2000 Tacoma SR5 XtraCab/ 2WD/ 2RZFE/ 5-Speed/ 131K miles/ No ABS
In the 2+ years of owning this truck, the brake pedal has always seemed a little softer upon initial application than what I would consider normal,
but a quick 2nd tap of the pedal has always brought it to where it should be, and it has always stayed firm.
Lately under normal braking, as the truck slows the pedal sinks farther down. It has gotten to the point where the pedal is nearly at its max travel by the time the truck stops, and I have stopped driving it for now.
It can be pumped up, but quickly sinks back to the floor when held down.
I already changed the brake fluid and bled the entire system (the old fluid was black and obviously no good.)
The fluid level is holding and there are no apparent leaks. I've double checked the intake manifold bolts for torque, and replaced the OE vacuum line to the booster along with new clamps, but nothing has made a difference.
Through my diagnoses I followed the service manual procedure for checking the brake booster, and it seems to fail the tests.
With the engine off and the pedal held down, it slowly sinks to the floor, With the pedal held down while starting the engine, it drops slightly (normal) but continues to sink, and with the engine off the pedal will pump up but sinks to the floor under constant pressure. Seems simple enough, but in 18 years of driving I have never had to replace a booster in any vehicle I've owned vs. many replaced master cylinders.
I've been anticipating a master cylinder replacement for some time, as it seems to have classic MC symptoms, but now I am not so sure.
I don't mean to make this more difficult than necessary, just looking for some pro opinions before I tear into the wrong repair.
Many thx! YotaTech rocks!

 

It sure sounds like the master cylinder is not sealing inside the bore allowing fluid to return to the reservoir. with the engine off and after you pump the pedal a few times there is no vacuum in the booster and it will have no possible way to draw the pedal down, replace the master cylinder.

 

Definitely master cylinder. A bad brake booster will just not assist and often you'll hear hissing air either all the time or while on the pedal

 

Ty akwheeler & Kolton5543. This is just the confirmation I was looking for!
Much obliged!

 

If you haven't done it already try setting your back brakes manually as tight as you can so the drum turns but is almost impossible to remove or put on drum, sometimes the adjuster stops working or you stop using the e-brake and you get to much brake pedal travel creating the same symptom.

 

Malcolm, having the pedal slowly go to the floor has nothing to do with the rear brake adjustment

 

Are you sure about that, what if the wheel cylinders are maxed out and your brakes go closer to the floor and you have really bad braking,.. been there before with poorly adjusted or adjuster or e-brake issues causing similar symptoms, just a suggestion before he goes throwing money at parts, remember what they say, if you don't learn something new every day....

 

I learned this a long time ago Malcolm, like in the 1980s, if you push on the brake pedal you will not feel pressure until the shoes/pads are touching the drums/rotors, (if you don't have a blockage in the brake lines). when that pressure bleeds off and the pedal goes to the floor you either have an external leak or the seal in the bore of the master cylinder is bypassing fluid. So if you are not having to refill the reservoir you are not loosing fluid so it MUST be the master cylinder. So, Yes I am sure about that. the difference is "˟˟˟˟ty braking pedal goes to the floor" and "pedal slowly goes to the floor".

From the first post by automan "as the truck slows the pedal sinks farther down."

 

Sounds logical I won't deny, I just learned a short time ago when a friend brought me his Taco and a master in hand for the same symptoms as above, I pulled the rear brakes apart and it had no e-brake or adjuster locks in the drum, no back brakes, brakes would practically go to the floor. Been 5 years now and it fine now, but hey it was just a friendly suggestion, I have also learned that ones description of whats actually going on may differ from another, and "as the truck slows the pedal sinks farther down." is identical to what was happening but to each his own, I prefer to check the obvious before I buy parts.

 

I see where you were coming from. I think your friend's Tacoma was more a case of "there is nothing keeping pressure in the system" problem. The pedal travels until resistance is felt via pressure in the system. Since your friend's Tacoma didn't have resistance (pressure) in the rear brake hydraulics, the pedal just went to the floor.

I did drive my truck for a day after changing my brakes without adjusting the rear brake shoes all the way. Of course I made sure it would stop fine before taking it out on the road. I still had good brakes, but the brakes engaged lower in the pedal travel. After adjustment, the pedal travel was back to normal.

 

Akeheeler is correct. If the brakes are out of adjustment then the pedal will travel until the shoes reach the drum. At that point the pedal should firm up and hold pressure. An internal or external leak will cause the pedal to slowly fall to the floor with a constant applied pressure to it. Another thing that can cause an identical symptom is a leaking ABS dump valve. They will leak fluid past into the accumulator and cause the pedal to fall too. I don't know about the Toyotas but I know on the 90s Chevys with the Kelsie Hayes ABS there is a small vent going into the accumulator. I'll put a paper clip into the vent hole then apply the brakes. If the paper clip moves out any then the dump valve is leaking.

 

So funny related story, I'll keep it short. Friend bought a 99' 4R a month ago, Brakes worked perfect but fluid was old looking, and we replaced the 3rd member differential as the pinion gear was smoked.., axle seals and rear brake lines needed replacing among other unrelated items to this story, it had new brake shoes and 200,000Km on the clock. We finished the work a week ago or so. He texts me yesterday saying his brakes where going to the floor and he bled them again still going to the floor or creeping to the floor when pumped running or not, but had brakes up front still sort of no leaks anywhere. We bled them again in the proper order a few times still same issue and no leaks anywhere. Pulled the drum and somehow when I wiggled the tight fit drum on or off with the brakes adjusted to tight I must have knocked the adjuster rod from the groove in the 1 shoe and it was sitting in there loose.., put it back together and brakes are better than ever, end of story,.... but yes it could be the master cylinder as well but you really need to check a few things before you replace parts just incase, I make mistakes though obviously as I did when I put the drum on/off or when i put the axle with brake assembly in or took it out somehow, in my defence it was dark on that side and I wasn't working in my own shop.

 

Apologies for the very belated follow up; I totally missed the ensuing debate on this. However, akwheeler & Kolton5543 were spot on with their advice. The master cylinder was indeed the culprit, and a new one fixed it right up.