Proportioning valve
 

Could this be possible?:

A malfunctioning lspv such that with a little extra weight in the bed of the truck such that the truck gets a bit better gas mileage.

I ask because I swear that with a 400 hundred pounds in the bed the truck seems to accelerate easier.

It's just a feeling so I know I could be wrong but man it's a strong and frequent experience.

My thought is that the lspv is clogged or something. If so, maybe the rear brakes never fully or cleanly let go so creates a drag.

The LSPV doesn't actually send pressure to the brakes...
It could, however, be that your brakes are dragging. Jack up the wheel and spin the wheel. There should be a slight amount of drag, but not much. Should make atleast 1 revolution...

Well, right. I've done that and it spins okay. I know that it's possible that the front calipers won't release completely if the lines are clogged.. that's what I'm wondering about in this valve. That it's clogged, but with a little weight the valve opens a bit and the rear's work a bit better.

It's hard to explain the experience of having an empty truck strugglin' to stay at 60mph on the highway only to easily pass people while carrying a load.

I would think the 400 pounds is enough weight that the truck rides smoother giving the illusion of more power.:fire2:

Just a thought

Sorry dude, that's not how it works.
If it were a residual pressure valve i could understand, but it's not.

If theres no pressure to the rear, theres no fluid movement.
Basically, the brake fluid is inert without the brake pedal being pushed.
Essentially, the LSPV doesn't retain or hold pressure on the rear brakes, just limits pressure.

If you want to try, take the bolt out of your axle that holds on the LSPV arm, and then zip tie it to your crossmember or frame.. If it changes, report back. Otherwise, i'm calling placebo effect. My guess is that when you have it loaded down you're probably mashing the skinny pedal, and it just feels more powerful.

LSP-BV does this:
The "LSP" section- "Load Sensing Proportioning" section limits pressure from the master cylinder to the rear brakes dependent upon the load- that's what the arm across the rear axle is for. More weight in the rear lets more pressure from the master get to the drums.
The "BV" section- "Bypass Valve" does what it says. It senses pressure from the front brake circuit and if the front brakes fail, it "bypasses" the proportioning valve section allowing full brake pressure to the rear wheels.

I might attribute better mileage with some weight to the shift in vehicle "attitude", or how it is influenced by airflow, and the perception of better acceleration to the vehicle leaning "back", changing how the acceleration is felt by your body.

X2. I couldn't put it better Abe. I am thinking attitude also. Brakes have nothing to do with this.

I appreciate the posts.

I'm certainly not hitting the gas pedal any harder. That's something I've been paying attention to.

The trouble shooting guide in the manual suggest that brake drag can be caused by several things, one of which is clogged lines. I've been trying to rule out various parts of the system and I'm now in the LSPV area.

If I find a stretch of flat highway, maintain a speed of say 60, with and without weight in the back, Shouldn't the RPM's be a bit higher if hauling weight?

I'll try disconnecting the LSPV arm control. Not sure when I'll have a chance, but I'll give it a go.

With a manual transmission the RPMs at a given speed should be identical given the same gear as there should be no slip in the system (like a worn out clutch slipping and letting the engine rev higher). With an automatic, you might see higher RPMs under load if the torque converter is not locked up due to the extra load. I recall once towing my dad's pickup truck with our old Dodge convertible and the tach was reading twice the normal RPM pulling up a fairly steep hill. It normally read 1000 RPM / 10 MPH in low and that time it was double that ratio, but that was with a heavy load up a steep hill. On the flat, the RPMs read normal for the speed.

takes 5 minutes, and one wrench and a zip tie.
just go do it and put this to rest lol

yah, I hear ya.

An automatic transmission can vary a bit in RPM's with relation to speed as 4Crawler mentioned, due to how the torque converter works when it's not in "lockup" mode, and due to slippage within the clutch packs and drums within the transmission itself. That slippage would show itself in increased coolant temps (since ATF goes through the radiator), and the transmission holding a gear to higher rpm's than "normal" before shifting.

If you've a 5sp, it's a "straight-through" sort of thing; nothing can slip except the clutch. Gears mesh and nothing can slip so rpm's are directly related to speed. So if empty you do say 50mph in 4th at 2600 rpm's, you'll alway do 50 in 4th at 2600 rpms: changing the load won't change anything other than may make you take a bit longer to get to speed, but your rpm's in any gear will always have the same speed when you're in that gear... unless the clutch slips.