I've got a 95 YJ with a 2000 4.3L motor and 4L60E. Just went from 10" drums on a Toyota 8" to a disc brake set up on a Tacoma axle.

My current brake part set up is front and rear WJ calipers, 12" front and 11" rear Explorer rotors. Stock replacement master cylinder, original single diaphragm booster, and stock proportioning valve.

Brakes are bled, everything is looking good so I took it for a test drive. Got it up to about 25-30 MPH and stomped on the brakes, figured with 35's it should stop great. It did a fine job of stopping, but the motor stalled. Started it back up and it sounds like a vaccum leak under the hood, but the brakes still feel the same.

Repeated, and any attempt at a hard stop will stall the motor every time.

Anyone have any idea's on this one?

You're not locking up the wheels, right? Just a rolling stop?

And with the stock booster...weird.

My only guess would be something relating to the vacuum line going to the booster, like during hard braking it sucks enough so that you loose too much air going to the motor, but that doesn't sound real likely to me.

If the torque converter's locked up when braking it might do it too, but I would think that would just bog the motor, not stall it...

Right, it won't lock the wheels at all. But the rolling stop distance isn't what I'd call terrible.

Now I went and and made a couple passes, but pulled the vacuum hose and plugged it. Crappy brakes as expected and no stalling. So I put the hose back on and took a 20 MPH-ish (no speedo in the Jeep) and it didn't stall for once.

Took it to an empty warehouse lot and took a pass around 50 MPH and it didn't lock, didn't stall either. Took another pass at around 60-70 MPH with the same results.

Torque converter? Hadn't thought about the trans. There was one stop where it stumbled like it was going to stall and it recovered.

This one has me stumped.

I'm not too sure about how the 4L60E works...but is the torque converter lockup triggered by vacuum, by any chance? If it locks up automatically when a certain level of vacuum exists in the motor, you might just be having a weird interaction between the vacuum required by the booster and a vacuum sensor for the lockup. But like I said, I don't really know much about that trans, I might be full of shit on that one.

Just thinkin a bit more...do you have a vacuum reservoir in the system anywhere?

I'm not too sure about how the 4L60E works...but is the torque converter lockup triggered by vacuum, by any chance? If it locks up automatically when a certain level of vacuum exists in the motor, you might just be having a weird interaction between the vacuum required by the booster and a vacuum sensor for the lockup. But like I said, I don't really know much about that trans, I might be full of shit on that one.

Good guess but the 4L60E is all electronic...there is no vacuum modulator.

Booster is shot or the vacume line running to it is.

Booster is shot or the vacume line running to it is.


Either that, or as vetteboy79 posted, you may need a vacuum reservoir.

Do you have the VSS installed?

Low on gas so the motor runs out when braking hard???????? Just a shot in the dark at a wierd problem. ????

Good guess but the 4L60E is all electronic...there is no vacuum modulator.
correct, but i think the commetn was if there was a vac. sensor somwhere that provided this type of input ( for the computer)

Might also, look at other vac. hoses, air cleaner, valve covers etc.. (not sure whats on a 4.3) Possibly some other leak is pushing you over that stall point...

Also did you tear the gromet in the booster ? of crack that plastic fitting ?

Now I went and and made a couple passes, but pulled the vacuum hose and plugged it. Crappy brakes as expected and no stalling. So I put the hose back on and took a 20 MPH-ish (no speedo in the Jeep) and it didn't stall for once.

Took it to an empty warehouse lot and took a pass around 50 MPH and it didn't lock, didn't stall either. Took another pass at around 60-70 MPH with the same results.

Torque converter? Hadn't thought about the trans. There was one stop where it stumbled like it was going to stall and it recovered.

This one has me stumped.


Just a thought but if you are mix and matching break calipers and master cylingers, then the master cylinder could be over extending pulling the vacuum booster with it. hyperextending the booster could cause a major leak, hence the dying and the hissing on restart.
I am not the best to tell you how to match a master cylinder displacement to the caliper displacement but if the brake pedal feels really soft or really stiff then that usually indicate a mismatch. however only a soft pedal would cause your problem. Hell you could just have a bad booster thats is leaking under heavy braking ( ie. long travel)

Push the clutch in before you stop. :flipoff2:

This may seem weird but my blazer (4.3 vortec and 4l60e) does that when the EGR valve gets sticky. Its easy to pull and clean or replace (but pricey). It also did something similar when my cpi regulator (vortec only) was leaking. THey are both very common problems, first thing I'd check is the EGR. Its on the front of the manifold. Pull it off and see if its fouled or sticky. I think they fixed the regulator problem in the cpi by 2000 though.

correct, but i think the commetn was if there was a vac. sensor somwhere that provided this type of input ( for the computer)

Yep, that's what I was getting at.

In thinkin about it some more, is the fuel pressure regulator governed by vacuum? I believe Ford does this...I dunno on the 4.3L...any chance the symptoms you were having could be a fuel delivery issue? A sudden change in fuel pressure by a vacuum drop or something?

Again, might be related to a vacuum reservoir...

I 2nd the VSS. If thats not installed in a EFI system the IAC (Idle Air control) generally cannot keep up & open the air fast enough to keep the motor running on de-accelaration. The VSS will sense this & increase the fuel to keep the RPM up high enough to keep it running & long enoug for the IAC to catch up.

My TPI set up has the same problem sometimes with dying for no apparent reason coming to a stop. Generally on a trail when i need to run most. I installed a VSS & that problem seems to have gone away on the street. no trail runs yet.

But i dont know anything about the 2000 4.3L.

I 3rd the VSS.

And I'm back. The stalling issue cleared itself up. Don't know how or why.
The VSS is in and hooked to the computer, just nothing to the gauge yet. Torn between the stock gauge and a Dakota Digital box or a new gauge all together.
I'm pretty sure I'm not pushing enough volume at the calipers since the rear ones are now as big as the front ones used to be, and the front ones are now quite a bit larger (pretty damned techincal aren't I?).
Spent the day working toward replacing the stock A/C pump with a York, and realized that I don't have the room for anything bigger in the way of booster or MC.
Anyone have a pic of the brake stuff from an auto tranny equipped YJ? I was told that they don't have the stuff spaced off the firewall the same way since there's no clutch stuff to clear. That could be a fix. YJ dual diaphragm booster and a Grand Marqui or Vette MC.

is the park/nuetral switch hooked up so that the computer will actually use the VSS info? & do you have a scanner to actually check the VSS & park/nuetral to make sure its working right.

is the park/nuetral switch hooked up so that the computer will actually use the VSS info? & do you have a scanner to actually check the VSS & park/nuetral to make sure its working right.

Damned if I know. But since the problem is new, and the rest of the set up has worked fine for a year I'll say yes it's hooked up.
What scanner? I've got an OBD II scan tool that comes up with nothing.

I'm going to try a new MC