I have a lifted (~3") '80 pickup with 35" tires with a leaky load sensing proportioning valve. I'm trying to determine if I'd be better off replacing it or installing a manually adjustable proportioning valve near the master cylinder. I have an extension for the rod on the axle and between bending the rod and sliding the LSPV up/down I managed to get it adjusted so that all 4 wheels lock up at about the same time. I like that the idea that when I load up the bed and head for the trail the valve automatically adds more bias to the rear brakes.

The problem with keeping it is there doesn't seem to be any rebuild kits available for it. My valve has a single inlet and a single outlet. The LSPVs for my year are very expensive since they all appear to be NOS. However, there are a lot of aftermarket valves available with an inlet, an outlet for the rear brakes, and a return outlet for the bypassed fluid; and they appear that they would fit in the same location. Would I be able to use one of these 3 port LSPVs and just plug the return port?

If I went with a 2 port manual valve it appears I can just cut and flare the rear brake line near the master cylinder and replace the LSPV with a coupling, is this correct? Is it practical to assume I can adjust the valve for even brake bias when the truck is unloaded and then give it a few cranks when I've loaded it up?

I'd prefer to keep the stock design unless there are advantages of the manual valve on my truck since it is lifted and has bigger tires.

Edit: I found a document that describes the operation of the different proportioning valve systems:
http://gt4.mwp.id.au/Toyota%20Manual...%20Control.pdf

The 3 port valves are load sensing proportioning and bypass valves (LSP&BV). It appears that the LSP&BV is a fail-safe if the front brakes loose pressure. Without pressure from the front brake circuit the LSP&BV will apply unproportioned pressure to the rear brakes (like it wasn't there). It appears I would need to put a Tee in the inlet line and feed this bypass port if I want the load sensing function to operate properly.

Any comments? Has anyone replaced a LSPV with a LSP&BV? Does anyone think just plugging the bypass port would work?

 

You have pretty well figured out that plugging the third port is a no go. If you want the stock design, the NOS unit despite the cost is the only proper way to go. I suspect the lift may have had something to do with the leak; since you lifted it 3" you should extend the threaded eye piece on the axle 3" or so. Just cut the old length off and weld a longer piece of threaded rod on the eye.

If you go with the after-market manual unit you are correct, just cut and flare. Guys mount these valves under the hood or under the box. If your going to adjust it and leave it that's O/K, but if you're going to adjust it according to load in the truck that's rather cumbersome...why not mount it in the cab somewhere (under the dash) so its more easily accessed? That may not be totally legal, as in it may not pass a safety inspection.

Some guys bypass it completely with mixed results. I personally wouldn't do that, you could lock up the rear wheels. And again, that may not be totally legal, as in it may not pass a safety inspection.

If you haven't yet google the Wilwood site.

Try not to splice brake lines...far better to make a new line all the way from the master cylinder to the flex hose.

 

I have an extension mounted the axle for the spring rod. I suspect the leak is more related to age, water crossings (I've had stuff floating around in the cab and the boot was full of crud), etc. than anything else.

Under further investigation I don't think I found a NOS unit. The illustration looks different than my unit. Since I can't use my VIN to verify the part number (I have a 10 digit VIN) I don't even want to try that one. Does anyone know the part number of the LSPV for an '80 RN47 pickup? Or a source for a 2 port LSPV? I might still consider replacing it if I can find a replacement for a reasonable price that I'm confident will work.

I bypassed my LSPV and took my truck for a test drive. The rear tires definitely lock up too early. I don't see how some people can get away with just bypassing the LSPV on a Gen 1 pickup with stock brakes. So I'm off to figure out how/where to mount a manual proportioning valve in the cab. This is likely the path I'm going to pursue. I'm in California and don't have to worry about safety inspections, just smog inspections which is enough of a PITA.

 

Well I don't know if its the same one but toyotapartsdirect.ca has one for 81-83's. Part # 4791026010 and it can be yours for the low low price of only $526 CND retail; discounted to $327 CND...so that's about $2.50 USD! Seriously the exchange rate is about 25-30% in your favour!

 

That is the part number I thought I had found but when I looked at some sites that had illustrations it looked a little different. However, the illustrations looked like they were a blown-up view so it may be misleading. I suspect that is the correct part number but it isn't worth the risk and cost to me.