I've searched this forum and others quite thoroughly and have read everything that was posted about the power brake booster on my 2nd gen ('94 4runner). What they claim was happening to me is the same symptoms that i have and so therefore i believe its the brake booster.

I will just re-iterate my problem and my search for what it is/was.

First off I have a great '94 4runner. She has less then 150k and I bought here over a year ago. She runs great. Minus a few oil leaks, which are on my list of things to do, she has really only needed minimum work. I changed both front axles, all 4 tires, brake pads and rotors about 9 months ago. I put the best ceramic pads that o'riellys had and mid-grade rotors. She runs great.

Then last week I was driving about 120miles to go pick up a boat and drive her back home. On my way to get the boat, on the highway about 25miles in, I get to the toll booth warning bumps and slow down (55mph is first bump) brakes work fine. I get to the 35mph bumps and my brakes are heavy, hard to push down, then barely respond when pressed all the way down. I'm like crap, wth?
So I get her pulled over in the parking lot at the toll booth and inspect everything. I look for fluid, check the master cylinder, inspect all the lines and hoses, and find nothing. Everything checks out.

So I decide it has something to do with my master cylinder or power brake booster. I get her towed home and do a deeper inspection. I am missing no fluid. I search the forums and find the exact wording of my problem and they claim it is the power brake booster.

I did not find anyway to test for it? My brother is out of town until tomorrow and he has all my tools and stuff, so until then I can't get the wheels off and inspect the pads, but I just changed those 9 months ago, less then 10k miles.

I have been towing a boat on and off since I got the 4runner, doing all the maintenance that is suggested on the forums and the chilton. The boat weighs less then 1300 lbs.

You guys have any ideas on how to test if it is the brake booster or the Master Cylinder? and if it is the brake booster how to test it once you take it off? and from what I read, if I get the brake booster off without emptying the MC do I still have to bleed the system?

Thanks, any info would be greatly appreciated.

'94 4runner man in KC

Here's a simple test for the brake booster.

With the motor off, pump the brake pedal until it is firm. Once it's firm, hold the pressure on the brake pedal. While holding pressure on the pedal, start the engine. If the pedal does not sink down then the problem is your brake booster. If the pedal does sink down then I would look into your master cylinder.

Thanks for the test. I did what you said and it stayed firm. I did release after I had her running and pressed the brakes and I could hear what sounds like a whoosh as I press the peddle down.

After I replace the booster should I be concerned about it going again or the MC going out? From everything I read the booster is something that is not suppose to go out.

Thanks again.

The brake booster is nothing more than a vacuum actuated braking assist. When you push on the brake pedal the brake booster opens a diaphragm and allows vacuum pressure from the engine to push on a rod in the master cylinder vie the front half of the brake booster.

That whoosh you're hearing is a vacuum leak in your brake booster. Your booster is definitely bad. Another sure way to tell if you have a vacuum leak in your brake booster is with the engine running and you step on the brake, the RPM's will drop a little bit because of the vacuum leak. Once you replace your brake booster and you're careful not to damage your master cylinder you should be fine with no further worries.

I realize that this is an old post but I have a question regarding troubleshooting a brake booster/master cylinder. My '85 truck (22RE) has poor performing brakes and I'm having a hard time diagnosing it. The pedal relaxes when you start it, so I would assume the booster is working. On the other hand, it brakes really stiff, so something's not right. Occasionally the RPM will dip when I hit the brakes (but not consistently). This morning it did it twice when almost warmed up and pulling into a parking space, but did not do it again???

Any help would be appreciated.

Thanks in advance.

I just replaced brake booster on 86 4runner, started the truck mashed brake pedal and booster came apart completely...brand new booster. is this a faulty part or is there any way I did something wrong while installing? It was a fairly easy change out...thanks for your help!

Reviving an old thread here...

I am trying to replace the Power Brake Booster in my '95 4runner (v6 SR5) and bought part SKP SKBB048 from RockAuto, which is said to be a fit but the rod for the clevis attachment to the brake pedal is noticeably longer than the OEM I removed. So much so that on install the rod buts up against the brake pedal itself long before the mounting nuts are tight. Any body else run into this issue or maybe can offer me guidance? Thanks!

Hey VallejoRunner - I have the same issue with a booster from Rockauto for 95 4runner... shaft too long. how did you resolve the issue? their only option is to return (at my expense) for another.

I could be wrong but isn’t the rod adjustable?

I know this is an old post, but is it possible to just replace the brake booster pump in a 2010 LTD? The dealer wants to replace the entire MC unit, I think it is only my booster pump that has a leak.

On this forum folks have generally replaced the whole assembly. There was a thread for the abs pump repair kit in the 4th Gen section. You can ask the dealer to replace just the pump. Also you can buy a used complete assembly from eBay and do it yourself or have an independent shop do it, would be cheaper then the dealer.