affects the braking performance if the valve is spun to a certain (any) position? I can obviously remove/bypass it, but doesn't appear to be a need to currently.

 

[skysurfer]

I found that the valve is very sensitive to adjustments. Just a few degrees of rotation changed the braking bias tremendously, which makes perfect sense when you consider the small range of motion that suspension droop would have to act upon the linkage. Rotating the center stem of the valve counter-clockwise 90 degrees and then reinstalling the linkage made my truck un-driveable. Rear brakes locked up instantly when I touched the pedal. Final adjustments were a matter of moving the linkage just one tooth at a time on the center stem.

 

[Craig 85]

^ Interesting. My truck still has the LSPV in line. Linkage disconnected, although someone went through the trouble of making lift brackets for the linkage....

The question is does anyone know how much this theoretically affects the braking performance if the valve is spun to a certain (any) position?

I had an '87 K20 I did a 4" lift on back in the late 80's The 4WD shop lifted the bracket about 2" off the axle at the time. I didn't really notice anything after the lift as I hadn't hauled any loads.

A few months later, I had to tow my El Camino on a 16' car trailer. I was young and dumb back then and did not have the electric trailer brakes hooked up. Well, I had to stop suddenly for a red right and almost drove through the intersection. It felt like I had no brakes.

I later tried a full lock up and got no brakes out of the rear. I bypassed the valve after this and my braking returned to normal.

 

[Grit dog]

^
@skysurfer
That's what I would think, makes perfect sense as in operation it would only spin a few degrees with suspension compression.
Did it spin 360 degrees freely though? I feel like this one is totally inoperable as I randomly spun it a while ago and felt no difference and can virtually guarantee it was not in the same position.

 

[Grit dog]

@Craig 85
Interesting. I'll do a little testing when I get some time. See if it's doing anything besides hooking 2 brake lines together!

 

[Blue Ox]

It functions more like a switch, as in it's either on or off. When it's on it acts as a second proportioning valve. When it's off it's effectively not there.

There is a spring and cam arrangement between the shaft and the internal cam that controls actuation. The shaft can spin 360° without hurting the inside mechanism which can't rotate that far, and if the pin is engaged in the low part of the cam applying force would shear the pin off the valve. So the spring pressure orients the shaft to the cam, but doesn't apply enough force to damage anything.

Like skysurfer I find a significant difference between when the valve is functioning and not. With an unloaded truck the rears will lock violently under even slightly aggressive application. Forget wet roads.