stanz

iTrader: (1)

I'm a bit stumped at the moment and can use some wisdom from all of you.

I was starting with this as my current state:

1986 4Runner:
-Front calipers - good condition, fully functional
-Front pads - 1 week old
-Front rubber brake hoses - 5 months old
-Rear wheel cylinders - new
-Rear pads - 1 day old
-Rear rubber brake hose - 5 months old
-Steel lines - all sound, no leaks
-Master cylinder - age unknown, no visible leaks
-Brake booster - believed to be original
-Brake booster check valve - believed to be original, holds vacuum

The issue:
-Squishy brake pedal with inconsistent pedal height -- somewhat squishy then inconsistently drops almost all the way to the floor on random brake attempts, but then is taller on later brake attempts (repeat the random cycle over).

Today's fix attempt:
-New master cylinder #1: bench bled and then thoroughly bled on vehicle. Could not resolve above pedal issue
-New master cylinder #2: went back to the parts store and got another new master cylinder. bench bled and then thoroughly bled again on the vehicle. no change. pedal issue remains.

Further diagnosis:
-We noticed that the idle seemed to get slightly (minor) rougher when depressing the brake pedal and holding it down.
-We pulled the check valve off with the motor off and heard the rush of air from the vacuum release
-Can pump the pedal to get it firmer with the engine off. Pedal does then drop a bit once engine is started and then the random squishy rise/fall continues.
-Don't have a vacuum pump handy to test the booster

We're puzzled by this issue as we've always heard that bad boosters cause hard pedals, not soft and inconsistent pedal heights.

What are your thoughts? Booster? Anything else that can cause this? It's actually worse now than when we started. It is not safe to drive as is.

ksti

I know you replaced rear brake shoes, but are they adjusted correctly? Is the LSPV working properly?

I really think it still air in the lines, Maybe a pressure bleeder is needed.

timothy47129

I know your frustration.I went thought the same thing after new pads and rotors ,wheel cylinders and bleeding the crap out of it.It might not hurt to recheck your wheel cylinders,I have gotten bad ones and have found new ones leaking were the line screws in due to a bad casting?here is an example I found about boosters. if a driver applies 100 pounds of force on the brake pedal. The car has a pedal ratio of 7:1 and a 1” bore master cylinder. The car will make 891psi of brake pressure. If the same car were to add a 9” single diaphragm vacuum booster the brake pressure would increase by 899psi with a total brake pressure of 1790psi .so there could be a leak or air in it that pushing without the booster full wouldnt show.Also if your 86 has the load sensing valve in the rear like my 89 pickup I freed mine up and adjusted it and it made a big difference.Mine was dumping too much pressure to the front brakes.Also are you bleeding with a jar and a piece of hose?

stanz

iTrader: (1)

I do have valve. How did you adjust yours?

timothy47129

Well the proper way is to use a pressure gauge and a specific weight in the back of the truck.But I disconnected the bar that hooks to the axle.Made sure the valve was moving up and down freely.greased the bolt the bar attaches to on the axle and adjusted the nut on the top of the link that attaches to the axle then moved the rod up for more pressure.I adjusted mine and test drove and kept adjusting it up untill it felt how i wanted it.A warning though you dont want to rear to grab too much in a panic stop if the rear locks it could swing the rear of the truck around.its alot easier to recover from a panic stop locking up the front brakes than the back.I just adjusted mine untill i feel a little more rear brakes than there was and went out to test it the first rain we got after the adjustment.Got it up to about 20-30 mph in a parking lot and slammed brakes to make sure the rear didnt lock.I did mine about 3 months ago and my brakes are better on my work beater than my other 3 cars.And they have a good feel to them too.

timothy47129

And here's a pic I snagged off google

ksti

Yes I mentioned the LSPV, but IMHO it would be the last resort. Your issue is a squishy (spongy) pedal that sinks.
Try the simple things first. Check rear brake adjustment. I'am going to say it again, I still think you have air in the system.

lobukbuild

ive had/have this issue. never able to figure it out. heard of people eliminating the lspv and running one line fronts and one line rears with a manualy adjustable prop valve under the hood.

takeitoutgetitdirty

I agree with KSTI. In my 86 it took hooking up a vacuum bleeder to get the air out of the system. No matter how many times I tried to bleed the system manually, I couldn't get it.

stanz

iTrader: (1)

Update: I've got a friend working on this that is a mechanic on the side and he's baffled by our latest fix attempt. He hooked up a power bleeder to the master cylinder to push fluid through the system at 25lbs.

He gets 5-10 seconds of air out of all four corners and the LSPV. With the engine off, the pedal then feels good. He starts the motor and the pedal goes soft on the second pump.

When he goes back and tests each of the 4 corners and the LSPV, he gets another 5-10 seconds of air out of each again... and again... and again... (for hours). He's now run over a gallon of fluid through the system and the fluid coming out is as clear as going in.

He's traced the system and there no signs of leakage anywhere. We cannot figure out how air is getting in but no fluid is leaking out, especially with the entire system pressurized.

Please lend any thoughts you might have. Thanks in advance!

stanz

iTrader: (1)

I should also note that he reinstalled the original master cylinder as a retest and there is no change... air keeps re-entering over and over again. We're certain that it's not the master cylinder now.

wyoming9

iTrader: (1)

:wabbit2: You have a connection someplace that may only leak a slight amount of fluid and is easy to miss but this is sucking in air. The amount of fluid lost one would think was normal pad wear .

At what point did the soft pedal start?? Was this the whole point of redoing the entire brake system??

Did you install all new parts or new used parts??

With so many things made in China it is so possible for almost anything be bad.

stanz

iTrader: (1)

Thanks. I suspect a leak somewhere too, but we cannot seem to find it. The soft pedal has been getting worse recently and prompted the purchase of the new master cylinder.

The master cylinder is new, not remanufactured. The rubber brake hoses were replaced back in July and the pads and shoes in the past week.

I used to work in a parts store while in college so I too know that new parts can be bad.

waskillywabbit

iTrader: (-1)

You've got a bad bleeder somewhere is my guess. Not quite sealed all the way.

Doesn't dribble but enough to suck air when you pump brakes.

:wabbit2:

stanz

iTrader: (1)

We checked out and tightened up each fitting and put in new bleeder valves. Still no change. I'm taking it to a shop in the morning to see if the can find the source. We give up!

wyoming9

iTrader: (1)

i wish you luck.



Could also be a rubber hose with a bad crimp.

stanz

iTrader: (1)

Thanks! It's enough to drive you crazy. No more throwing parts at it. We'll see what the shop comes up with.

stanz

iTrader: (1)

Verdict: the shop was able get a little more air out and confim no leaks. The found that the biggest offender is that the wheel cylinders installed by the PO, although new are the wrong ones. They're for a later year and have a bigger bore. The 86 master cylinder wasn't moving enough fluid to get the right levels of pressure up.

jerry507

Very interesting! I had this exact same issue nearly word for word on my truck. I don't have air issues, because I've done repeat bleeds and never get any significant amount of air. However the PO did install a rear axle from a dual tire 2wd. Took quite a bit of searching to find wheel cylinders to replace the nearly shot ones. I was thinking it was my LSPV but now I'll probably just stop looking and live with it.

stanz

iTrader: (1)

Sounds like you need a master cylinder that matches upp with you wheel cylinders.