Brake Proportioning Valve

B

Bart_

Carpal tunnel level member
Location
GTA
Many moons ago cars had simple braking systems. A master cylinder had lines to the front and rear brakes. The designers sized the barking power and cylinder or piston sizes to get the front rear bias they wanted. Then came proportioning valves where you could tweak less pressure to the rear to prevent rear lockup before front lockup so the butt end of your vehicle wouldn't overtake you in a panic stop (the front would lock first so you skid in a straight line). Then came load censing proportioning valves I think in the 80's so if you had 1000 lb bricks in your bed you could up the rear brake pressure. In the 90's ABS kind of took over, sensing the wheels and diddling the pressures. My dilemma is from the load proportioning era.

Got a 90's 4runner with one of these valves mechanically spring linked to the rear sag. More sag, more pressure to the rear. The valve itself will act like an attenuator to the rear cylinders but also redirect what it attenuated back up to the front pistons i.e. there's two brake lines to the rear but one is actually from the valve to feed the fronts. I've got an air gremlin, can't keep the brakes bled. People usually see leakage when the valve fails, I don't see any. When I vacuum bleed there seems to be a finite quantity I can pull out, definitely not empty the reservoir. Oh and there's a rear ABS plumbed in, I'm not exactly sure but its been inactive for many years. Toyota calls the valve LSPV. Weird thing is it's non constant i.e. at low hundreds psi it's 50/50 bias and changes as you hammer the brake pedal according to FSM.

Some suggest Master failure. Can't convince myself yet but might have to throw money at it and see if I win. Master is way less $ than the LSPV. Lines cylinders callipers all look good to the naked eye.

Consider this a sheepish throwing of a fishing line out there for help. Maybe someone has been down this road already. ?
 
Well I've never dealt with your exact issue but I dealt with a similar one on my '92 F250. For me it turned out to be the RABS(Rear Antilock Brake System) valve that had sucked in some air and prevented a firm pedal until it was bled proper. I did look up the details on an 4runner brakes from mid 80's to mid 90's and the system looks pretty basic as far as possible failure points.

Since you don't say how/when the problem began I will assume it just kind of started doing it and this was NOT right after performing work on the brake system.

What are you bleeding that seems to only release a finite quantity of fluid? The LPSV? I assume brake fluid can be bled from all 4 corners--as in no plugged lines.

In the spirit of Occam's Razor I would just replace the MC, especially if it is very old or of unknown age. Remember to bench bleed. Also, you probably know that if the fluid level in the MC gets low enough while installed on the truck you will need to remove it and bench bleed it to remove air. Maybe a power bleeder can fix it without removing the MC but IDK.

You might also grab some new bleeder screws if the current ones are in rough shape. I tried the vac bleeder but found it more trouble than it was worth. I prefer the cheap-o DIY rig of just a clear piece of tubing, 3/8 or 1/4 I think, slipped over the bleeder with the other end inserted into a small bottle of clean brake fluid--make sure the end is completely submerged in fluid so you don't suck air into the system. Wrap wire around the bottle so you can hang it from the chassis while pumping the brakes.

Good luck to ya.
 
I've got an '89 Toyota one ton uhaul box truck with the proportional valve. Mine hasn;t acted up but I do recall other threads that sounded like your situation. The solution was varied. I can't recall one unique solution. Fron what I recall you're following the proper steps. Going from simple to complex.

Spending some search time on a Facebook fan page might help focus your next steps.

Ohhhh...I just thought of something. There is a unique pattern for bleeding the lines with a prop. valve. I don't recall what it is but it seems like its different than the Rt Rear/Left Rear/Rt F/L F. The valve changes something on that sequence
 
Update on the air gremlin. I've ordered a master (Advic is supposed to be renamed OEM Aisin) and two off brand wheel cylinders, least cost for starting shotgunning. Sat in the truck today, engine off, first three pumps pumped the pedal up high and solid, 4th and 5th pumps Lowered (!) the pedal and it went soft. Never seen that before!

Soft pedal was slow onset, driven. Previously driver rear corner bleed has found the bubbles and solided up the pedal. Lines route from LSPV to passenger rear, across diff to driver rear. On a different truck with perpetual minor air buildup and cross diff line the driver rear cylinder was the culprit. "Limited" fluid quantity was actually a lot, but I was perplexed why I couldn't just keep drawing out more, with bleeder open and substantial 1/2 gauge vacuum. Possibility - bit of crud blocked the bleeder? Reservoir cap was off.

I might put in another bleed session, maybe include a tube to reservoir bench bleed and/or a few permutations of order.

The ABS pressurizes off the power steering pump and has a modulator unit. In the past I've changed front and rear lines and callipers without special ABS bleed procedures. FSM doesn't seem to have special ABS procedures either. If case I didn't mention, shoes are fresh snugged up.

Thanks, I've gone blurry reading yotatech, 4runnerorg Ih8mud toyotanation etc as google found the threads:) The LSPV is an unrebuildable golden unicorn. $600!
 
Bart_ said:
Many moons ago cars had simple braking systems. A master cylinder had lines to the front and rear brakes. The designers sized the barking power and cylinder or piston sizes to get the front rear bias they wanted. Then came proportioning valves where you could tweak less pressure to the rear to prevent rear lockup before front lockup so the butt end of your vehicle wouldn't overtake you in a panic stop (the front would lock first so you skid in a straight line). Then came load censing proportioning valves I think in the 80's so if you had 1000 lb bricks in your bed you could up the rear brake pressure. In the 90's ABS kind of took over, sensing the wheels and diddling the pressures. My dilemma is from the load proportioning era.

Got a 90's 4runner with one of these valves mechanically spring linked to the rear sag. More sag, more pressure to the rear. The valve itself will act like an attenuator to the rear cylinders but also redirect what it attenuated back up to the front pistons i.e. there's two brake lines to the rear but one is actually from the valve to feed the fronts. I've got an air gremlin, can't keep the brakes bled. People usually see leakage when the valve fails, I don't see any. When I vacuum bleed there seems to be a finite quantity I can pull out, definitely not empty the reservoir. Oh and there's a rear ABS plumbed in, I'm not exactly sure but its been inactive for many years. Toyota calls the valve LSPV. Weird thing is it's non constant i.e. at low hundreds psi it's 50/50 bias and changes as you hammer the brake pedal according to FSM.

Some suggest Master failure. Can't convince myself yet but might have to throw money at it and see if I win. Master is way less $ than the LSPV. Lines cylinders callipers all look good to the naked eye.

Consider this a sheepish throwing of a fishing line out there for help. Maybe someone has been down this road already. ?
Click to expand...
I had a 1980 Toyota that had this, there was a spring arm which would push on a plunger. I can’t recalll what the problem was but the fix was just removing the spring arm that would depress the plunger, essentially removing the function. I really didn’t notice a difference but never took the truck onto the freeways
 
Google found this nugget on the mystery components, fyi if you're a Toyota of the era dude: (from yotatech)

LSPV and Rear ABS info
I have wondered two things about these valves since seeing them:

1. Why is the front brake circuit plumbed into LSPV?
2. Why is the power steering pump plumbed into the Rear ABS?

Last night I had the good fortune to get these two questions answrered. I'm a brake engineer at the Toyota Proving Grounds and have been waiting for my next trip to Japan to find an 'old' brake guy to ask. As luck would have it, he came to me.

We had a brake supplier now named ADVICS (a merger of brake divisions of Aisin and Sumitomo) in town this week. As it turned out, I had dinner with the original Aisin Seiki design engineer who was onsite at Toyota Japan and responsible for the LSPV and Rear ABS when it was later introduced.

He stated the following (not everything was a perfect translation due to the language barrier, but close):

LSPV

The front circuit to the LSPV acts as a safety bypass valve. There is a low pressure plunger or piston that is moved by front brake pressure and redirects fluid to flow into the 'business end' of the LSPV to get proportioned. Since the 4Runner is Front/Rear split its considered a failsafe if the front circuit fails. With no pressure from the front circuit, the plunger stays in place and is simply a though valve allowing full pressure to the rear brakes. Even though the rear brakes suck, they would suck much worse if you lose the front circuit *and* were proportioned by the LSPV. There is a chance though of locking the rears if the vehicle is unloaded I would guess.

Rear ABS
When I first started in brakes in '94 for Kelsey Hayes, I had responsibility for their rear ABS system. It was a simple valve tied to a sensor on the rear diff that read from a fine-toothed ring gear called a tone wheel. These systems were primarily on trucks with light rear ends and prevented the the rear wheels from locking up. All it did was isolate pressure from the master cylinder when the rear wheels locked, and dump pressure and dump pressure to get them spinning again. It was considered passive ABS. There was no reapplying of pressure like any modern 4-whl ABS. It was also a single channel system operating off of a single sensor which meant that the LR and RR wheels acted the same and were not independent of each other while in ABS. Imagine the diff was locked as far as ABS is concerned.

The Toyota Rear ABS is considered active because it can reapply pressure to the rear brakes. Enter the power steering pump circuit into the Toyota Rear ABS - its means by which pressure is reapplied.

When you apply the brakes and lock the rear wheels, pressure is isolated from the master cylinder and then its dumped to get the wheels spinning again. The power steering line is plumbed into a chamber in the ABS valve that has a piston: on one side of the piston is power steering pressure (controlled by a regulator in the ABS valve) and on the other is brake line pressure. When pressure is dumped from the rear brake circuit due to wheel lock, it is reapplied by the pressure from the power steering pump side of the piston pushing the now low pressure brake fluid side of the piston. That piston force from the power steering side of the piston builds brake pressure back up until its dumped again. This whole process is repeated many times per second until the brakes are released.

That's why the power steering pump is plumbed into the Rear ABS.

So now I know...

- end quote

so the actuator might be failing and causing my soft pedal, looks like it's a series element I can try bypassing.
 
2005 4Runner Sport had brakes redone last year practically from ground up. I told them it was master - pedal fade is typical - Toyota has a diagnostics procedure before you go to - you guessed it - the master. Two shops diddled around till I took it to a Toyota dealer out of town that a lot of the climbing guys swear by - they were right on it, after contacting California Toyota mothership, but the master was almost $7000 paltry devalued tattered and debt ridden Canuck bucks. Ouch. But to be fair to them . . . the master is like the space shuttle, with the ABS integral to it. So maybe kinda same issues as yours? I so long for the days of 55-57 Chevies and simplicity (even did Rochester 3 two's once).
This and a busted ring gear tooth within a year old, on warranty, have been the only issues all this time. Fingers crossed. Cuz man are these expensive to fix, at least up here. Enjoyed your note. Cheers.
 
I'm now going dinosaur on it, after bypassing the inactive ABS actuator made no change. Decoupling front and rear, stage 2 is after market proportioning valve. Fun fun fun in the winter weather :)
 
You must log in or register to reply here.

New threads New posts

Back
Top Bottom