fastgtfairlane

I have a 94 4runner 3.0L, 4x4 5spd sr5. it has 298k on it. I recently replaced my rear axle bearings and seals. i replaced both rear brake shoes and wheel cylinders 2 years ago. had it apart for a few days with some rubber vacuum caps covering the rear brake lines. put it all back together and when i tried to bleed the rear brakes, starting with right rear, I got a burp and nothing else. since then i have no fluid to either rear wheel. I checked fluid at the LSPV and was able to bleed it, but nothing coming out of the top of the valve that goes to the rear wheels. I can bleed the front wheels fine. the fluid was dark. It appears that the LSPV is stuck closed, bypassing the rear circuit. any way to unstick it or is it bad and needing replacement? id like to know for sure its bad before replacing it due to cost. ive been doing searches on this for a few days and have found several threads related but i didnt find any difinitive answers. thankyou again for your suggestions.

2ToyGuy

I'm pretty sure you can pull off the LPSV and clean it out. Maybe a little compressed air, low pressure. Maybe a little brake cleaner all through the LPSV, followed by compressed air to get the cleaner out.

Did you maybe try disconnecting the rod from the LPSV to the axle at the axle end and moving it end to end of it's throw? Might just be a small chunk of blech in the LPSV that will break loose as you move the axle rod. Worth a try, anyway. Or the compressed air into the valve where the brake lines come in from the front end, and movement of the axle rod at the same time. Again, worth a shot.

I wish you the best of luck, whatever you do.
Pat☺

fastgtfairlane

yeah thats what i was going to try next. unfortunately, its out in the weather on a gravel/dirt driveway and we had rain all weekend so i wasnt able to really do anything. i considered trying to back feed the lspv with pressure from the front brakes. i was going to try removing the bleeder and making some brake line to connect it to the outlet side to the rear brakes to see if i coudl free the valve up. also concerned the master cylinder might be worn and cant build enough pressure to pump the rear circuit up now that i opened the circuit up.

akwheeler

Are you doing this with the truck jacked up (axle drooping) or at ride height?

fastgtfairlane

vehicle is jacked up on jackstands which are under the rear axle so technically it would be like having a load on the axle that way?

akwheeler

Yes, if it's supported under the axle the vehicle's suspension would be at ride height and the brakes should bleed properly.
That is assuming the the arm on the LSPV isn't bent or the valve is faulty.
If you were to disconnect the arm from the axle and raise it toward the underside of the vehicle you may find that you can bleed the rear brakes easier.

fastgtfairlane

i was going to try that next. i did buy a replacement lspv on fleebay im not sure if it will fix the issue or is the correct valve however they do look the same. looking at a line diagram of the brakes, it appears that the front line off the master, goes under the master and tees off to the front calipers. on the line going to the passenger side has a 2nd tee which feeds the passenger front caliper and also feeds the top fitting to the lspv. the rear line on the master goes straight to the lspv. and the top fitting feeds the rear brakes. is that correct?

im also concerned i may have a bad master so i wanted to check flow at the lspv from the master. im not positive that it wasnt replaced before i bought it and could be bad. now that i released pressure to the rear cylinders, the master is too worn to pump back up. similar to a auto trans with old fluid can have problems with clutch pressure after the old fluid is emptied and replaced. its just a theory on the master at this point.

2ToyGuy

Did you adjust the pedal correctly? It doesn't take much of a misalignment of the pedal to make the master cylinder SEEM bad.

Is the vacuum to the booster good? Does the booster hold vacuum after the engine is shut off? See the FSM for how to check the booster.

Also, if the pedal is adjusted correctly (see the FSM, as always ), did you bleed the master first? If there's air in the rear brake portion of the master, well... I mean the piston that feeds the rear brakes, not necessarily the piston at the rear of the master, although I believe they're one and the same. I might be wrong on that, though. If you think that the master isn't making enough pressure for the rear brakes, it's easy enough to check. With the engine running, pull off the line that feeds the rear brakes, put your finger over the hole, and have someone press the pedal. You'll be able to feel how much pressure the master puts out. Be careful, it could get messy Make sure that you seal the hole in the master the rear brake line connects to with your finger, when the brake pedal is released. You don't want to let air into the master when the pedal is released.
You can do the same checks at the LPSV, too. Just pull the line that feeds direct to it from the master. Again, though, make sure you don't let air into the system. AND bleed after, so any that did get in is bled off.

Edit: I forgot to mention, you can get rebuild kit for the master cylinder, or a whole new master, for that matter, at RockAuto. New master is anywhere from about $40.00 to about $80.00.
Also, if the o-rings on the master's pistons are worn out enough to let fluid flow past them when the pedal is pushed, making the pressure and fluid volume to the rear brakes/LPSV low, or leak down if the pedal is held, I believe you can replace them with o-rings from a commercial source. Harbor Freight, McMaster-Carr, something like that. Just like the idle adjust screw. Did you check to see if the pedal leaks down? Ie: with the engine running, press the pedal hard, and hold it there. If it goes any lower over time, the o-rings are shot. Good test of the master. I just wanted to throw this in...

Yes, you've got the lines correct, as to what goes where.

Make sure the LPSV is adjusted on it's mount correctly, and that the "spring" that goes to the shackle that goes on the axle is adjusted correctly. Not bent, and so on.

Let us know what you find
Pat☺

fastgtfairlane

updates- finally got an opportunity to work on the 4runner again. disconnected the arm to the lspv from the axle and lifted it up. it sorta popped like it was frozen. ziptied it to the tailpipe and started attempting to bleed the rear brakes. finally started getting fluid to the rear cylinders finally! so i was able to bleed my brakes. the LSPV was the issue as i thought. glad i didnt have to replace it at least yet. we'll see after a road test.

87-4runner

Good deal let us know with an update how this story ends. 👍👊

2ToyGuy

After it popped, was it easier to move up and down? I would move it several times, while bleeding the LPSV. Any gnrrr that might still be in there should come out when you do that. Same with one of the rear brakes. Move the bar up and down while bleeding a brake, to clean out the LPSV.

Remember to bleed each brake, and the LPSV, until the fluid comes out clean and new, not just until the bubbles stop. Very important.
Make sure the Master Cylinder stays full of clean, new fluid. Don't let it get so low it draws air.

Glad you got it going right!
Have fun!
Pat☺

fastgtfairlane

yes it moved up and down easily after the pop. i did bleed all 4 sides and the LSPV last. after i freed the LSPV and had fluid at the cylinders again, i went to get more brake fluid to fully bleed the brake system. since i didnt have anyone to help bleed the brakes, i made my own bleeder. 1 liter coke bottle, 3/16 ID clear hose, drilled a hole large enough hole in the bottle cap for the hose, submerged end of hose into bottle with some fluid in it, drilled a second 1/8 hole as a vent hole. crack the bleeder and slowly pump the brakes until clean fluid comes out. pedal feels good now.

akwheeler

This method usually will only work if you have a check valve involved. You can get replacement bleeder screws designed for one man bleeding that have check valve in them.
Once you let off on the brake pedal any air still in the wheel cylinders or calipers is drawn back into the brake lines and just moves back and forth more than being forced out the bleeder.

fastgtfairlane

i would agree with out with the exception that keeping the line attached to the bleeder screw is sitting in brake fluid inside the bottle so any air in the circuit gets pushed out to the bottle and bubbles out and no air can get in. i continue pumping the brakes several times after the line was completely solid with fluid indicating the air was gone. im aware of the bleeder screws you mentioned, i just had this available to me at the time.

akwheeler

I guess you missed the point, if you don't get all the air out on the first pump when you let off on the pedal any air still in the brake caliper or wheel cylinder gets pulled back up the hose or tube towards the master cylinder. on the next pump it moves back toward the caliper or wheel cylinder and just goes back and the cycle repeats each time you pump the brakes.
eventually either the air bubbles will rise to the master cylinder or get pushed out of the bleeder screws, but it's far from efficient and not 100%.
I am sure it only worked for you because you didn't have a large air bubble to start with or you gravity bled it for a while.
If your method was a good one don't you think it would be the recommended procedure rather than using 2 people?
If your brakes work well without a spongy pedal think you just got lucky, even a blind squirrel can find a nut once in a while!

scope103

I don't think it works that way. The master cylinder is a pump; it pushes the fluid one way and is not designed to "pull back," any more than bicycle pump would. The spring on the brake pedal has nothing to do with opening the brake pads, the pad return springs do that. If the brake pedal "pulled back" the brake fluid, disk brakes could not be self-adjusting.

That's a little bit of an over-simplification; there is friction throughout the system, so if the bleeders are open to air the pedal spring will pull back a little fluid, and pull in a little bit of air. The real test is using a clear vinyl tube into the container of new brake fluid. On the first pump or so, there will be a bubble of air in the tube. With each subsequent pump, does the bubble just move back and forth, or does it progress toward the container?

akwheeler

I think you need to do some studying. If the fluid only goes one way where does it go?
your "bicycle pump" theory doesn't hold up, they have a check valve!
there are no "return springs" on calipers, only drum brakes. there are only anti rattle springs. the only "return" action is from the square cut seals around the caliper pistons.
when you release pressure from the brake pedal the piston in the master cylinder returns "pulls back" to it's at rest position which will pull air back through the hoses and lines if there is any in the calipers or pistons or if a bleeder is open. otherwise why doesn't every repair manual or tech school teach you to open the bleeders and just pump all the air out?
if you watch your clear plastic tube in your description above you will see fluid pull back when you (or a buddy) let off the pedal.
Trust me, I've been working on brakes for 40 years, the best way to bleed brakes is to keep the fluid moving in one direction by opening/closing the bleeders, using check valve bleeders, using a one-man pressure bleeder attached to the master cylinder or by using a one-man vacuum bleeder at each brake.
I had to learn the hard way but google is your friend now, start here:
How the braking system works | How a Car Works

akwheeler

Here's another link, if you perform steps 6 and 7 correctly the bottle and hose are not even needed.
How to Bleed Car Brakes (with Pictures) - wikiHow

scope103

I must not have explained this clearly. Let's look at just the front (disk) brakes. Say you're replacing the pads. They have 1mm friction material left, and the new ones have 10mm friction material. After removing the old pads, you need to compress the caliper to make room for the 18mm (total) thicker replacement pads. (You can do that because, as you pointed out, when the master cylinder is not compressed, there is no return check valve.) But you don't compress them exactly 18mm, you compress them "all the way" to give enough room to get everything back together. Let's say you leave only 1mm of space next to each new pad (2mm total).

After bolting it all up, you climb into the cab and step on the brake pedal: it goes all the way to the floor. You pump the pedal a few times, and it gets firm. What you're doing is pumping fluid out of the master cylinder into the caliper pistons, to take up that 2mm of space. (Because of the hydraulic ratio, the pads move very little compared to the stroke of the pedal.) If the piston in the master cylinder "pulled back" the brake fluid, the caliper pistons would move back and forth but never touch the rotor. And never provide braking. Similarly, as these new pads wear, more fluid leaves the master cylinder to keep pushing the caliper piston out farther and farther. It couldn't do that if the master cylinder pulled back all the brake fluid that was pushed out.

Why do we customarily close the bleeder on the "return" stroke of the master cylinder? There is some friction in the system, so the valving in the master cylinder will exert a tiny amount of "pull back" (not enough to move a caliper piston). The "pull back" should never be as much as the fluid pushed out of the master cylinder. The amount might change depending on the condition of the seals in the master cylinder. If the bleeder was open to air, it might, or might not, pull back a little air (a little is all it takes). If the bleeder is connected to a tube into a container of brake fluid, all it could pull back is fluid.

Sorry I didn't make that clear.

akwheeler

If we need to explain in intricate detail so be it, the piston in the master cylinder pushes on a rubber cup that faces the fluid side (inside) of the chamber so that when pressure builds it forces the edges against the cylinder bore thereby sealing the system (after it passes the internal port to the reservoir) and transferring pressure (force) to the entire hydraulic system. when the pedal is released and the piston returns to it's starting point it pulls the cup back with it, if the bleeders are closed and there are no leaks the edges of the cup will allow fluid to pass by since they are not forced against the cylinder walls. If there are any bleeders open or leaks in the system air will be drawn in when the pedal returns.
this is simplified somewhat because there are actually two pistons in the modern brake master cylinder, one for the front brakes and one for the rear, if either loses pressure (or has air in the system) that part of the master cylinder will not build pressure, but the other will after significant pedal travel.