ahickman

Alright, can't find it in the FSM or through a search. I have an 86 runner and have had to replace the LSPV and want to make sure I have the plumbing correct since the hardlines were replaced by the PO, I am not sure which is which. The LSPV has three ports (not including the bleeder). Two ports on the forward side and one on the rear side. The rear side is, I believe tied to the rear brake line. The forward two are not clear as to which one is the high pressure side and which one is the low pressure (return) side. Going from top to the bottom, which one is which?

thook

Give me less than an hour and I can look at my 4rnr and tell you.

ahickman

You da man! Thanks.

thook

*sigh*.....okay, decided I better feed the dogs, pigs, and donkey before it got dark.

Anyway, the bottom line on the rail is the pressure line/comes from the MC. The top returns to the front. And, the rearward port goes to the rear brake line. However, you might want to trace the lines starting from the MC to make sure the PO didn't flip things around. Don't really see that happening, but you never know, eh.

ahickman

Matthew, if I understand correctly, the bottom port on the LSPV is fed by the same line that feeds the (through a T valve) front right brake caliper..which is fed by the MC?

abecedarian

Contrary to popular belief, there is no return.
If there were a 'return' or any way for the fluid for cross from the rear to the front circuit... it sets up a situation where total brake failure is possible and which is also in violation of FMVSS standards.
Many people think excess pressure from the rear is bypassed to the front so the rears don't get full pressure... and that's actually quite an absurd thought since FSM tests have the front pressure set higher than the rear. So how do you divert pressure from a lower pressure system to a higher pressure system? That's like connecting my garden hose to a fire hydrant and expecting my garden hose to push water into the hydrant.

What the LSPV does is allow full rear brake pressure in the event the front brakes fail regardless of the load in the rear, and also allows variable pressure to the rear depending on load (provide the front circuit is fully opeartional).

One port is from the MC front brake circuit and the other is from the MC rear brake circuit.

So I think thook answered the question:
bottom forward line comes from rear circuit on the MC
top forward line comes from the tee behind the right front wheel
rear line goes to the rear wheels

thook

Dang dude, you'z impressive as always. Thanks for the clarification.

abecedarian

hehe... I was only looking over your favorite site: autoshop101.com

(not really but they have it there too)

thook

That site is so packed with info. I've yet to even look at it all. There's more there than I need to know about my vehicles, but even with them I haven't studied it all. Yep......I do like that site.

ahickman

After reading the as100.com section on LSPBV (which I incorrectly referred to as LSPV above) I have a clearer picture of the plumbing routes. Abe, would I be correct in suggesting the following in an effort to further troubleshoot rear brake (lack of) action:

1. disconnect front brake plumbing at the LSPBV to simulate a front line failure.
2. Plug the removed line to lessen the unecessary lose of fluid
3. Leave the LSPBV port exposed
4. Apply brakes and check for rear cylinder actuation

If the LSPBV is fully functional, the rears should receive full fluid pressure having bypassed the LSPV?

dropzone

iTrader: (6)

OK, since you and the majority of us have been incorrectly referring to the valve as a LSPV, enlighten us, what is the 'B' for. Looking at a Toyota FSM now and they left out the 'B'.

ahickman

"B"ypass

thook

Hahaha.....load sensing proportioning valve, bypass, biotch..........whatever. Atleast we all know what it is.

I'd plug the exposed port too, Aaron. You're going to lose fluid and suck in air with it open.

abecedarian

If you want to simulate a full front brake system failure, leave the port from the front brakes unplugged, or plug the port and open the bleed valve. Otherwise you will be building pressure in that part of the valve as the rear applies pressure: if the front port is plugged, it may allow whatever fluid and / or air is in there to build up enough pressure to prevent a full bypass.

thook

Shiznit..........guess I need to a little rereading on that valve.

abecedarian

front brake pressure is on one side of the valve and rear brake pressure is on the other side of the valve... the "bypass" valve, if you will.
if there's enough pressure built up on the front side when the rear pressure pushes the valve in the front side's direction because the front side is closed off (plugged?), it may think you have some front brake pressure.

abecedarian

and if the front side is plugged off and full of brake fluid, which doesn't compress, will the valve work correctly and apply full rear brake pressure to the drums?

ahickman

Abe/Thook, I replaced the LSPBV and the rears are locking up. However, I now have a different problem in the fronts. Both calipers are sticking if I mash down on the brake pedal. I have to bleed off the pressure (through zerk) on either caliper. Seems like the master cylinder is building pressure and not releasing it? Can only assume that the MC is bad and needs to be replaced since both calipers are experiencing the residual pressure?