brake pedal drops slowly under pressure??

1997 4runner 3.6 v6 auto 4wd stock rims and tires with abs. I have replaced the rotors with oem and added a set of duralast creamic based pads with new shoes and drums on the rear. Master cylinder and reservoir are new also. I have visually inspected all the hard lines and checked each wheel for leaks and the level in the reservoir hasnt dropped.

The entire system has been bled completely a couple of times using new fliud and no air bubbles remain at the wheels. When the engine is off you can pump the pedal 3 times and stand on it and it does not move. With the engine running if you let it sit for a short time and go for the pedal it drops almost to the floor (within an inch or so) then if you push it again it firms up but then slowly drops to toward the floor under pressure. The truck stops very well when you have a chance to pump the brakes first but that first push it feels like theres nothing there. This is my wifes daily driver and I have the same setup on my taco with no issues only diffrence is I have no abs. Please if you have any suggestions let me know!!!!! Im not sure what to do next.

 

Check for loose wheel bearings. "Play" in a wheel bearing can allow the rotor ( and hub and wheel) to deflect from its normal rotation. This will push the break pads further back into the caliper than normal. Your first pump makes up this distance and the second gets the job done. (Assuming your calipers are tight) Sometimes wheel bearings can get pretty sloppy without making a lot of noise and a symptom like this can be the first hint that something is wrong. Unfortunately these bearings are a pain to change ( pressed in non adjustable and "sealed") If this is your problem, and you want to "DIY" , do a search, there are several REALLY GOOD write ups.

 

Did it do this before you did all the brake work? It possibly could be the brake booster, the diaphram is ripped/leaking.

 

This is what I was thinking, this or the master cylinder seals are leaking by. If you don't have a leaking wheel cylinder, and your pedal slowly makes its way toward the floor at a stop light, one of those two things are bad.

 

This^^

 

The front wheel bearings are less then 6 months old. Yes the pedal has been spongy since i bought the truck. Everything Ive done has made it better but its still not right. Is there a method for testing the booster or the seals in the master?? Thanks for your replies guys!!!!!

 

There is another thread that is the same issue. In my case, it was the actuator. Read up on this thread:


http://www.tacomaworld.com/forum/2nd-gen-tacomas/22021-spongy-brakes-2009-taco.html

 

ok so it ended up to be your abs actuator? I will go and check if the rear shoes are close enough to the drum hat. That is something I had not checked. The master cylinder is less then a month old and is new not refurbished. Is it possible I got a bad one???? Again thanks !!!!!

 

Did you install it ??? If so, did you bleed it properly ? A master cylinder needs to be bench bled before installing, then I usually go around and bleed each wheel to be sure any and all air is out. Boosters rarely go bad, less often than master cylinders anyway. I suppose it is possible you got a bad master cylinder. But I would bleed the hell out of all lines first. I'll have to get back to you on a possible bad booster, I've never experienced one myself, I'm sure there is probably a vacuum check you could do on it, maybe someone else with experience could chime in.

Edit: Forgot to add... A wheel bearing, brake rotor, etc etc... has nothing to do with a spongy or fading brake pedal, I don't care what anyone says, it's completely and totally unrelated. However, a bad bearing could cause a vibration in the pedal as your are slowing down, and a warped rotor will cause a pulsating pedal which most people have experienced at some time or another. Once you are stopped, and the pedal starts going to the floor, something is leaking by.

 

Ok, got confirmation on the booster... Get a little vacuum hand pump with a gauge if you don't already have one and make sure it's holding vacuum.

Adjust your rear shoes as mentioned. Make it so they're JUST free enough to allow the drum to spin by hand, even if they're rubbing just a touch that's ok as long as you can spin the drum.

If your cylinder was properly bled and you're sure there is no other air in the lines, you have a defective master cylinder.

One other suggestion is use a hand pump to do your bleeding if you're not already. It's the best way to make sure you're getting all the air out.

I hope this helps you bud !!!

 

Yes I installed the master and bled it before according to the instructions included in the box.No air remained in it when reinstalled. I will go get a vacuum gauge and chek the booster for pressure. Thanks everybody TW rules!!!!

 

If you go to sears, they have a nice little kit that includes the pump and some attachments, you can use it to check vacuum as well as bleed brakes or whatever. Harbor Freight has one for cheaper, but that one didn't last me very long.

 

It was the rear brakes not adjusted properly. I jacked up the rear and pulled the e-brake and spun the tires and thet just rolled. Thanks to everybody for the suggestions.

 

Hey all!

Sorry to revive this thread but I've walked into a very similar issue!

The description of my problem would sound just like the OP.

I've replaced the front calipers, rear cylinders, pads, shoes, rotors, drums, bled-bled-bled, then had an actual mechanic bleed them again and adjust the rear brake shoes to catch right.

I still get intermittent spongy brakes where I need to let off and push again to gain brake pressure. not fun when you're pulling a small trailer in NYC!

(most of these parts were replaced just to do the job right and not come back again and The truck needed new calipers as they were seized)

Is it likely that the master cylinder needs replacing to gain full strength brakes again?

 

if you have a hard pedal when the truck is turned off then the master cylinder is good, if the pedal sinks to the floor with the truck off then its bad

 

so if my pedal is solid when the truck is off after I give it 2 pumps, but is mushy and shit-tastic when i'm driving....
but when I think about it, sometimes the pedal just goes soft before I start the truck and I can't really get solid pressure......

whats your verdict?

 

Anyone else have a suggestion?

 

with the truck off you should be able to give it some pumps but should harden up, if it sinks to the floor then its bad master cylinder from what I have heard, I had the same issue as you after replacing the front pads, its much better after adjusting the rears, and I think bedding the pads helped also, when I initially replaced the pads it was very noticeable. but I think as another guy stated there is buildup on the rotors that needs to be burned off by proper bedding procedure I didn't do that but after time the brakes have seemed to bedded themselves and are working good now on mine.

 

that kindaof sounds like a Master cylinder I would have it checked out, I took mine to a dealer and asked them to test drive mine and they said the soft pedal is a norm but to adjust the rear and maybe cut the rotors if I wanted better, if it comes to replacing the master cylinder it doesn't look that hard to do yourself but lots people say to use an OEM Toyota master cylinder not an aftermarket one