I just bought this 4runner last week so im not too familiar with it.

I drove for and hour and a half on friday night heading out of town and had zero issues.

Then sunday morning i get ready to head back towards my house. After a mile of driving i notice the brake pedal getting firmer and firmer. Then eventually the car just came to a stop on its own so i pulled over. The rotors were red hot. I let it sit for about 45 mins. Tapped the check valve and removed the vacuum hose. Then eventually i was able to drive it back home (1.5 hour) with no issues.

Then this morning im ready to head to work and i only get 3 miles until the brakes lock up and the pedal is firm as a rock again.

I tried waiting it out and removing the vacuum hose again but this time nothing. So i unscrewed the breather valve and let some fluid out of both front calipers. Then i returned home. But by the time i made it home the problem was back. Ive tried driving it around my neighborhood but the issue now returns every 2 to 3 miles or so.

Ive searched and searched and understand hard pedal means booster but no one ever mentions front calipers locking.

So far ive replaced the check valve, bled the brakes and replaced the flex/soft lines on both front sides.

Is it time for a new booster / master cylinder? Or is there something else i should look for?

Thanks

I just found in an unrelated forum that it could be an overfull reservoir.

Im going to check that tomorrow morning.

On my friends motorcycle it was his master cylinder.


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Removed some fluid. Still has he same issue.

Guess ill try the booster

From the symptoms, it really sounds like something happening at the master cylinder.

Probably the master cylinder piston isn't returning fully when the pedal is up. The way the seals work is that they can suck some extra fluid past them when the pedal comes up, as part of a 'pumping' action. But the fluid can only return from the system back into the reservoir when the piston fully returns all the way out, and the seals go past small holes that lead to the reservoir.

If the piston is prevented from returning fully, it can still pull fluid into the system via the pumping action, but the fluid is then 'stuck there.

A couple of reasons why the piston can fail to return fully:
1) The pedal isn't coming up all the way, and it is preventing the piston from returning the last little bit.
1a) Someone was messing with the push rod attached to the pedal which reaches into the master cylinder and pushes on the piston - and left it too long. This is possible if someone who didn't quite know what they were doing replaced the master cylinder,
2) The master cylinder is old and gunked up*, and crud and contamination is making the piston sticky and it's just getting stuck before residual pressure and the spring in the master cylinder get it all the way back.
3) It's probably possible, but I've never seen it, that the vacuum booster somehow is keeping slight pressure on the system?


* - Brake fluid really likes to absorb water. This is good in the short term - because any water contamination is absorbed and not left to rust things. But over time the moisture level builds up. And eventually the fluid, in addition to boiling much more easily (surprise!!! no brakes) it becomes corrosive and makes things like pistons stick in their bores. It should be flushed through the system from the bleed valves every 3 or 4 years, until fresh fluid comes out the bleed valves.

That's the first thing I'd check, based on the symptoms.

I must add that bad brake lines (rubber hose) will also do what you are describing. The inner lining delaminates and can ball up and block the line which will not let the caliper piston float.

But if both your front brakes are acting the same then I’d look at the master cylinder


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Drive around the block until you feel the onset of locking up appear
Loosen Master Cylinder from booster ( back nuts off ) see if it rolls then.

brake pedal pushrod is too tight? I'd check that first - there should be a mm or 2 play at rest there before it activates the MC.

I had a bad booster a while ago, and it wound up locking my driver's side caliper.

I attributed the lockup to the fact that I nearly had to stand on the brakes to limp home when the booster died (I went like 5 miles at maybe 10MPH, not *too* dangerous, ha), and that I'd just pushed the piston too far out of the caliper, but maybe not.

Once I changed the booster out with an Advance Auto one, I had to crack the brake line at the caliper to release the pressure, then press the piston back in. The passenger side was fine.

I took that time to install speed bleeders on each side and run about a pint of brake fluid through each.

It's been about a month and the brakes are still great.

Sorry forgot to follow up.

Before replacing booster or MC, i had already pulled the calipers off a parts car because before this brake issue i noticed 1 of the caliper's pistons had seized.

I had also replaced the soft lines on both sides.

I ended up swapping the MC from my parts car and that did the trick. Ive put about 300 miles on it since then and everythings been working great.

Thanks

Axle leak on the rear drums. Had it on my 3rd gen. The symptom for me was a skid at the end of a stop and brakes that were clamped when I tried to leave after being parked. You have a great truck but leaking axle seals contaminating the rear drums can cause weird front brake issues.

Just look behind the rear wheels for any sign of oil dripping and collecting dirt. Dont even need to remove the wheel, although you get a much better look if you can.