Brake Booster Failure *SOLUTION FOUND*
 

Vehicle: 2001 4runner SR5 4WD with 250k.

The other day while driving, a few lights come up on the dash, and a loud chime sounds. Still drives fine for 2 blocks, then no more power brakes (i.e. brake pedal really hard to push).

Read up on forums and factory service manual, and it's the brake booster pump that isn't running. That explains why it still drove fine for two blocks; it had enough pressure still built up. I disconnected the electrical connections to the unit and put power straight from the battery to the pump (recommended by FSM) and nothing happens. I removed the whole assembly (master cylinder and brake booster), thinking I need to swap in a new unit. Once it's out of the truck, I try connecting power from battery straight to pump, and it runs fine. So, I reconnected all the electrical, turn the key in the ignition, and the pump starts up no problem.

The only difference between the pump working and not was the process of disconnecting it, and I'm thinking that maybe there was a loose connection that got knocked around. Also, I had to disconnect the brake lines to remove the unit and they are still disconnected, but I don't think that should make a difference. Other than that, haven't taken apart the unit at all.

I don't really want to put it all back together not knowing what the problem was, in case it randomly comes back. I'm also questioning whether the problem is with the pump itself, since it does work sometimes apparently.

All the following is fine:
- Vacuum hose has no obvious cracks/leaks
- Battery has plenty of juice (12 volts)
- Idle is unchanged when pump runs (saw this was a problem for other peeps)
- Brake fluid was topped up/clean
- All brakes were at least 80% and worked perfectly before this problem
- Manual brakes still worked even once pump malfunctioned (i.e. hard pedal, no lack of pressure)

Does anyone have any ideas? Appreciate any help.

No ideas with your actual problem, but I can tell you the lowest price I've found online for the complete master cylinder/pump/booster assembly is $1,900.....

My setup works fine, but has been slowly leaking fluid from the 3/4" round, recessed 'plug' at the front of the cylinder casting. I rigged up a small dish underneath to catch the fluid as is was dripping on the inner fender sheet metal. :(

Used ones I've considered on Ebay all seem to show similar signs of corrosion at this 'plug'.


Andreas

Thanks for the heads up. The replacements online are pricey, I'm thinking of going to a wreckers to pick up a replacement. I'll keep an eye out for the corrosion

Well I think I found the culprit.

Took off the cover off the pump motor, and found a really fine brown dust covering everything. It looks like normal wear and tear from the brushes. Seems like too much though for 250000km (155000 miles).

Attachment 143839

Using a multimeter on the electrical connections to the pump, I could get a short just by moving the probes around a little.

Attachment 143840

Attachment 143842

We used a vacuum with a straw attached and some compressed air to get the dust out.

Attachment 143844

After getting all the dust out, I couldn't get a short no matter how much I moved the multimeter probes around. I'm convinced it was the cause. Hooked it up straight to the battery aftewards and it works. Putting it back in this weekend.

If you want to try this, taking the pump motor off is really easy. There are 5 bolts you can't miss that you need to remove. Once removed, the pump motor will still be difficult to take off because of the strength of the magnet. I unbolted waaayy more than I needed too, but I don't think you need to disconnect anything else.

This is an awesome thread! These things fail all the time, and Toyota wants a MINT for these parts.

If this fix works for anyone else, feel free to post up. May make it a sticky if enough people can confirm this works

Thanks! Yeah this fix cost me let's see.... the power to run a vacuum, and a few q-tips. :bigok:

Well the problem came back after a couple of weeks, so it was time to dig a little deeper.

This time I took apart the pump itself.

Attachment 142738

On the part of the pump that the multimeter probe is pointing to, you can see some of the copper coating has worn off. That was the problem all along, and it explains intermittent operation that some other people have experienced.

After the motor stops spinning, if one of the brushes ends up on this dead area, the motor won't be able to start up again. Sometimes I think it will get jolted back into place and can fire up again. The problem wasn't continuity, it was lack of continuity.

I have no idea how to fix this. Might be time to throw in the towel and go to the wreckers.