You may just need to adjust the nipple depth on the booster. The part that goes into the master cylinder.
You can simply pull the master cylinder back off the booster and use a small socket (I don't know the metric size; I have a 1/4" drive small socket set for work and just went down the list till one fit) and some small (preferably right angle) needle nose pliers. The pliers grip the base and the socket threads in or out. Just a few turns.

That, or the replacement booster is bad too. I had a bad one out of the box, but my 2nd one was fine.

Also, if you stand on the brakes *too* much, you risk getting the caliper pistons stuck. I had to crack the brake line, then compress the driver's side pistons back in, as they were stuck out and gripping the rotor even when not braking.
Then bleed the brakes, etc.

Thanks for those tips AnteredRuin. That gives me somewhere to go from here. I sure hope this remanufactured brake booster is ok. Don't want to go through changing that again.

I'll adjust the nipple out maybe 1/8" and check results.

I have suffered through the exact same problem..

Replaced booster with a used one ... did brakes all the way around
better but still... I seen brakes on a fully loaded 15 ton straight truck
fully loaded that were more responsive and worked better..

Lately brake light was coming on on occasion added a small amout
of brake fluid .. Now brakes are hard as a rock... tried bleeding them
no joy... guess I will put another booster with check valve and master
cylinder from pick a part.. Anyone have great brakes on their 4runner?

Any ideas about it?

Best Regards

MotorChris

Update

Replaced the master cylinder .. And bled.. That makes replacing all of the normal parts including 2 leaking rear wheel cylinders originally.. The good news is I found a Chinese company on ebay and I purchased a new booster with a check valve made in China for 49 bucks shipped .. Installing this morning at first light.. New Booster and check value looks good . claims to have a 2 year warranty .. Compared to 450 from advance auto or 967 from toyota its worth a shot... I will update .. Quite a while ago I did the 140 amp Cadillac alternator upgrade rocks.. Also put the Bosch 1.8k starter on it and a honker of a deep cycle battery. 1/0 battery cables.. 😆

Regards
Motor Chris

okay
The Chinese booster is still working good.
It came with a new check valve everything looks
Identical to the stock Toyota booster.
I did adjust out the nipple on the booster.
I still have some lag in the system but it is much
Much better... Thanks for info in the nipple I think that
Helped also.. The Chinese booster is okay two months later. While I do prefer oem parts a new booster from Toyota is 1000 bucks ...

Best Regards

Motor Chris

Glad to hear it. Once you've replaced the booster a couple times, it gets pretty easy. I can probably do it in under 20 minutes now.
Glad that the Chinese one worked. I got a Chinese eBay special for $60 that was bad out of the box. Better than my dead OEM one, but not by much. Still not driveable and I needed my truck back so I just bit the bullet and got a remanned one from Advance Auto.
FYI, they have pretty good coupons floating around regularly. 30% off, $40 off orders $100 or more, etc.

Do you have a link fort hat specific brake booster on ebay?

For what it's worth, I recently had my brake booster go out with an intermittent hard brake pedal, and after replacing it it was still pretty stiff until I replaced the master cylinder as well. Used a remanufactured booster from O'Reilly's since I was on a trip and didn't have a lot of options. No issues with it so far other than a burning hole in my wallet.

Adding to this thread as it came up when I searched for answers to a similar problem. I've been dealing with an intermittent hard brake pedal as well. Usually my first 10 stops or so are pretty hard/weak, and it gets better after that. I bled and bled and bled, to no avail. Switched the check valve - no improvement. Finally, bought a remanufactured booster from Napa. That did the trick. Braking is restored. No more hard brake pedal.