Brakes: Load Sensing Proportional Valve LSPV and LSPBV

[straygoose]

I just got mine back from some brake work and I still seem to be having a problem with the fronts locking up too easily. At first it was the left front locking and pulling the wheel over, so I put her in the shop and found out the right caliper was bad. So that got replaced. Now, both of them seem to be locking just way too easily for comfort.

Could it be that I just need to back the adjustment off to more of a rear-brake position? Could the valve be seized? Or is it something else entirely? I had the hoses checked out and they seem to be OK, and the mechanic didn't believe it could be the master cylinder, so what gives? I looked under there when I got the vehicle back, and it looked to be about neutral, maybe set a little toward the rear brakes. I'm waiting for the rain to stop so I can go to look again.

The mech said that I had strong pressure going to the fronts but not so much to the rear drums. I assume that is normal, being that drums don't need as much hydraulic pressure as discs do. But he repeated that 3-4 times in the half-hour we talked so I'm wondering if he meant that it was lower than it should be?

Of course I have had this RV for about a week and a half now, and it is my first Toyota, so this may be the norm for them and I just don't know it. If its not though, any information would be useful.

[waiter]

Adjust the proportion valve wide open, disconnect it and wire it open. remove it and replace it with a "T"

any of the three options will get you maximum braking to the rear wheels.

John Mc

88 Dolpnih 4 Auto

[straygoose]

Sorry I took a bit to answer. By the valve you mean the lever mechanism on the rear axle? I'm a little confused. I think it's time to download that manual.

[Derek up North]

Yes, you want the valve wide open. The LSPV is the simplest attempt at making the braking system 'smart'. It's only got 1 'input' of information, the ride height. Add a set of air springs to the rear axle and this 1 'input' is all screwed up, fooling the valve into thinking you're not heavily loaded.

[straygoose]

OK, so I'm going to back the lock nut off to the top of the post. I don't have air bags so I don't have to worry about that.

Is it possible to go too far and end up with rear bias? That's probably just as dangerous as the problem I have right now.

Edited November 2, 2012 by straygoose

[Derek up North]

I don't think you've got to worry about too much rear bias. But certainly, find a nice empty stretch of road and test them before going out onto a busy freeway!

[Vanman]

Best way to check brake bias for rears try braking down hill, if too much rear brake they will lock up as we don't have antilock. Another check is try stopping on a hill going backwards. If fronts lock up too easily try again and add rear braking via the parking brake, thats what I have to do with mine. I remember somewhere here another poster saying that's what they do. Now that all the brakes are done on mine will see what's what. When we did the rears at 85 K the shoes were 1/2 down.

vanman

[straygoose]

I don't think you've got to worry about too much rear bias. But certainly, find a nice empty stretch of road and test them before going out onto a busy freeway!

I kind of figure as much, since the locking up comes at slow speeds. It probably also happens at high (+35mph) speeds but I haven't had to get on the binders real hard out on the highway.

Sometimes though, I feel like it's still just locking up on one side (front left). Maybe its just because I'm on that side of the vehicle too, but it just feels that way to me. And like I said it seems to be locking up at slow speeds (10-20mph).

I'm not really up for testing her by slamming on the brakes at 40mph, seeing as I've only had her a couple of weeks and I'm still getting familiar with her handling, or lack thereof. Don't really feel like laying her over on her side just to see if she wants to stop quickly.

That would suck. Big time.

Edited November 3, 2012 by straygoose

[WME]

You can feel the difference driving normally. You can NOT adjust the arm to far, mine is wired up to the frame.

[Derek up North]

It probably also happens at high (+35mph) ...

I think only in a Toyota motorhome would 35+ mph be considered a 'high speed'!

[straygoose]

/\ This is funny! +1 to DUN

/\

You can feel the difference driving normally. You can NOT adjust the arm to far, mine is wired up to the frame.

I'm going to back mine off either sometime today or tomorrow. If that fixes the problem great, but disabling it is something I don't feel comfortable doing. Thanks

[straygoose]

OK, I crawled under there, and once I realized it was a five minute adjustment (DOH!), I grabbed a couple of wrenches and commenced to cranking. I thought I might need to get her with impact tools or dynamite or something, but the whole assembly came apart in a minute.

I backed her off a good 1/2" as you can see in the photos. The shiny area on the threaded rod is how far I backed her off.

Took her around the campground here and she seems to be more balanced than before. The fronts still locked up, but it seems to take a lot more pressure on the pedal than before, so that's a good thing. I have to go into town tomorrow, so I'll check her out a highway speed then. I figure if worse comes to worst, I can always crawl under her in the WalMart parking lot and either back her off more or bring her back into a more neutral position. I do have about another 1/2" of adjustment to make if I need to increase the rear bias.

In the photos at the top of the post, it shows that the lever is held in place by two opposing nuts with lock washers. On mine there is only a lower nut with a lock washer, and the top nut is actually welded into the lever itself. Obviously an engineering change. I'm sure others have the same setup, so does anyone know when they changed it? Just curious.

This has been very helpful. Hopefully the problem is solved with just a little more adjustment to the bias. Thanks to everyone.

[Derek up North]

I don't remember if you have air springs or not, but if you do, change the air pressure will throw things off and you'll have to start adjusting all over again. That's why most advocate simply just adjusting it all the way.

[straygoose]

No, no air bags on mine. It does have spring helpers though. Will that change things?

[Derek up North]

No, since they're not adjustable. Possibly when the helper springs were installed the valve wasn't adjusted at the same time. The helpers would fool the valve into thinking that you were lightly loaded, even though you aren't.

[straygoose]

That's what I'm thinking too. Well' I'll find out tomorrow if everything is OK. I have about six miles of state highway back into town to check her out on. I think I'll probably just have to back the valve back a little further and that will be that.

I've probably got 300lbs. of personal gear on board. I'm not sure if that's an overload or not. It better not be because I have a few things to add to that and a couple of things that need to be added in the way of amenities, like solar panels, a microwave, beefed-up batteries and such. Maybe a lightweight scooter of some kind.

[straygoose]

Hey! Things are much better. Still not optimum, but I'm going to back the setting off a little more and leave it at that. I'm still pulling left just a bit, so as soon as I find someone that can make me some new brake hoses for her, I'll swap those out. That should cure the problem.

Thanks again to everyone. I may not have kept her if I couldn't get the braking a little more up to snuff.

[straygoose]

OK, quick update for anyone who wants to know...I backed the valve off to full rear bias and the braking finally seems to be acceptable. I still wouldn't want to follow anybody at closer than 6-8 car lengths, no matter what speed I'm travelling at. But, I can live with what I got, at least until I investigate the one-ton hubs for the front end.

I don't like the idea of having two different lug patterns anyway. It was not one of Toyota's brighter ideas, if you ask me.

I have one more pending issue: I am still pulling just a bit to the left under hard braking. I suspect I have a brake hose collapsing on the right side. Is there a company out there who can make the hoses for me? Can't seem to find anything in the local parts stores, and no one around that could do custom work, so If anyone has a web address, I would be most grateful. Thanks

[waiter]

A lot of us just disconnect the proportion and wire it to max braking.

As for the pull,

Brake hose shouldn't do it, the pressure will be equal on the left and right.

Could be pads or rotors, one side a little more varnished or hardened the the other

could be a soft spring on one side, The nose dives a little more on one side

could be a minor camber adjustment on one side. When the nose dives, it changes the toe in on one side.

I'd recommend looking at the pads and rotors first.

John Mc

88 Dolphin 4 Auto

[straygoose]

Nope, got new pads and calipers, rotors are good, and the system has been bled three times over. Just got the job done about ten days ago. The only reason the hoses weren't done then is I that neither I or the mechanic could locate a set in Baltimore, and I had to get out of town before that storm, so I couldn't wait around to do some research and mail-order the parts.

Alignment is good. I don't sense or see any spring sag, and it's not visible to the eye. My bumper is level. Matter of fact I was putting bubble levelers on my dash today, so I fiddled around with her until she was absolutly level, both front and back and both sides. Actually took a tape measure from various points on the nose and tail and the heights were symmetrical, so the chassis is as level as she's gonna be, considering her age. BTW, neither bumper has been banged around, so I know they're straight.

The left front locks up while the right front doesn't, which is why it pulls. Granted, it locks up a lot less than two weeks ago, when it was probably providing 75% of the braking, but it will still lock up when you lay on the pedal. It's simply getting more pressure than the right side is. That hose has to be at least semi-collapsed.

The only thing on the front brakes that weren't swapped out were those hoses and the master cylinder. The master is good, so it has to be that hose. I'm just going to have to find someone that will make custom hoses and then I'll just put the new ones on both sides. Should solve the problem, or at least as much as you can solve the braking problems on these things.

So if anyone has a fave place where they order stuff like this, I'm all ears.

[Derek up North]

So if anyone has a fave place where they order stuff like this, I'm all ears.

This might be a source:-

http://epc.uhaul.com/LinkOne/Viewer/Default.aspx?ReturnUrl=/LinkOne/Viewer/default.aspx

[straygoose]

Oh hey, I hadn't thought about U-Haul. Thanks Derek.

[straygoose]

Hmm. gonna have to get in touch with them. They only have parts from '89-'93, and I have an '87. Maybe it will bolt up, maybe not. Forty bucks for one hose! BTW don't break your Load Sensing Valve. It's almost $150!

[Derek up North]

U-Haul only started with the Toyota in '89. Guess they figured it wouldn't work without a V6.

[WME]

Does this help?? Most seem to be under $10

http://www.rockauto.com/catalog/x,carcode,1277502,parttype,1792

[waiter]

Tire pressure, tire difference might also do the one side.

John Mc

88 Dolphin 4 Auto

[Maineah]

This ^ check them all.

[straygoose]

DOH!. I was going to have my tires rotated on Monday or Tuesday, so I haven't really checked the pressures lately. For some reason I am really bad about montioring tire pressure. Thanks, guys.

Does this help?? Most seem to be under $10

http://www.rockauto....2,parttype,1792

WOO HOO! I forgot all about Rock Auto. Those guys have everything, it seems. Thank You very much.

[straygoose]

U-Haul only started with the Toyota in '89. Guess they figured it wouldn't work without a V6.

Little did they know, the straight 4 can pull a house!

[zero]

I don't see any problem with increasing rear wheel bias in our rigs due to the constant loading.

The issue is when the LSPV is eliminated entirely from the braking circuit. Disc brakes by their design require higher fluid pressure to operate as compared to a drum brake. The reason being the pads in the calipers clamp the rotor, more stopping power requires higher and higher pressures. A drum brake is "self energising". This means that when the brake shoe is pushed into the drum, the rotational force actually pulls the shoes more tightly against the drum. I've read references that stated a caliper/disc brake requires two to three times the fluid pressure of a comparable drum.

So what this means is by eliminating the LSPV you have equalized the front/rear braking pressures in the system and the rear drums are exerting a larger braking force than the front discs. I personally feel you have just negated a substantial portion of your braking power by doing this.

I don't believe that is completely true. The majority of motor vehicles with disks in front and drums in back do not have any sort of load-sensing proportioning valve in back - and work fine. The different pressure needs for the shoe-brakes in back versus the disk brakes in front is usually taken care of by the design of the two stages in the master-cylinder and/or a fixed proportioning valve somewhere. The load-sensing valve on back found in heavier trucks is an added feature. I've had the one on my 1 ton truck disconnected for years and my brakes work fine. Granted it may be that Toyota trucks with the load-sensing valve on back may work better with it when varying loads are involved - otherwise it would not be there. I just drove home a U-haul type box truck that was minus that valve and the brakes felt fine to me. If the rear was over-agressive the back wheels would of skidded in hard braking. If underagressive - the front wheels would of skidded (which happens often in many of my GM cars and trucks at times). I guess that's why ABS was invented. And yeah - I'm somewhat guessing at the specifics for the Toyota. The 1 ton has heavier brakes in front and back so I can't say for sure how bad the need actually is for the load-sensor. I live in the rust-belt of NY so in heavy trucks - the rear load sensor is often the first part to get ruined by road salt and removed. I've noticed much difference without it.

[zero]

The left front locks up while the right front doesn't, which is why it pulls.

I've had that happen on several cars and trucks. Caused by the brake hose the came apart in the inside where you could not see the problem and blocked off flow. I also had one do the opposite on a GM Safari van and it caused a caliper to lock and stay locked. The bad hose-on-the-inside thing is somewhat rare but it happens.

I've had several Toyota one-tons dual-rear trucks apart in front -with the big brakes. The hoses on all I worked on including the 1986 four-cylinder box truck I just took apart - have hoses that measure 14" end to end. Each end has a female inverter-flare female threaded port with 10 mm X 1.0 threads. Also has snap ring grooves to hold in place. Very common part when you look buy size instead of odd-ball Toyota application. When not in a hurry I buy from Rock Auto but any NAPA also has the hoses. NAPA #s are - 38916 for the 13.5" hose like Rock Auto sells as Bendix # 77664. Dorman sells the same hose as # 36738. Other hoses that work include: NAPA # 381612 at 13.8", NAPA # 381377 at 14.1", and NAPA # 38802 also at 14.1"

[straygoose]

Excellent! Thank you very much. I was going to go to NAPA either tomorrow or Tuesday for some other things, so I'll just add those numbers to the list.

[Lee & Joan]

AH, the light goes on, GREAT POST. Now I know why both my front tires skid after the first rain in the fall, when backing up, in panic stops. The whole front end has new everything and the back end is in good shape but this has been scary. Of course I have my airbags nearly maxed out, DUH. Got to check that valve tomorrow. Never felt much front end dive, but man this rig would lock a front wheel on a hot tar snake in the summer & on an wet oil contaminated road in the fall, and anytime on gravel. Just the other day at 1/2 MPH I almost plowed into my 2 ton flatbed when the fronts locked while turning on wet gravel in the driveway and we were headed straight into the truck. I got OFF the brakes (counter intuitive) and was able to steer out of it. With dirt bikes you use almost all front brakes unless you want to slide the rear around something. This is just the opposite.

 

[linda s]

I adjusted mine all the way and it made no difference at all. Worth a try but

Linda S

[Lee & Joan]

Are we looking at LSPV where the proportioner limits fluid pressure to the rear brakes or LSPVB ? I see mention here of "blocking the return line to the front brakes" as part of eliminating the valve. Is this as simple as replacing the valve with a "T" fitting going to the two rear brakes and to a single line to the master cylinder, or is there another line that returns to the front? Others here have stated that you can disable the valve by detaching the control rod/spring and "wiring it open", just how is that done? I am finally getting around to doing something about this, have read the entire thread here and the service pdf's given in the first post. I would rather just put a T in thus bypassing a valve that would seem to restrict flow to the rear even when new and "properly" adjusted. Has anyone put a T in?