I'm a little confused about how the brake circuits work in these trucks (1990 pickup).

I see one line from the master cylinder T'd to the front left brake and a line running from front to rear to the load sensing proportioning valve.

The other line from the master cylinder is T'd to the front right brake and a second line running from front to rear to the load sensing proportioning valve.

But then there's only one line out of the LSPV that gets T'd to both rears.

I'm not sure what's going on inside the LSPV, but I thought that these circuits had to be independent of each other. Wouldn't a failure in any of the rear brake lines depressurize both circuits? Wouldn't a failure anywhere?

http://imgur.com/dlUGhcQ




Am I reading this right? How does that work?

 

Does your truck have the early anti locking brakes with the valve on the right side on the frame in the front ??

I have older and newer and hate to steer you wrong

On mine both lines from the LSPV tie into a tee fitting on the front brakes if I remember from when I removed the LSPV

 

There's different kinds of LSPV. But they essentially work the same. I have a hard time keeping it all fresh in my mind as to how they all work. But I've done a lot of reading up on it.

Short answer: Toyota has you covered. You've got nothing to worry about.

More info here: https://web.archive.org/web/20141220...utoshop15.html

 

I looked at it more closely today and it looks like a front/rear split for the two circuits, not a diagonal split.

So I'm guessing that what happens is, is that you have your front system and your rear system, and two lines running front to rear because one of them is for the rear brakes and one of them is to send fluid bypass up to the front (it is a Load Sensing Proportioning *and* Bypass Valve after all).

The two lines had me confused. Makes sense.

The reason I ask in the first place is because I replaced both of those lines recently and didn't pay any mind to which lines went to which ports on the valve I plugged into because I didn't think it mattered. It looks like it matters.

 

Yes it does

If you got them swapped you might have some interesting braking .

Those valves are a pain I don`t think I have any that still work.

Then trying to get the bleeder valve broke loose was so very expensive .