mawk

I have a 91 4runner. The brakes are horrible. No pressure to the front discs. I replaced the master cylinder, but same thing. I noticed that when I bleed the master cylinder when the peddle is pushed down it starts to push fluid out to the rear channel. When the peddle is half way down it is fully pushed the rear channel fluid out. For the front channel it is under vacume until the peddle is pushed half way then it starts to move fluid but doesn't "squirt" until it's all the way down. When I drive I have no front brakes. My questions is "is this normal?" BTW this is the 3rd master cylinder I put in! Does the rear LSPV push extra fluid up to the front to stop the front when the brake peddle is less than half way down?

mawk

Can anyone that has bench bled the MC tell me if fluid moved out of both ports at the same time? Even knowing that would be a BIG help. Thanks.

abecedarian

fluid should be pumped out of both ports on the MC.
the LPSV does not divert any pressure or fluid to the front wheels from the rear circuit.

mawk

Just so I'm not retarded... You mean the fluid should come out of both ports at the same time right? Not the back, then the front? I guess I'm off to advance for the 4th MC.

drew303

yea it does Theres a return line that diverts to the front. the LSPV limits pressure to the rear brakes to prevent premature lockup and the actual LSPV allows more pressure for when the bed is low/sagging under additional weight.

the T on the right... the line coming into it from the right is the return, or loop line from the LSPV.


Lemme get this straight, you have rear brakes but not front brakes and a new MC. And you're sure you bench bleed it right?

Are your front flex brake lines in good condition? If theirs any bulging/crack (could be internal) then that could result in no brakes. Anywhere fluid is allowed to decompress... or accumilate... how do i say this. The system has to be 100% pressure resistent. If a rubber brake line is expanding, even just a tiny bit, you won't have enough pressure to have brakes.

You'd be surprised at how little fluid the MC actually moves when you depress the peddle.

drew303

I honestly can't remember if it came out both at the same time or one before the other. Its been awhile since I bleed one. I've never really looked anyway. Regardless, you should have fluid out of both at some point and no air. ...

you are bleeding it right tho? run two lines back into the resevoir (under the fluid level) and pump till no more bubbles.

allnmstkn313

Fluid should come out of both ports in the master when you pump it...

Bleed the Master, Driver Rear, Pass Rear, LSPV, Pass Front, Driver Front.

If that doesn't work, either the master is dead, or you have a leak in your lines somewhere.

abecedarian

yes and no.
if you look at the test procedure for the LPSV, the rear brake pressure is lower than the front under all circumstances making it impossible for the lower pressure rear brake fluid to enter the higher pressure front brake circuit. the lpsv uses the pressure from the front brakes to limit / modulate the rear pressure in accordance with the load.

drew303

In the FSM the rear brake pressure is different than the front because of the LSPV. per the FSM you measure rear brake pressure from the wheel cylinder. Then adjust the adjusting arm accordingly till you get the right pressure as noted in the FSM.

Doesn't say that the pressure is different to begin with.

I'm trying to understand this, could you elaborate? Doesn't make sense to me that you'd have two seperate systems applying force to the braking system that needs it the least. I see kind of that you'r saying the front's superior pressure is the acting force on the piston that regulates rear pressure (ie pushing it against the rear pressure) but I don't believe thats how it works.

Manual proportioning valves are just needle valves. They limit pressure.

It makes sense to me that the stock LSPV has a return line to revert excess pressure to the front where it's more needed. It's still proportional because it's a looped system. When the LSPV is first adjusted you're accounting for the extra pressure going to the front brakes.

The way this works in my mind is, by allowing rear brake pressure (ie the first piston in the MC) go to the front brakes it's more efficient. You don't need to press as hard (or need as much pressure on the brake pedal, ie piston rod) to apply enough leverage to lock up the brakes.

I noticed my brakes are stiffer after I added a Manual prop valve, vs just using the stocker. ( I have disc brakes ) but I experimented with using one or the other, or both. I'm now using both. With the manual LSPV, because it doesnt allow fluid to bypass it I think this accounts for the stiffer pedal. I'm having to press harder on the pedal to get enough force into the second piston (in the MC) to create enough leverage/pressure to stop the truck. This is with a 1" MC as well.

Makes sense to me at least. Especially since the argument that the new larger rear calipers require more force is ludicrous because the stock LSPV doesn't do enough on it's own to limit enough pressure to keep them from locking up.

**Also, would make sense for my case because when you press on the pedal, the first piston is what applies pressure to the rear brakes. As that pressure builds, it begins to push on the first piston (front brakes). By allowing pressure to be applied to the front brakes from both pistons, you're making the system more efficient. The rear system is secondary to the whole closed loop system via the LSPV that regulates how much pressure acts on the rear cylinders.

According to the FSM, when the front is at 711psi the rear should be at ~512psi .... now bring the front to 1,422 psi, the rear should be around ~782psi. The harder you press on the pedal, the pressure in the front increases faster than in the rear. From 28% more than the rear to 46%. Could these be because both pistons are applying pressure to the front? Via that return line? As leverage increases, the first piston begins to act more on front than the rear?

i dunno... my mind just keeps going lol

abecedarian

http://www.autoshop101.com/forms/brake08.pdf
scroll to pages 13 & 14 where the lsp-bv on the toyotas is addressed.
The proportioning function of the valve is operated by the arm to the axle (yes, I was wrong about the modulating effect of the front brake pressure).
The bypass valve only serves to allow full rear brake pressure in case of a failure of the front circuit. Without the bypass valve, if the lspv operated the way you describe- that other line is a return of excess pressure to the front circuit- the loss of the front circuit would also cause a loss of the rear circuit.

mawk

Great Link! After reading that I think I have a good understanding of how it works. Since it has to have front pressure to work the lspv I'm assuming that fluid should move from the front and back of the master cylinder at the same time. I picked up a new one this morning. If the thing doesn't push front fluid until it's half way in I'm going to freak out. Thanks for you help guys.

drew303

awesome! Wish it would have that info in the FSM. That makes sense now enitrely.

this is excellent, its always been refered to as a return line.. it so isnt. thanks abe!

drew303

you are correct. It should move fluid at the same time equally. Both the F/R pistons of the MC are the same. But you may not get fluid equally at first when you bench bleed (half the reason you bench bleed). The pistons will be full of air at first so you gotta get all of that out before you get a nice steady stream of fluid.

goodluck

Bigblock

On all vehicles I've ever M/C'd the rear always got fluid first then the front.Your M/C's are OK.You need to make sure you have no air in the lines.If you have a lot of air it will be hard to bleed because of the reduced effect of the air in the system.Patience will help alot as well as a buddy to pump the pedal.
Bigblock

mawk

The old master cylinder must have been bad. I got a new one last night and after I bled it I had pressure at both ports at the same time. But I have a new question. Since I put the new MC in I have great line pressure to the lines going into the caliper (sp?) but nothing on the bleeders. I thought the bleeder might be clogged so I took them out and they are fine. Even with the bleeders out and my finger over the hole I don't get any pressure. If the caliper was bad shouldn't I still get brake pedal pressure? I have very little pressure. The back brakes work fine and no air in lines. Any thoughts?