Hard to explain this but after my 3.4 swap (every thing is hooked up correctly no vac leaks) if I'm in the throttle and brake abruptly the pedal becomes hard and truck will not stop. It has happened a few times times now and I have avoid crashing but luck does run out. So I need to fix this. The truck run down:
89 extra cab sr5 originally 3.0 now 3.4 5spd NO abs (didnt come with it factory)

I'm almost wondering if my booster is going bad?
Brakes are good all the way around and properly adjusted, no pinched lines ect and has plenty of fluid.

Like I said in the beginning it only happens when braking after hard acceleration.

 

Make sure you have the check valve between your booster and intake manifold.

When you hit the throttle you lose vacuum. No check valve means no vacuum at booster either. It will come back when you let off throttle but, will not maintain it without the check valve.

My 22RE had the check valve but my 99 4Runner donor did not. Must have been built in the booster. I had to use the check valve I had with my 22RE.

 

Even without vacuum you should still have brakes.

It might not seem like it trying to stop after being in a WOT for any length of time.

I had a booster fail I just plugged the line on the engine side and drove for a few weeks till I got around to installing the new one.

the hard pedal does lend itself to the booster not working like it should be if the vacuum is leaking out from lack of a check valve or other problems.

 

if the check valve is there, you can try pulling the vacuum hose off of the booster, it should make a loud popping noise, if things are holding vacuum like they should be.

this could be a good opportunity to step up the large dual booster, if you don't already have one in there... i got mine off of a t100.

 

i remember the good old days when power brakes where just on the high end cars .

 

So after playing around with the truck if I WOT and instantly hit the brakes the pedal is hard. And if I shut the truck off with the brake pedal pressed it will push the pedal back up once the truck is shut off

 

So did you put a check valve in the hose to the booster? There have been others that posted almost the same question and the check valve fixed their problem.

If the brakes were working fine before the swap, what is different now?

 

I had the same problem when I did my 3.4 swap. Brakes would go hard as soon as I shut off the engine. Was a real pain when trying to start on hills.

x2 on the check valve. Put it inline with the vac line going to the power booster and make sure it's installed in the right direction.

 

Well I'm not sure how I missed the check valve but that was the culprit. Thanks every one I musta just over looked it!