Last weekend I swapped in a 96 T-100 dual diaphragm brake booster into my 1986 4Runner while doing an install of a new master cylinder. At first I was loving the new set-up. I had a very firm pedal, pedal travel was greatly reduced from before (prior state traveled almost all the way to the floor) and I brake response was awesome.

But after 10-15 minutes, I started noticing a problem: the pedal travel seemed to decrease more and more with each depression of the pedal, coupled with increased brake sensitivity (a little dab of the pedal was enough to bounce your head off the steering wheel) and all was also noticing brake drag the more I drove. It got so bad so fast that I could barely get to 3rd gear. In the half second gap between second and third gear, the truck slowed drastically as if someone was also applying the brake.

I pulled over and could smell my front brakes burning. I then tried to back up a few feet and could not. The front calipers were locked and I peeled out just trying to back up.

I'm in the process of moving to a new house and was out of time to figure this out myself. I called a tow truck and waited an hour, also letting the brakes cool down. When the truck showed up, we noticed the brakes had released and seemed to be normal for the time being.

I had it towed to the only place I knew was open on a Sunday - Firestone in Portland. They confirmed the front lock-up and bled the system thoroughly. At first they thought the bleeding resolved it, suggesting that perhaps some dirt got into the system and it was preventing the front brakes from releasing.

I almost got home that night before experiencing the lockup again.

I took it back to them this weekend (again, I have no time right now). They were able to isolate that it was indeed the booster but then we got into a debate whether the booster was bad or whether it was a rod adjustment issue. There was a new service manager working there and he played the corporate line that they will not work on or adjust parts that are not OEM spec. I pleaded for him to evaluate the rod adjustment, but he would not.

Desperate to get my rig back so I can use it to move, I had them put my stock single diaphragm booster back on. The lock up issue has definitely gone away but the brake pedal and overall system performance is now back to the state that I had prior to the swap.

Note: with the motor off, the booster would build pedal pressure but would also make a loud 'swooshing' sound with each pedal stroke.

...so my questions are:
1) What's the best way to test the booster to see if it is defective?
2) Does this sound like a rod adjustment issue? If so, the rod lined up with the pedal when I put it so I'm wondering how I go about figuring out the correct adjustment should I reinstall it

 

Did you do the bigger master as well? bigger calipers?

It works as a system. You don't just upgrade one component.

 

The FSM gives the details about testing and adjusting the booster. It could also be the T100 curse.

Sounds like a rod adjustment issue or your new Master Cylinder is bad for some reason the pressure on the front brakes was not releasing like it should.

So every time you applied the brakes more fluid is held in the calipers till they are locked up

Then as the truck sits the pressure slowly releases .

I can well understand the not working on oem equipment with all the law suits we are not allowed to use any but factory parts so we have a paper trail

 

HighLux - the reason I went for this swap is I had heard that others had done this with success without upgrading the hydraulic components. Perhaps it is not a viable swap?

I had always thought that if the linkage travel was within spec and mounting surfaces match that practically any booster can work with any MC since it's a mechanical device.

wyoming9 - since the issue went away with the stock booster went on, I'm certain it's not the MC. I'm just bummed that I am back to performance that I'm not happy with.

 

It's been nearly seven months since I abandoned this swap. I decided to give it another go yesterday. My assumption was that the pushrod coming out of the booster was adjusted too long, causing it preload the master cylinder. As the brake continued to drag, the fluid would heat up, expand and ultimately lock up the front calipers until it cooled down.

I confirmed that the pushrod was adjusted out too far. I dialed it back to match my stock booster, popped it on and I have no more issues.

Braking with the dual diaphragm booster has increased significantly (stock 4 cyl calipers w/31s). I had thought about a V6 caliper upgrade but I'm now feeling like the braking is more than adequate. I still think about replacing the rear proportioning valve with an adjustable one.

The other think I noticed was that the seal around my stock booster pushrod was shot, creating a leak. Thus, it's hard to say how much of the improvement is from the dual diaphragm vs. a properly functioning booster. Either way, I'm very pleased.

 

I've never heard anyone say "gosh, my Toyota pickup is great, but you know the brakes are just too weak by design." It sounds like you had a leak in your stock booster, and had you fixed that you probably would have been quite happy.

Thanks for the report back.

 

just a thought ... I don't know if this would be the issue your having , but more a "theory" I just though of after reading your post .
there is a 1 way vacuum valve on the booster . if this valve is sticking or blocked allowing vacuum into the booster but not out .. every time you stepped on the pedal it would just increase pressure on the break pads ?

just a thought



.

 

This swap is on my list. I got the whole shebang from the pull a part. Rotors, calipers, master and booster. Did the 22re electrical plug plug into the t 100 mc. I haven't looked at mine and forgot to snip the plug at the yard. Always thinking the next guy might need it. I think they crushed it the next week. My rotors were below spec but the local guy turned them for cash...liability. My assumption is there is a 20% fat margin there. ideas? A picture of the rod adjustment with measurement would be nice.

 

There's no such thing as a fat margin. There's a "machine to" or a "discard at" spec on rotors and drums. What's the worst that could happen? I've seen just about everything, including the guy that did that and wondered why the caliper pistons blew the seals as the pads wore down because the pistons were pushed out too far.

 

I haveDone the complete swap I have disc brakes in the back V6 brakes in the front with venting holes in them I did the 1 inch master cylinder upgrade now I am wondering will the 96T 100 dual brake booster fit my car