1994 Toyota 4Runner SR5 4WD (2nd Gen)

Background problem....
Out of nowhere, after owning and driving it daily for 6 months I hooked up my utility trailer and didn't get 15 miles down the road and noticed the engine bogging down. I was in 4th gear and all of a sudden i can't get it to accelerate so I kept down shifting until everntually I pulled into a gas station and came to a screaching hault. The pedal was as hard as a brick!
I had tools, and I could smell the front brakes were really hot (both sides) so I slightly open the front brake line on the Master Cylinder and it sprayed brake fluid past me 5 feet. I tightened the line back down and had someone pump, pump and hold the brake pedal down and while holding I did the same thing and it again released all kinds of pressure, I tightened down the line again and brakes were back to normal. It happened 2 more times before I got it back to the house.

Round 1...
Master Cylinder? I thought so...I was wrong. I replaced it with one I got from Advance Auto and after pain stakingly learning how "bench bleeding" is done, it had no affect. I figured I had gotten a faulty MC so I returned it and got another one and with still would lock up after only driving for a few minutes. (Bench bleed both correctly and re-bleed the whole system starting at the furthest wheel RP,RD,FP,FD) It didn't make the problem worse it didn't change anything.
Round 2...
Took it to a local mechanic that has a ASE certified shop. I got the suggestion that it was probably the front calipers were both bad and the front rubber lines must be bad also....he was wrong! I went and picked it up having to bleed the MC twice on my 8 mile drive back to the house, only stoiopping back at Advance Auto to pick up the front rubber lines and new Calipers with new pads in them. I figured WTH I'll throw some cash at it.
I started with the front rubber lines going from the body to the frame (cheapest first) and no change at all!!! So then I thought, fine I'll do the calipers. No change, but I new all along the Mechanic was just throwing money at a problem because the symptoms were not text book. I shouldn't have followed that lead!
Round 3...
Brake Booster? Not sure of all the symptoms of this, but read in a few places that it can cause the pedal to get hard. I now am on a strict budget cause I foolishly wasted my budget on Calipers I didn't need. (Not bitter, just want this fixed, I really love my Toy!) I found one that was parted out of a '94 4Runner so I bought it cleaned it up made new gaskets for it between the Firewall and the MC and that seemed to fix it. I locked up twice after the booster replace and then nothing I have put 500 miles on it hauled a couple tons of mulch behind it without issue and then 4 days ago the problem came back like it had never gone away!!! WTF!!!
On a side note... The dreaded LSPV Valve. I removed it during this also and put in the Manual Proportioning valve from Jeg's. I mounted it just before the rear line connects to the MC. Why did I change it? I could bleed the valve on the LSPV, but not the rear brakes. I tried it with the key in the on position also (some out there had suggested that) before anyone asks. After the bypass the rears were bleedable again.

Hmmm...interesting problem. It seems like you've replaced just about everything that can go wrong with the braking system. A couple of questions:

Does your 4Runner have any other symptoms that may or may not seem related to your brake problems? Anything at all?

Have you checked your ported and/or manifold vacuum at idle and under load?

Do the brakes only act up when you're towing or hauling? Both times you mentioned that you had problems, you said you were either towing or hauling mulch.

Were you heavy on the throttle immediately before the front brakes started locking up?

One way or another, something is building up hydraulic pressure in the braking system. What makes this strange is that it only seems to affect the front brakes, right? You have drums on the rear, right? Since the brakes are cross linked, are you sure that there's no excess pressure in the rear as well?

Jed, first, thanks for responding so quick! Answers to your questions below.

Does your 4Runner have any other symptoms that may or may not seem related to your brake problems? Anything at all? Idle, it has a rough idle where it is normal then once run long enough to warm up it the idle will drop to 400 RPM and then back to 800. It never stalls. I've run Sea Foam into the intake some in the oil and in fuel a couple of times in the last 10,000 miles. Thats the only thing I can think of.

Have you checked your ported and/or manifold vacuum at idle and under load?
No, how would I do that?

Do the brakes only act up when you're towing or hauling? Both times you mentioned that you had problems, you said you were either towing or hauling mulch.
They will lock up with or without load. The first time I was under load with my trailer. It onloy weighs like 600 lbs. The other day though I was by myself (no passengers) and not hauling anything.

Were you heavy on the throttle immediately before the front brakes started locking up?
It's very possible. Even though it stopped locking up after I replaced the booster it still seemed like maybe the brakes were still gripping a little because the power was less than it used to be before this all began, so the booster may have just helped, but not fixed it.

One way or another, something is building up hydraulic pressure in the braking system. What makes this strange is that it only seems to affect the front brakes, right? Correct. You have drums on the rear, right? Correct. Since the brakes are cross linked, are you sure that there's no excess pressure in the rear as well? I can try and bleed them again. But no I am not sure.

Jed, I have read in other posts about a check valve in the vaccuum line that is running to the booster. Do you know much about how those are working and I wonder if it may not be a culprit.

It's unlikely the booster has anything to do with your problem. It just makes applying pedal pressure easier (less effort). It's not going to have any effect on a caliper or wheel cylinder.

A bad residual valve in the MC would be a possibility, but you replaced it.

Does it have ABS?

Yeah, that's kinda what I was wondering. That's why I asked you about your vacuum. Check valves are sorta like "one-way" valves, where they will allow vacuum to be pulled, but block the passage if pressure somehow builds up in the line.

The first thing I'd do is get a simple vacuum gauge and check the manifold vacuum. Generally, with the engine warmed up and at idle, it should pull around 20 in. hg. on the gauge. The needle should be dead steady too. If you apply throttle, you should see the needle move towards zero. Under wide open throttle, it should be at zero.

I may be totally off, but I'm trying to figure out what could possibly be causing hydraulic pressure to build in the braking system. I also think your idea about the check valve is something you should check. One quick way to check it is to remove it and shake it. You should hear the ball rattling around inside. If nothing is rattling, then the valve is definitely faulty. It's actually very similar to a PCV valve, which is nothing but a check valve for crankcase ventilation.

Fourwd1, It does have ABS and the light on the dash has always been on since I've owned it.
Jed, I'll pick up the vacuum gauge on my way home tonight and check the vacuum. I'll also check the check valve.

The "check valve" is the residual valve I mentioned, and is usually part of the MC.

I think your problem could be related to the ABS system, which is why I asked if you had it.

Ok, so how much do you know about the 94's ABS? I honestly as at a loss here any good reads on how it works I read some stuff about the ABS front wheel speed sensors and I need a whole picture. I think your on to something though!

Maybe a dumb question but what type of brake fluid are you using? If its not something mechanical then maybe you're using the wrong fluid?

Nodd, I use DOT 3.

Jed, below are the vacuum pressure ratings.
At Warm Idle: 19 Hg
At 2800 RPM: between 22 and 23

i would look into the proportioning valve...also a SMALL chance it could be the plunger on the booster incorrectly adjusted especially since the Mc and booster have been replaced. double check your hard lines to the front...make sure there's no kinks or signs of the lines being rubbed on

did you adjust the plunger depth to match the new master cylinder. if not get a proper service manual for procedure. did you flush brake system? could be junk in system that has now contaminated your new master cylinder. p.s. plunger is also called brake booster push rod. good luck, i will try to follow your thread. if push rod is to far out it is like driving with brakes on!

I just bought it - the previous owner already replaced the Master Cyl - he gave up on the problem and sold it cheap to me...

I replaced the Hoses - Then the Calipers and pads - and rotors -

Now I'm wondering what to replace next...


I'm surprised to see someone with a 4runner having the same problem which has stumped 12 different mechanic shops in my area.


My brakes will slowly build up pressure until they are nearly locked up.

Only the front two brakes

Not always Both - sometimes just one.

It happens when the car is turned on for the first time of the day - and it just sits there idling.

It happens after driving on the highway for 60 minutes - just slowly builds up pressure (not even touching the brake pedal) until i have to pull over and bleed them.

Lately it seems to happen every 5 minutes - I gave up driving it about 8 weeks ago.

Oddly enough - sometimes if you stop - and push REALLY HARD on the brake pedal - hold it for 5 seconds - then release (Repeat this 3 or 5 times) the brakes will unlock themselves and you can continue to drive for 5 more minutes; or 30 minutes...


Sometimes I can apply the brakes 10 or 20 times through stop and go traffic - red lights - stop signs - and they will work perfectly.

Sometimes they will lock up on their own for no reason.

Sometimes they will unlock themselves for no reason.


I anxiously await any other comments here... I suppose buying an Anti-Lock brake control module will be the next step (550 from the dealer; 50 from a junk yard... lol)

I am curious to know how this 'proportioning valve' works - or what other things are in the brake system creating / releasing pressure. It would sure be nice to see a flow chart for a brake system to understand where that fluid actually goes!

-Brian