ign_cas

87 4runner. My truck has a problem with the brakes. My brakes work fine half of the time and the other half the pedal goes all the way to the floor. I noticed that if I'm at an idle and I pump the brake a lot of times, the rpm starts to drop dramatically.

From what I've searched I think it may be a break booster leak? Does this sound right? Any other opinions or any way to diagnose it? Thanks for the help.

nv4runner

You can check for brake booster leak by depressing the brake pedal after the 4runner has been shut off and holding it for a few somewhere around 30 seconds to a minute. Release the brake and then hit and hold it again. Repeat 3 or 4 times. It should get progressively stiffer as you press and release with the engine off. If after the first press it is as stiff as can be you may have a leak.

RustBucket

iTrader: (1)

Brake pedal going to floor doesn't feel like a booster to me. If anything it should make the pedal harder as it fails.

Sounds more like a master cylinder leak, but the booster could be wonky too.

ign_cas

Finally got around to checking the booster.

1) There is no change in pedal reserve distance when I press the brake pedal many times with the engine off.
2) I pressed the brake pedal down then started the engine and the pedal went down slightly.

As per the FSM, this shows that the brake booster is operating. I also did the tests that confirmed the booster is air tight.

Does this mean it's the master cylinder? Any other suggestions or tests?

My 4runner drops RPMs drastically if I pump the pedal fast and dies out. And every now and then the pedal goes down to the floor, but brakes still work.

Munkymag

i had the same thing going on in my 87 4runner. Mines was leaking alilo but i put in ah new 1" bore MC and now its no longer touching the floor, and braking better then before. I still gotta get the v6 brake calipers then it should be better i wanna say its your MC.

ign_cas

Munkymag: What year MC did you get? And did you replace the booster? And was it an external or internal leak you had?

Munkymag

Got mines off off lowrangeoffroad.com. The junk paid me nothing on ah good unit so i bought one from them (lowrange). Cost me like 120$. If you can get ah used one i guess but i didn't want too deal with a no good unit too so i got ah new one (i wanna say if i'm right 96 4runner or truck and down) you should be able too use the MC from. Look it up too be sure. my unit was leaking from under the plastic bottle that held the fluids. The part were it connected too the pump for the MC if you know what i'm talking about. Not too sure what year they give me but it should of been from one of those model v6's. The booster i did not have too change.

ign_cas

Ah see. Thanks for the input. Will look into that. Oh and where in Southern Cali are you in?

Anyone else have any input? Or tests for internal leaks?

Munkymag

San jacinto, ca (riverside county)

RustBucket

iTrader: (1)

Just get a stock replacement Aisin (or Advics, same thing now) brand master off rockauto.com. Should only run you $50 or so. No need for the 1" one unless you intend to upgrade the calipers, it will just make your pedal too hard.

Definitely rule out the MC before you look further. Internal leaks are common and result in squishy pedal.

ign_cas

Would a faulty master cylinder cause my RPMs to drop drastically if I pump the pedal fast?

Munkymag

Still new too the game. I can't answer that question. Hopefully some one else can. Good luck

scope103

"Pedal goes to the floor" is the classic sign of a bad Master Cylinder (unless you've got a leak, but then you'd have a pool of brake fluid on the garage floor). I wouldn't call it "squishy"; that's more a description of air in the system (usually called "spongy"). That's particularly true if it goes to the floor and the brakes fade out, only to be revived (temporarily) by "pumping" the brake pedal.

Since yours is intermittent, you may have dirt in the master cylinder, or a slightly torn seal. But it's not worth rebuilding; buy a reman and leave that the experts. RockAuto has them right now for as little as $20!

I agree with RustBucket; the stock Master Cylinder has adequately stopped millions of these trucks for the past 25 years. There's no point in experimenting by putting in a non-stock brake component unless you REALLY know what you're doing.

RustBucket

iTrader: (1)

The drastic RPM drop may be a symptom of something else. You have a hydraulic pressure brake problem -- fix that one first with (likely) a new MC. Then check what's going on with the RPM thing. Could be there's also vacuum leak somewhere or an idle problem.

nv4runner

The drop in RPM comes from the massive vacuum leak you cause by rapidly applying the brakes. That booster runs on engine vacuum. If you disconnect the hose that attaches to the booster at the intake side I'd bet good money that you'd have similar symptoms (loss of RPM/engine dies).

ign_cas

Thanks for the help. I'm going to pick up a new MC today.

If I disconnect the vaccum hose to the booster, my car stalls out right away. Isn't that normal though?

RustBucket

iTrader: (1)

Never tried, but it stands to reason that a huge vacuum leak could do that.

ign_cas

This weekend I installed a new master cylinder. It helped a lot. But when I pump the brake rapidly I still get a drop in RPM (not nearly as drastic as before). Is this normal? Can someone pump their brakes 10 times fast and tell me if their is any RPM drop? Thanks again guys.

ign_cas

Finally fixed the problem.

It turned out to be an internal leak in the brake booster.

My initial problem was that my RPMs would drastically drop if I pumped my brakes more than once.

First I checked the check valve along the vacuum line to make sure it worked properly. Then I unhooked the vacuum line to the booster and plug it up with a bolt. With it unplugged, I pumped the brakes and I had no change in RPMs.

Therefor the brake booster turned out to be the problem. I installed a new one (from a V6) and now it works perfect and doesn't drop RPMs when pumping on the brake.