Hey everyone, I got a 90' 4runner 3vze 5speed 4wd with rear anti lock, 222k stock.
Recently it has developed this pita problem where the front brakes will grab hard at low speeds mostly when slowing to a stop, pedal feels normal. sometimes when going slow enough a hard clunk will happen and the whole damn truck will come to a complete stop without even touching the brake pedal, this will happen EVERYTIME I brake coming to a stop. But sometimes rarely it wont happen at all and all will seem normal.. also while going 20mph or more a loud "purging" sound will come from either the left front or right. it will go on for a good amount of time until i stop the truck at a light. it happens randomly but doesn't seem to slow me down. loud and annoying more than anything, can only really compare it to the sound of an ac compressor purging freon but amplified times ten! one side of the front wheels will completely lock up but will bust free after undergoing a load. this is very dangerous for me because of the recent ice and snow here in kansas city..
Very stumped on this, I took the runner all the way up in the mountains in colorado and it was perfect all the way to breckenridge and back home. brake pads are good and its always been topped full with brake fluid. anyone else had a similar issue with these trucks? Again this started happening out of nowhere, just hopped in one day and it wanted to be an ass. its traveled over 10,000 miles since I bought it and its been as good as gold until now.

 

Are you positive it is the rear brakes? I have had a bad booster give me the same results you are experencing. It affected my front brakes. In a safe area, what I did was disconnect the vacuum line to the booster and plugged the line. It will only give you 25% braking power but see if the brakes no longer apply themselves.

 

The rears do not have any issues, only happens in the front. I will test plugging the line to the booster here in a bit and see if it makes any difference.

 

I plugged the line and have only driven down the street, haven't noticed much change aside from a super stiff pedal. Stopping seems somewhat smoother but still unsure. Leaving my neighborhood was nerve wrecking! the front driver side wheel completely locked up and had to punch the gas in 4wd to unlock it, and with all the ice it was tough to regain control. I will update any differences. But if this booster is not the issue... What should i check out next?

 

Sounds to me either calipers hanging up

A crushed brake line causing the caliper to not release as fast as it should

Grease on the Rotors

 

Calipers hanging up? could you be more definitive?


And as far as updates on the brake booster... There is no difference, still having the same symptoms I have already explained earlier. Loud purging noise emitting from either side of the front wheels that comes and goes at random. and the truck still wants to stop itself at low speeds.. When I changed my front pads the other night i did not notice anything unusual, everything seemed good. it feels like the system is building up too much pressure. ideas ideas?

 

Brake assembly actuator part4510-35010 is the culprit. It is powered by the power steering pump . It could be clogged or the power steering pump is weak.
Part of the anti- lock brakes in your truck.

 

Where is this actuator located?

 

actuator location is under the power steering pump attached to the frame. follow the hard lines form the pump. Usually the pump itself is weak which affects the brakes. I had the same conditions as you describe and replacing the power steering pump fix it.

 

Hm. How strange, never on any vehicle I have owned and operated have i heard of the ps pump majorly effecting the brake system. I will look into this, but if my pump was weak... wouldn't the pump itself show any signs of stress? like whine or difficulty in turning the steering wheel? just trying to get some more insight on old toyota mechanisms.

 

have you replaced the steering pump?

 

You can first flush out the power steering pump. remember to use ATF not power steering fluid.

 

Thing is that is piggy backed to the Power Steering Pump and only effects the rear brakes .

I know mine has been removed after the rear brakes quit working all together

It really was a strange idea and only around a short time.

Curious how this will play out

 

On the off chance that it is the rear ABS actuator, which runs off the power steering pump, here is the service manual section that covers that system. There is a diagnostic procedure described that might be of help. Like Wyoming says, the system is a bit unusual, and was only used in the 2nd gen 4runners. They needed pressure from somewhere to pulse the rear brakes, and they chose to get it from the PS pump.

 

Just another thought. I'm wondering if a check valve has failed between the front and rear brake circuits, such that the pressure from the ABS system is now being applied to the front brakes and causing them to lock up. The "purging" noise you hear could certainly be the ABS actuator operating when it shouldn't.

 

Kind of curious just how this played out??