Last week I made some changes to my '86 4Runner's braking system. The braking seemed week and I think the servo wasn't working so I replaced it and the M/C with ones from a '91 4Runner.

I'd read about the benefits of this because the later V6 servo is a dual diaphragm and the M/C has a 1" bore as opposed to the 22re equipped 13/16th " bore. Plus it's a direct bolt-in swap. The stuff I picked up for the swap was absolutely mint looking and was in perfect working order...



I've done the swap (easy) and bled the system a few times to get a full fluid flush and the braking is definitely better...but it's not nearly as good as my 1990's braking.

I thought for sure that these changes would make the '86 perform equal to the 1990 since everything else (to my knowledge) is the same.

The problem is that it's still relatively difficult to push the pedal in to get the kind of stopping power I'm used to. The pedal doesn't want to travel down as far either before I start to feel the stopping effect.

Any suggestions here?

Is it possible I'm not getting enough vacuum to the booster? Also, why would the pedal start braking so much sooner than the pedal on my 1990?

Thanks much for any feedback,


Warren

 

Sorry, this is going to sound like I am an engineer techno geek, but I am not.

The new master cylinder has a much larger diameter. The piston of the 1" cylinder is .78 sq inches. The piston of the 11/16" cylinder is .37 sq inches.

So, with the new cylinder (1") you only have to push the pedal half as far to move the same volume of brake fluid as the old cylinder (11/16"). This is why the brakes engage sooner.

But you will also need to push twice as hard to get the same braking effort (assuming manual brakes, the dual booster probably helps a lot towards this, just not enough)

The design of the master cyinder is matched to the design of the calipers and wheel cylinders (big cylinder and small brakes= fast action/low pressure::::small cylinder with big brakes=slow action/high pressure) If you want a little volume improvement, you might try to find a 3/4" master cylinder

Does your truck have good engine vacuum? I just worked on my dad's diesel that had bad vacuum to the booster (it uses a vacuum pump instead of engine vacuum). The brake pedal felt solid, but didn't get enough pressure to the pads.

You might try putting your 11/16" cylinder onto the new booster and see the improvement of the dual diaphram with your old cylinder. I think it will bolt on.

Was the 91 (donor vehicle) a V6 engine? They did use bigger brakes I am told. Might be why they have a larger volume of fluid being moved

 

I'm not sure if this is it, but if the calipers are not the same as V-6 models, then that might be the cause. I know on some of the trucks and runners the calipers were smaller. If that's the case you will have a faster engagement like yoda said, but if the calipers are smaller than what the V-6 models are, then you'll get very little braking performance.

 

"If you want a little volume improvement, you might try to find a 3/4" master cylinder "

Did you mean wheel cylinders here?

"You might try putting your 11/16" cylinder onto the new booster and see the improvement of the dual diaphram with your old cylinder. I think it will bolt on."

and here you're saying that I might try my old M/C with the new booster to see if it aids braking power right?

I'm pretty sure the '91 donor vehicle was a V6. I didn't realize that the calipers and wheel cylinders might be different. I'll have to check into that. If they are, I think a good route to go would be to put the larger calipers and wheel cylinders on my '86 to boost the braking performance. This will also help with the larger than stock tires.

Thanks for the feedback guys.


Warren