Been working on the '90 for several months, few hours a weekend when home from work on the road, and now really need it for winter. Did wheel bearings and a caliper, and then tried to get fluid to rear brakes for the first time since I've owned it.

Cleaned LSPV, swapped out for a used one, nothing. Replumbed both lines from just behind the fuel filter straight to a "T" fitting, and from there to the rubber axle line. Usually loaded up with weight anyway, so hopefully no safety concerns. Bleeder assistant went a little crazy, overextended (blew seal in?) empty master cylinder so I replaced that. Thoroughly bled with syringe, no air coming out, hooked all up, and bled multiple quarts through the system. Fluid to four wheels now, but no pressure at first press, minimal pressure after pumping for 5 seconds, loses it quickly.

Desperate for my winter work vehicle, finally took it to Midas. Guys said they've seen this several times before, the stock master doesn't have enough force to build pressure without the LSPV. They tried to bleed anyway, said the system was airless (no charge, thanks) and I needed a prop valve.

I know lots of guys use the manual bias valves, and at least one claimed he was getting stopping pressure without any rear prop valve. Has anyone else heard of this issue I'm having? Will a manual valve from Summit solve my problem? Or is it possible I still have air in the ABS unit up front somehow? (no bleeder I can find there...) Huge thanks in advance for any input.

 

LSPV isn't the end all be all. I've run stock systems without and had plenty of brakes.
Manual prop valve will fix it.

:wabbit2:

 

I'm tempted to switch over to a manual PV. The LSPV on mine isn't very effective. The rear end is very prone to lockup under heavy braking.

 

Thanks for the replies, I'm looking at a manual valve from summit racing, and have no prop valve whatsoever at the moment (open lines from master to rear drums). But I'd hate to replumb yet again and still have no pressure if the problem turned out to be air in the ABS unit or something. Has anyone seen LSPV removal causing no pedal pressure?

I'm unsure, because at one point Numbchux claimed to have bypassed his buddy's LSPV and improved braking performance, though this is seeming increasingly unlikely.

 

With no proportioning valve you should be getting full fluid pressure to the rear at all times. Any type of valve is only there to limit fluid pressure, decreasing braking performance to the rear when it's not needed and/or would cause the rear wheels to lock up prematurely.

Removing the LSPV could, in theory, increase braking performance depending on the vehicle's configuration. Meaning, in stock form, it's not likely going to. But on a modified(even slightly not stock) vehicle it might.


Brake booster...

 

Just out of curiosity, I've seen posts here and there saying that you can replace the 4runner master cylinder (13/16 bore) with a later year and/or FJ unit that has a 1" bore. Is this a direct bolt in? I'm also curious to know if the stock MC on the 88 has the residual pressure valve for the rear brakes built into the MC. The brakes act like there isn't one.

 

Thanks Mudhippy, that was also my thought with the LSPV removal, usually traveling with a load in back. But also makes some sense that the master bore isn't large enough to fully pressurize all four calipers/cylinders at once, and relies on some back-pressure from a prop valve, resulting in no pressure without several pumps. I wish I knew...

IRK, can't say for sure on your questions, but if you're asking about the resid pressure valve because you're getting excessive pedal travel, you may just need to tighten up the rear adjusters. Tried pulling and releasing the e brake several times while backing up?

 

"resulting in no pressure without several pumps. " Just had this problem.
You have air in the front to back line if you retained that line. Need to bleed bleed bleed.

 

In my case it was a poor performing power steering pump that caused low brake pressure to the rear cylinders. The ABS( rear Only ) is powered by the power steering pump pressure- that the way they did it. My symptoms were the same as yours- couldn't get pressure to the back brakes- couldn't bleed them. Low brake pedal at low RPM's. When I revved up the engine the brakes took hold. Changed out the PS Pump, and my brake pressure came back.

 

i think here's your problem:
"Replumbed both lines from just behind the fuel filter straight to a "T" fitting, and from there to the rubber axle line." hint being you said "both lines"... if you loose the lspv, there will be only one line to the rear axle. one line from the master cylinder to the pipe under the chassis to rear to the rubber hose that lets the axle go up and down, then a tee on the axle out to the brake cylinders.
the 2nd line to the lspv comes from a tee on the right front brake line. remove that line from the tee and plug the tee.
bleed the brakes and adjust the rear cylinders.
a good stab on the brakes in the wet and you'll lock up the rears and spin.

 

Thanks for all the help. Didn't know about PS-ABS interaction, but since my pedal is dead at all RPMs, sounds a little different. Need to weld up some plow braces for the soon to be here snow, and shock brackets for a trailer, but hopefully this PM I'll be able to round up a 3/8-24 bolt to plug that front T, and bleed bleed bleed...

 

hello, manual pv from summit p#260-8419 list a min of 100psi and max of 3000 psi, what is the stock or unloaded psi setting?
thanks in advance for any help

 

Generally about half that I believe, that one will work.

I plugged that front T, and redid the rear line setup. Now have fluid at the front, but not at the rears. Nothing even at the first "L" fitting above the pass wheel, 3 ft from the master. Assuming it a bad master, or maybe having it routed like I did, without a P valve blew the rear cylinder. At least the truck is back on the road, I'll return the master someday soon and hopefully that takes care of it.

 

A couple things:

I cannot imagine how changing the power steering pump fixed a lack of rear pressure to the rear brakes except coincidence. A rear only ABS system is passive and closed. The sensor in the diff reads speed from a ring gear only. When the ring gear decel is much greater than the vehicle decel (different sensor) all it can do is dump pressure to the rear brake circuit. There is no motor like in a modern 4-wheel ABS system so it cannot re-apply pressure or do anything but dump pressure. Its fed ('powered') by brake line pressure only and the dump valve has a 12V power supply.

Since the LSPV is gone you should have free flow to the rear brakes. Open up the bleeder on one of the wheel cylinders and pump the brake pedal slowly until fluid comes out the bleeder. You have to move a lot of fluid to get it to the back. Keep your eye on the reservoir level too.

Once fluid comes out of bleeder, close it and do the same with the other side. THEN bleed the rear brakes under high pressure.

I'm not so sure that the m/c is bad yet.

 

I was getting very poor pressure with two lines running to the rear, but had fluid there after several pumps. I plugged the first T fitting after the MC, coming from the front MC piston (which was feeding both front and rear brakes via the T). Now, front pressure is great, but there is no fluid at the rear. Which indicates to me that the fluid was getting to the rear before only via the T, and the front piston in my MC was trying to pressurize the whole system.

Right now, with each MC piston running front and rear separately I have full pressure up front, (front piston in master) and nothing in rear (back piston in MC), even at the first fitting off the master from the rear piston. Lines are already full of fluid. I can't think of anything that would cause these symptoms aside from a blown seal in the rear piston of my MC. Different thought?

 

no lspv on mine with a Summit proportioning valve opened 100% mounted at the master

No issues

 

Did you also plug the T fitting above the front passenger wheel to separate front and rear circuits? Front MC piston/front brakes, rear piston/rear brakes? And do you also have ABS? Just trying to eliminate variables, thanks.

 

I'm confused.

Are the rear brakes properly bled?

 

http://www.ncttora.com/fsm/1990-1995...e/absactua.pdf

 

I bled 1.5 quarts through each rear wheel, and nearly that through the fronts before I plugged the T by the front pass wheel which connects the front MC piston to the rear brakes. The pedal used to go to the floor first push and would build up only a little pressure, no change through all that bleeding.

Now with the T plugged the front MC circuit pushes front brakes, rear MC circuit pushes rear brakes. Bled that little bit of air from my front pass wheel after plugging the T and front brakes work great, but rear brakes are not getting any fluid at all. So seems to me like the fluid that I was getting before, after much pumping, must have all been coming from the front MC piston/circuit, before the T was plugged, leading me to believe that a seal on the rear of my newly reman MC piston is blown.