I have a 1996 4runner 4x4 3.4l engine. My problem I'm having is that the brake pedal becomes hard (no vacuum) randomly while driving. In order to fix it, all I have to do is pump the hard brakes a few times and it softens up (vacuum returns). It's happened at 45mph before, but I was idling at 45mph, and I notice it happens mostly at lower speeds and rpm. The problem started around the same time I replaced the idler pulley for my A/C line, not sure if there's a connection; the engine was squeaking really loud because the idler was seizing and replacing it fixed THAT problem.

I checked the valve check valve and it is only allowing air one way (out of the brakebooster, not into), and I did a brake booster test where I held the brake pedal all the way down when I turned off the car to see if the brake booster is holding vacuum and it IS holding vacuum, the brake pedal did not decompress when I turned off the engine.

I've replaced a lot of hoses to try to see if there's a vacuum leak. Any suggestions are appreciated, I have a camping trip I want to do this weekend so I'm hoping to fix the car before then.

 

I had a similar issue with my 95 right after doing a smoke test on the intake looking for vacuum leaks.
Random loss of power brakes, works one time then nothing, then it's back etc.
I am sure that the issue was the oil from the smoke in the system was plugging a tiny port inside the brake booster.
I replaced the booster and all is well. I doubt that it would be a good idea to open up a brake booster to attempt a fix, too much liability and it may just fly apart under spring pressure if you tried. I have always just replaced them if there is an issue.

 

Closest symptoms I've had to that were that my check vale would stick in very cold weather (single digits or lower). So you'd have a pedal press or two of power brakes from the previous days vac still in the booster, then nothing, MASHING the pedal to slow even a little. Then it would warm up under the hood and start working again. Swapped the check valve (it's cheap) and now it works perfectly in the cold. And the rest of the year too.

 

Did you ever figure this out? I have a very similar problem. Just swapped brake booster and check valve, but am still experiencing and intermittent hard pedal. TIA!

 

It was fixed for me after I replaced the brake booster. You're probably experiencing low vacuum in some capacity, and may need to look around a little more, or raise idle with your idler screw and see if it persists.

 

Can you elaborate a bit more on your fix? I swapped my booster and check valve, and shortened the 11/16 vacuum hose. But still get it randomly for the first few stops every once in a while.

 

Not much else except for replaced my brake booster and the problem went away. I used this one