Fixed:Brake woes. 2nd master cylinder and still popping and dropping brake pedal

1997 taco with the 2.7 4x4 non ABS.

I replaced the master cylinder once and could not get the rears to bleed and the fronts were never right. Installed NEW MC after bench bleeding. After some reading I found that the Load Sensing Proportioning Valve (LSPV) in the back of the truck could be the cause. I zip tied the spring arm to the bottom of the bed (simulating a full load) and I was able to bleed the rear brakes. Front brakes had no fluid coming out until I dropped the LSPV spring back down.

Now my brakes are bled for the most part with only micro bubbles coming out of the front calipers(I am hoping this is a frothy fluid situation as I cannot find any leaks).
My main issue is the brake pedal is stiff but if you apply the brakes hard (sitting in the garage) the pedal will seem normal followed by a "pop" and slight drop of the brake pedal. This only happens with a ton of pressure applied. I have been bleeding the brakes over and over but no dice. Ill post a video.

https://www.dropbox.com/s/qhvfz9cjr17rv0i/VID_20150630_140812.mp4?dl=0

 

start over and bleed the front brakes first until completely clear fluid comes out, do the closest wheel first then the other side and do the same for the rear

the way its acting it is like the pedal reaches a point where it has gone too far and the fluid bypasses around the piston and lets the pedal drop, cant say I ever saw that before with a new MC

are you using name brand parts or cheap aftermarkets or reman stuff

 

I re-bled the front brakes and they still had the micro bubbles coming out, but just a few every time I cracked the bleeder. I checked for leaks again and was unable to find any. I can't remember the name brand but it is whatever company O'reilly auto parts uses for their new MCs. The MC is new.

ETA: I am bleeding the brakes with a helper pumping the pedal, not a mityvac or something else. Mityvacs always suck air into the tube when bleeding so I avoid them.

Thanks for the help!

 

I've never seen that behavior before, either. Did it do that before you replaced the master cylinder?

It looks to me like I imagine it would behave if you're pushing the master cylinder too far and getting the piston past the hole to the brake lines. That's a wild ass guess, though.

 

The brakes were doing something similar but not like that. Pedal became spongey as well. I dunno.

 

I finally caved and had a shop do diagnostics on it. They said all lines were good. MC was good. They had another guy listen for the noise while somebody pushed the pedal and found the noise was coming from the LSPV. It seemed as if under heavy braking the valve would "pop" and make the pedal drop a bit. They "evacuated" (whatever that means) the LSPV and bled the brakes again and the noise did not come back. They said to watch it and if the noise comes back to replace the LSPV. I guess it can stick in certain situations.

 

That's great. Thanks for the reply. I don't the proportioning valve would ever have occurred to me. Having someone else listen was a good idea.

 

Sure thing. Hopefully my experience will help somebody else in the future.