Brakes went soft completely out of the blue

Within the last three years I have replaced front brake pads and rotors and the rear drums and shoes. My brakes have been firm and effective until a week or two ago. The pedal has gone soft. On first push of the pedal it very nearly goes to the floor. at any decent speed on the road I would definitely not be able to stop suddenly. I have parked it until I can figure this out. Let me add these facts and observations:
Front pads and rotors are in great shape
Rear Shoes and drums are in good shape, one drum may be ever slightly warped
No leaks anywhere I can see. it stays in my garage which has a painted floor and would notice a leak
Brake reservoir is absolutely full and has not been topped off since the brake work a few years ago
The brakes will pump up and actually seems like they will work, but a firm push on them 1) wont stop the truck effectively 2) pedal goes to the floor after "breaking through" the initial firmness i was able to get by pumping. To be more clear it initially seems like it will be a crisp braking action then the pedal breaks through something and goes to the floor.
I am about to dig into the problem and thought I'd float it here first. I intend to bleed the brakes tonight as that seems to be the fist action prescribed in the service manual.
Any thoughts will help!








2003 Tacoma Extended Cab 4x4

 

Vacuum pump failure maybe?

Funny how the 3rd gen manual says to replace that part like it's a consumable. Wonder if it's the same on yours.

 

Master cylinder gone bad? Time for a rebuild? Disclaimer: shot in the dark, no actual experience or authority to diagnose on a hunch.

 

Usually brake booster or master cylinder, assuming no air in the system.

If I had to guess, probably master cyl.

 

Thanks for the feedback. another quick question: what is the function of the booster, beyond the obvious? how does it work?

 

Booster uses vacuum from the engine to enhance your input to the brakes,giving you more braking force than you would have otherwise.

 

Engine intake vacuum will provide brake pedal force amplification via a diaphragm booster.

https://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake1.htm

 

With no fluid loss evident I'd lean towards the master cylinder. I haven't done one on a 1st gen but the part is less than $100 and usually only takes about an hour from start to finish, including bleeding the brakes.

 

Sounds like the master cylinder took a dump. I had an old Honda that did that. I took it in for a smog test and as I was leaving the shop the brakes went soft. Changed out the m/c and bled the whole system out and brakes were solid again. Remember to bench bleed the m/c before you install it back in your truck.

 

If you are losing fluid and its not on the ground, then its draining into the brake booster....
Master cyclinder rebuild kit is about $70.....
If that brake fluid gets into the booster, those are many hundreds of $ to replace....
So neglecting a failing master could get very expensive.

If the booster is going out, you would of gotten a hard pedal instead.

 

Can the load sensing proportional valve over the rear axle cause this? When I was looking into replacing mine, I seem to remember some reports of a soft pedal indicating a problem with it.

 

If you got air/moisture anywhere in line, the brakes get super soft when all that heats up and expands.....
Is there a difference between cold and hot?

Any chance you need to rebleed the master.....

 

No they are soft pulling out of the garage and soft putting it back. I have ordered a new master cylinder. My plan is to remove the old one, bench bleed it, install, do a system bleed and see where i am. Thanks to everyone for the input.

 

OP..what is result of your brake problem?