nomadzoul

Hi , i have a 1981 Toyota 4x4 and i change to rear disc brake conversion. I have a 1 Inch master, a manual proportioning valve with a 2lb residual valve after it and the stock LSPV in the back. Up front i'm running a non vented rotor with 4 pod calipers (2 pods are a bit larger then the other 2)

I bled the brakes a couple of times but the problem persists, if i pump the brakes a couple of times they do hold (didn't test going at a speed but had a friend try to turn the rear wheel by hand.) but after some time i get a soft pedal once again. I checked multiple times for leaks and cant find any. I plugged the rear port from the master and left only the front and brakes work fine but with the rear it seems that there isnt enough pressure to push all 4 calipers. If i switch on the engine the pedal goes to the floor and have to pump it to get something.

Any ideas what i could do?

nomadzoul

I read all the write ups and they all mention the same things, Manual proportioning valve so you can adjust rear brake bias , 2LB residual valve for the rear and 1' Master, and some say they removed their rear LSPV and still worked fine.

muddpigg

iTrader: (1)

Is residual pressure valve backwards?

some 1” MC don’t need a residual valve. Don’t remember years.

nomadzoul

at the very first i tried with just the 1inch Master (didnt work), then i bought a proportioning valve since i had removed the rear LSPV (didnt work), then i installed the residual valve right after the front proportioning valve (i got a bit of pedal resistance but not much braking power). Yesterday i installed the factory LSPV as well. So at the moment i have 1' Master with a proportioning valve and residual valve in the engine bay then the rear LSPV and im still not getting much. 100% sure the residual valve is installed correctly as i blew air in it to check correct flow

Bingle

LSPV will reduce pressure to rear brakes. I don't believe I've ever seen someone do rear disks and keep it in line.
Drum brakes take less psi to engage than disks.
Not sure what other issues you may have going on, but I'd ditch the LSPV.
Do you have the manual prop valve cranked down too much? Try opening it up a few turns.
Test with engine running (if you aren't already. Brake booster makes a big difference.
Get back to us so we can try to get this sorted out or so others may know what a potential pitfall may be.
Good luck

PS. Mudpigg - It's the landcruiser 1" MC's that are set up for rear disks "stock". The 4Runner ones are all drum rears IIRC.

nomadzoul

I did try without the LSPV in the beginning but nothing. i adjusted the brake prop valve in a couple of setups. all closed , all open and in between (with and without the LSPV). I even tried the LSPV attached the to axle (so mostly open) and all the way up to simulate having a load in the rear to get more brake bias in the rear. What i was thinking of trying was to move the residual valve more at the back but 2lb in the front and 2lb in the rear is the same + i think having it in the front is better so this way you already have 2lbs of pressure in the line so with that you don't need to pressure the line.i tried as well with the engine running changing the front proportioning valve setup but still issue persists. i'm really at a loss.

Bingle

Hmmm.
I assume you 81 is the same as mine and the LSPV does NOT have a return line going back up front, correct? Later models do have this and require mods to the Tee below the master or something of that sort.
Again, the LSPV should only hinder the operation of the rear brakes.
I'm running a Landcruiser MC (so no residual valve, though I believe a 2lb might be in order to make it "better") with the manual PV right off the MC.
V6 rotors and calipers up front, stock fronts out back. LSPV deleted. Works great.
You're not loosing and fluid, right?
Bleeder valve on the rear calipers is on top right? (making sure you don't have any air trapped in calipers)
Double check the routing of the lines? Visually trace it all the way from the MC to the rear. Pinched or clogged line?
Not trying to insinuate you haven't checked easy stuff or that you're dumb or anything. Just spitballing ideas and seeing if anything sticks.
I'm a bit at a loss myself.

nomadzoul

Yes the LSPV i have only has 1 line to it..all calipers are in order and when bleeding i am getting a good amount of fluid so i don't think there is an issue there,for me it seems as if my master cylinder isnt pumping enough fluid to all 4 calipers.and when plugging the rear line i get good brakes up front so this is why its very weird as everyone seems to have this setup but for me it isnt working... now im thinking of a GM Master as it has an even larger bore but i wanted to see if there are other options then changing the master again (since the 1 i have is new)

nomadzoul

how did you mount stock front calipers in the back if i may ask?

Bingle

Here's the review of the conversion brackets.
https://www.yotatech.com/forums/f105...s-lror-288399/
It appears that LROR doesn't offer these anymore though. I guess one could call and see if it was an in house thing or perhaps it was a different MFG and you could still get them.
Unfortunate if it's no longer available because it really is the cheapest way to get disks out back and it works really well.
My T-case e-brake is from Sky and it's been working great for about a decade.

Something has got to be wrong with how you set yours up but I'm just not seeing what it is. Should be no need for the GM master.
Rear brake line hooked to MC close to firewall, fronts at the "front", right? (I'm pretty sure that's right for the V6 MC) If you had that backwards, you'd have no residual valve for the fronts and be "doubled up" for the rears.
I didn't notice, but did you use a V6 truck MC or the landcruiser one (both are 1" bore)? I assume it's the V6.

In my experience, start with the simplest, totally bonehead thing you could have done wrong/forgot to do. That's usually what the problem is. Work up from there.
Hopefully someone else chimes in with the "magic wand" that straightens this all out.
Gotta be something simple we're missing.

nomadzoul

Yes this is my setup. the one furthest to the firewall is going to the splitter for the front brakes then the 1 closest to firewall goes to the proportioning valve , a residual valve just after it and keeps going to the back.

dropzone

iTrader: (6)

bypass the LSPV

did you properly bench bleed your master?
Converting to dual diaphragm brake booster will also help

nomadzoul

I didn't actually bench bleed the master but i have bled the brakes so many times that its impossible there is still air in the system.I recently just re installed the LSPV as before that i didn't have it, i only had the manual prop valve and the 2lb residual valve but that still didn't work for me. Also upgrading to a dual diaphragm wont solve my issue as that would help only if my pedal was very hard to press but in my case as soon as i switch on the engine the brake pedal goes to the floor with very little effort. Im thinking of upgrading to a GM master as it seems to push much more fluid then the Toyota 1''

muddpigg

iTrader: (1)

Did you have the bypass line that connects front circuit to rear? Did yo blow lines out?? I’ve had bad MC out of boxbefore. Are pistons on calipers pushing out? Maybe another component is bad, manual prop valve, or residual valve. Try removing and see, need to narrow this down.

nomadzoul

Since mine is a 1981 i dont have a line going from the LSPV to the front i only have a line going from the front to the LSPV. MC works fine if i plug the rear outlet, so this would mean that my master just isnt pushing enough fluid the move all 4 calipers (mostly the issue is because of the rear since those calipers are HUGE). i did blow air through the residual valve and it did restrict flow so that seems to be working. The manual prop valve i cant really tell but in reality that shouldn't give me ˟˟˟˟ty brakes, if anything i would have my rear brakes lock up before the front, while my issue is that none of all 4 calipers are pressing that hard to lock the wheel.

I also did test adding each item 1 by 1 and bleeding every time but nothing made the brakes work 100%. Actually when i have front prop valve , residual valve and the LSPV the brakes got a bit better but when starting the engine i get a very soft pedal until i pump it a couple of times.

muddpigg

iTrader: (1)

If you have to pump it a few times to get a good pedal; i’m Thinking 2 problems:

1-Air in the line. You’ve been into system multiple times and bled multiple times so I kinda doubt it.

2- residual valve is not holding sufficient pressure so pistons son’t retract back.

I’m really curious about problem.

nomadzoul

im actually thinking of moving the residual valve at the back just before the splitter which feeds both calipers, maybe this might help in keeping the 2lb just in that part of the line only and not the whole line (which could be the reason why i have to pump a couple of times everytime).

muddpigg

iTrader: (1)

Let us know