New Master Cylinder, brake pedal slowly goes to floor

1998 4Runner 2wd, automatic, 342,000 miles.
My new master cylinder was replaced by another brand-new master cylinder because pedal would slowly go to floor. The problem remains so the master cylinder I replaced wasn't defective. No fluid leaks observed. I have not had to add fluid. 4runner stops ok in normal use. Pedal feels good with engine off, only slowly sinks to floor with engine running. Will stop & hold on a red light (for instance) but pedal will slowly sink. Took off master cylinder top and observed no movement in fluid while pumping brakes. In the last 2 years I have replaced master cylinder, booster, pads and brake shoes. Where else might I look?

 

I don’t see any mention of bleeding the system, and while I’m sure it’s just an oversight on the part of your typing, this sounds a lot like air in the system.

 

Sorry, yes, it feel like air in the lines.
I bled it three times at the wheels with a hand vacuum pump. No change. I took it to a friend and we bled it the old-fashioned way at the master cylinder & wheels. No change.

 

Strange, because All signs point to a bad master cylinder.

 

Had an issue recently with a Mazda 5. Similar issue but not exactly the same. Soft pedal at all times.

Replaced everything...New MC, New Calipers, bled the system many times, etc.

Wound up being the ABS unit had a stuck internal bypass valve. Which is actually something known that can happen but it's hard to diagnose since there is no real test for it and it's a hella expensive part.

Do we know if the ABS system in these cars could cause a similar issue?

 

i dont know if this is helpful, i have 1999 service manuals and it says to bleed the rears first, or booster push rod adjustment

 

Yeah... the ABS is about all that's left. Which is gonna be beyond my personal capabilities. I'll have to bring in professional auto techs.

 

Try Bleeding it with the engine running RR, LR,RF, LF. Make sure that the hose from the bleeder screw never sucks air back in. Bench Bleed the Master Cylinder BEFORE INSTALLING> Make sure your rear brakes are adjusted properly. IF this fails then yes take it to a professional. But Unless the ABS is Fubar which is probable but not likely you just did not get the air out. I have had to pump almost a Gallon of Brake fluid thru my truck after replacing all the lines and rear wheel cylinders before getting all the air out. It also helps when bleeding the brakes to OPEN the bleeder Slow push to the floor. Close bleeder pump the pedal, Start over. Two person job take your time.