Hyper-sensitive brakes, 1st 500' only

Hyper-sensitive brakes, 1st 500' only...did a search here, not much other than trucks w/'VSC' issues, which I dont believe I have [SR5]. Feels like braking smooths out after a few 100 feet, and all good unitl the next trip, if less than a few hours elapsed time since last trip.
I dont drive more than about 500 mi/month, so not sure that has any bearing on this.
Most reliable and noticeable when I first back out of driveway and at least for 1st 100 to 500', I have to drive as though Ive placed an egg between the pedal and foot. Super light pressure nearly locks up, close to lurching an unsuspecting passenger. Definitely subsides after 1st 1/4mi and then always brakes predictably and smoothly, as in no noise whatsoever.
This AM, just moved truck to park on street and just in that 20-30' of 2mph movement, I could hear mild chirp of tires. Its been like this since Ive owned [May2024] and only brake service Ive done was brake fluid flush at my 1st 1000 miles.
The brake booster/master cylinder is the 'Normal' looking design [vs the weird unit on some FGTs].
No leaks from MC, Booster, frt & rr wheel cylinders. Ive not checked rotor thickness, but can feel smooth rotor all around.
Truck history suggests well cared for, but Im guessing in 173k mi, maybe on 2nd or 3rd set of brakes, but just guessing. Might be ready for complete rebuild?
Anybody been there, fixed that?

 

Two things I've read that could be the issue:

1. Organic pads (especially cheap ones) can absorb moisture and become "grabby". Driving a short distance and heating them up expels the moisture. This would be the easiest/cheapest thing to try to fix, but I'm skeptical.
2. Brake booster or vacuum line failure. That could include the vacuum check valve. I don't know of an easy or cheap way to test for this.

A quick google search (because now I'm curious) came up with lots of people with the same complaint but no solid answers or fixes. It seems like I've had a car in the past that had this same issue but I don't remember how (or if) I fixed it. I don't recall ever changing a booster though.... I may have sold the car before fixing (haha). One other thing you could try is to turn off traction control (if your truck has it) to see if that makes a difference. Hopefully someone else is familiar with this scenario and has fixed it before!

 

No more than you drive it are you sure it’s not you?
That’s about how much I drive wifes 4Runner and Everytime I get in it for the first few brakings I almost send everything, myself included, through the windshield.

It’s me not being used to driving something with such good brakes , and very little pedal slack, nothing wrong with the 4Runner.

I almost do the same in the tundra, but i haven’t adjusted the play in drums yet so their a bit more slack in the pedal otherwise I probably would be doing the same as 4Runner.

 

@Teutonics and @Redoak...fair observations.

I otherwise drive a '20 CorollaHB and before Tundra, '01 Tacoma SR5 AC. Also wifes' '18 Prius.
Have never had this much brake sensitivity [even w/3 past BMWs, or my Resto'd '70elcamino].
Seems telling that its only sensitive in 1st 1/4mi or few 100 ft, depending on level, downhill, or uphill 1st 500'. [3 ways to exit my neihhborhood.]
Im thinking the 1st thing is to ascertain if the sensitivety issue is Frt, Rr or both brakes [given no apparent failing wheel h/ware].
I guess if it appears to be both, I would think then to check booster vaccum?
If Ok, then on to MC.
I remember going from standard rubber caliper hoses to the SSreinforced type on my ElCamino; that change created a noticeable decrease in pedal effort; almost feels the same
As I stated above, this appears to be repetitve, but also a 'initial', non-continuous issue.
I may drop into dealership and request a general brake sys check next time im in for oil change. Not that their analysis would be absolute; I still subsidize a reliable Toyota indie-shop. BUT, Ive put a bit more trust into this particular 'stealership', mostly b/c they listen to what I want and dont want.

BTW: Unrelated, but matching the brake sensitivity, one of the 1st things I noticed about this truck [not sure if its a common FGT characteristic], is that the throttle is also sensitive. By that I mean that unless Ive got the egg between my foot and the pedal, this truck will lunge fr/ a full stop. Ive had to train my foot quite a bit, especially b/c I try to be a bit mindful of fuel consumption...OTOH, I do like the freeway climb to 75mph.
I even asked a friend who worked as Toyota dealership mechanic before going on his own, if that throttle [pedal/cable/sensor or whatever triggers MAF response] could be adjusted to smooth out the bit of lurch as MAF opens. He said not possible [ECU totally controls]...not calling him wrong, but find that hard to believe, given thats a pivotal part of user interface safety that could be easily mis-managed by user; IE, inadvertant heavy foot in frwy stop-n-go, etc.

 

Internal leak in the master cylinder? Recently bled brakes? Pushed the pedal down too far?

but mine didnt let go, i had to crack the bleeders every 100 miles or so & only used the shifter & e brake for stopping.

 

@Redoak , this got my attention.

"..but i haven’t adjusted the play in drums yet..."

Wonder if my rears are the offending component? I think I recall reading that this is not as simple as it seems; involves LPV position, etc.

 

@Sirfive ...would think a hydro leak would create reverse effects: max pedal effort, spongy pedal, lack of full lockup..

Could trapped air bubble [poor brake bleed procedure when flushed 1k miles back] cause this?

 

My rear drums squeal when I first start driving it coming to a stop a nice loud squeak every time.

 

Mine was leaking fluid past the piston seal, so the first touch was ultra tight, because it didnt let the pressure out. Even not touching the brakes, as things would warm up, they’d get tighter & tighter until it would kick me out of OD.

luckily i found this place & they helped me replace the MC & warned me about pushing the pedal too far.

 

kinda interesting @KTM_AJ421 ...
Only sounds Ive ever heard fr/these brakes is some brake squeal when stopping a 5k# tandem trailer load. Super quiet otherwise.

 

https://www.tundras.com/threads/rear-brake-adjustment-theory-and-practice.99575/

 

Usually this causes the opposite problem though.

 

True. A person could still use it to back track issues though.
Check all The tolerance parameters in the thread to ensure everything is in tolerance, and go from there.
For instance: Is the bell crank moving freely? perhaps sticking? Not enough play?

I'm curious which brakes are locking up.

If it is the rears only then perhaps the LSPV is causing an issue, or wear in bell crank area, If back brake settings within tolerance perhaps there is moisture in area causing it to lock into reading wrong until it breaks free.
Also rotors and drums on vehicles not driven much in humid climate can begin to rust and cause grabbing for a short time until rust is gone.

I'd drive it a bit everyday and see if it does it every day or just the first day driving after it has set for a while.

In an unknown like his we would always start with looking at hardware first to diagnose such a problem.

We Had a truck come in once that didn't want to release brakes until they had gotten hot. And since it wasn't fully releasing it was making brakes touchy.
Found its was the wheel cylinder starting to rust internally due to moisture in the system and heat was helping it break loose. Honed out the wheel cylinder, and installed new cups, (in those days you could get rebuild kits for almost everything) and solved the problem.

I have also seen Front brake calipers causing issues from moisture in the system and doing the same as I mentioned on the rears.

One thing I would definitely check if I were him is the heat as soon as it stops being so touchy. Stop and check the heat of all rotors, and drums. If one is noticeably hotter than the others it could be a telling sign.

I had a front brake caliper lock up on me in the 90's on an old chevy. Had to knock it back with a hammer so I could move.
Brakes had been touchy, and pedal hard on starting to drive because contamination (moisture. really humid here, and used to have to ford over quite a few flooded bridges) in the system was causing them to stick 'open' and was moving forward with too much force when it initially engaged until it had been used for bit.

 

I have a bit more time now:
Also @ps8820 What did your brake fluid look like?
Was there debris, lot of trash in the bottom of catch container?

By its nature the old style brake fluid absorbs moisture, ((not sure about the new synthetics) and can cause rusting in the calipers, and wheel cylinders which can causing rusting and sticking issues.
See my post above about my old Chevrolet.
If you have a 20 year old truck and brake fluid never been changed, before you flushed it, I would think you will have some corrosion starting in your wheel cylinders, and calipers.

Does your pedal travel change after they stop being so touchy?
Does it have a bit more travel, or feel less hard?
If so it could possibly be, not saying it is, contamination in the system, or rust in calipers and wheel cylinders affecting them until it has been freed up.

Sometimes a telling sign is what I called brake pop. If a wheel cylinder, or caliper is sticking in retracted position, and someone hits the brakes since the brakes are sticking when they release they release with force and will sometimes make and audible pop as it breaks free.
Don't know if Toyota's will do that or not. I've seen it on many others in past though.

People used to never change it, and I used to see many wheel cylinders on 10-30 year old vehicles that were corroded internally beyond repair when I worked as a mechanic.

 

First rule of first gen brakes, adjust the rears. The whole system depends on proper adjustment of the rear drums. Start there, then trouble shoot your way from there.