4Runner: Now it's the brakes

You all were so much help in my earlier thread for my 4Runner, I thought I’d run another by you.

To revisit: it’s a 1990 4Runner with 240,000 miles, auto-trans, v6 3.0 engine. It was recently, heavily rebuilt (engine, trans, suspension), by the previous owner and then sold to me about a week ago.

This Saturday, I was driving up the mountains for the first time since I’ve gotten the car. The engine is small so I spent a lot of time downshifted and climbing slowly up the mountainside (to 9300 feet alt).

I was following another vehicle and when we got to the top, the vehicle in front hit their brakes for some reason. I went to hit mine and “Whoops!” the pedal slid nearly effortlessly to the floor. I swung right, along-side the car and kept pressing the brakes. When the pedal was nearly at the floor, the brakes grabbed, I slowed quickly, and I pulled back into my lane.

When I reached the top and the engine dropped back to a more normal RPM, the brakes returned. A couple times this happened. When the engine was laboring for a longish period of time, the brakes would go away then return fairly quickly as the average RPM dropped back to normal.

The 4Runner has a history of a slightly squishy brake pedal but this seems ridiculous.

I asked a mechanic friend at the mountain top (group camping weekend) and he said that he thought I was losing my master cylinder. If it heats up, as the engine is working harder, then the braking strength drops. When I returned to a more normal RMP, the master cylinder would cool and brakes would return. He said if it was a vacuum leak, then the pedal would get hard, not soft.

The brake fluid is full, even slightly over the “Full” line but not badly so. Also, FWIW, the engine runs a little hotter than normal because the previous owner replaced the overhead cams, breather, & exhaust manifold with “performance” parts.

So - any other theories before I drop $130 on a new master cylinder?

I’ve never heard of a brake master cylinder bypassing due to an engine working hard/getting hot. Not saying it’s impossible, but it sure isn’t typical.

Most often with a faulty master cylinder the pedal will start out having resistance then fade away, dropping to the floor while not adding any braking effect. It’s often noticed while sitting at a stoplight.

Hydraulic cylinders (more often clutch than brake) have been known to get temperature-sensitive when old, but it’s cold that makes them act up. I’ve seen cars where the clutch master cylinder offers no pedal resistance after sitting out in the cold, but will recover when it warms up a bit.

Now, if the brake fluid is getting hot, that can cause the symptom described. This typically happens when descending long steep hills, where the brakes are used a lot and the heat generated from that use gets transferred to the fluid. Brake fluid lines should be routed so as not to be vulnerable to engine and exhaust heat, but I’d look at yours to make sure.

I wouldn’t be inclined to replace the master cylinder without some corroborating evidence. It just doesn’t sound like the way faulty master cylinders usually act.

Nope - exact opposite - no resistance at all, then a sudden grab. Works fine down here in the flats.

Well, I downshift a lot on hills - standard technique in the mountains. The brakes worked fine all the way down the mountain for me. I was doing some reading on generic brake systems and it made me think that perhaps I’ve got nasty brake fluid and it’s boiling on me. It’s certainly hot enough under there. Low speed, high rev. There’s not much carrying the heat away.

Perhaps replacing the brake fluid with a high-temp variety?

It is very possible that the “genius” that engineered the “performance” (or lack there of) mods to you car wound up routing the exhaust too close the brake lines. Hell he might have even done away with a heat shield or two to save weight. :rolleyes:
It is also very very possible (bodering on a certainty) that you fluid has some to a lot of water absorbed into it.
Given these two conditions, and the fact that your engine does not run right, you could be throwing so much heat on a long uphill climb that you are boiling the water in the brake lines.
This would cause a no pedal conditon until either you pumped enough fluid to compress the air bubbles, or the brakes cooled down.
Look at your fluid. Brake fluid is a light amber color when new. If it looks either muddy or like engine oil it is time for a flush.
I would suggest using DOT 4 brake fluid, my personal favorite is Castrol GTLMA.
Silcone fluids can be a bear to bleed, and you probably don’t need the extra headache, or expense at this point.

Thanks guys - I’m going to proceed with a fluid replacement. It’s the cheap way out.

I’m like to try to find a way to post some interior shots of the engine compartment. Would you be willing to let me know what’s missing and what heat shields, etc, I should try find/manufacture and put back?

The most expensive way of fixing a fault like the one you have described is to replace parts until the symptoms go away. If you have a pedal that goes to the floor but then recovers quickly, this doesn’t sound like it’s a heat-soak issue (which would cause the vapor problems hypothesized above) - unless you mean to say that if you pump the pedal repeatedly you get recovered braking ability - which could be vapor lock.

I’m trying to think how the condition you describe could be attributable to low manifold vacuum at high rpm and WOT (low in this case meaning not much negative pressure in the manifold) - or by a vacuum leak.

I am still suspicious of the “performance” camshafts that were installed - these might also explain why you have to rev the engine to get any power. Although this would seem like a change (the cams) that would be counter to the off-road 4x4 mission that you said the previous owner was targeting - for that one would presumably want more low-rpm torque, a wide power band, and reliable operation. Also, if you are running hot at highway speeds, it’s just going to get worse at rock-crawling speeds. Although you said this was at altitude going uphill.

My theory would be that you have a vehicle of unknown provenance, and you might be well-served by having a reliable mechanic or Toyota dealership perform a thorough mechanical inspection.

Sure I’ll look. Not to say I will have much clue as to exactly what I am looking at. :slight_smile:
E mail them if you want. E mail is in profile

Just a quick update.

I posted to the same question to a 4Runner specific board I found yesterday. Another Colorado poster with the same model says when the throttle is open for long periods of time, like climbing a mountain side, the car loses engine vacuum. If the check valve in front of the vacuum assist is bad, it’ll result in the same problem.

He suggests replacing the valve & the fluid.

That doesn’t make sense.

Any engine has little or no vacuum at full throttle; however, engine vacuum is created the instant the throttle is closed. Now, it could be that there’s a leak in the brake booster (and a faulty check valve is one such leak), so it might be that after a period of full-throttle operation there is no reserve vacuum in the booster, and yes it would take a little time to build up enough vacuum to operate the booster.

But…loss of booster operation means no power assist. It does not mean no brakes. The pedal should be higher and harder. You can duplicate this by pressing the brake pedal a few times with the engine off. After a few strokes, the booster vacuum is depleted and the pedal is high and hard. Keep pressure on the pedal as you start the engine and you’ll feel the pedal go down a bit as the booster operates. Note that it takes but a second or two to generate power assist, even though you had zero booster vacuum to start with.

I’ve never heard of a brake power booster vacuum problem resulting in the symptom you described. I read the reply on the forum you linked and didn’t see that he had actually solved such a problem by replacing a booster check valve, he just seemed to think it might be related. I disagree.

Anyway, I think it does make sense to flush and fill the brake fluid. It seems fairly likely that it’s contaminated, and might have some air in it as well.

I also recommend DOT 4 fluid - it has a higher boiling point than the more common DOT 3. It is also somewhat more hygroscopic, but that’s not really an issue in a dry climate like yours.

And you can test your booster check valve rather easily. It should allow flow in one direction only. If the vehicle has been sitting for a while (overnight would be most helpful), pull the check valve out of the booster. If there’s vacuum in there, you’ll feel/hear air being sucked in. If it retains vaccum overnight, or after sitting all day, it ought to retain vacuum during a hill climb.

Alternatively, you might be able to hook the engine vacuum hose to the booster side of the check valve (probably different size nipple, but maybe not). Pull it out of the booster while hooked up right with the engine running, and you’ll have a big engine vacuum leak. The engine will probably die, but you can feel the vacuum if you’re quick - even plug the valve with a finger and keep the engine running. Then with the engine off, connect the hose to the booster side of the valve and start the engine. There should be no detectable flow throught the valve. If you can feel any vacuum then, the valve is faulty.


Hi - reopening for an update.

I had all the brake fluid replaced a few days ago and it’s been behaving itself since. I ran it up the largest, longest nearby hillside and it was OK at the top.

Our local Brakes Plus franchise said that that seals around the rear brake cylinders are leaking. When you pull back the boot, there’s seepage around the piston. The leaked brake fluid doesn’t seem to be leaving the boot under normal use so I’ll monitor for the moment.

Anyway - the old nasty fluid looks to have been the problem.

Good deal. It’s got to be something simple once in a while.

I’ve seen this condition go on for over a year before getting significantly worse. However:

It’s hard to predict when it will become a real problem. We don’t know how long it’s been leaking, and it doesn’t always last a year before turning into a major leak. When it does get bad, brake fluid will contaminate the shoes. This can cause squirrelly braking which can be dangerous and/or loss of enough fluid to affect pedal response. The fix requires replacing the shoes (they cannot be cleaned) which might not be necessary otherwise.

So, it may well go a month, six months, a year before it has to be dealt with - or it may not. It’s a calculated risk. Just be aware of that risk, as we are talking about brakes.