brake MC problem

[Rich]

I wanted to do a brake fluid change as I have done many times before on my supra (and other cars). Because I had been working on the car mostly last few years in stead of driving it I hadn't done a change for a few years and the times I did drive it was on the tracks, so I decided to let all the fluid run out first, then clean the fluid reservoir and flush the whole system with fresh new fluid.

So after I did this and bled the lines the pedal kept sinking down when pressed and only when pumping it a few times it would build pressure. So after bleeding it a couple more times and the pedal feel not changing I concluded I screwed up the MC seals.

So, I replaced all internals of the MC with OEM parts (f*cking expensive!) inspected the inside of the MC for scratches but it looked smooth and fine. The old set came out straight as well. I compared the new set with the old one for seal-order (sometimes the are on backwards) and check if I got the right one and it was an exact copy, seals are on the same way.

I pre-filled the MC on the bench so the bleeding is easier. Bled the whole system using different methods; vacuum set (like a mityvac), two person bleed and as per tsrm; clear tube to a half filled jar. I did a couple of bleeds but I still have the same pedal feel.

It feels like the MC is leaking internally past the seals, there's no external leaks anywhere, also not in the brakebooster.

The brakes worked perfectly before I changed out the fluid; hard pedal feel and the car has ABS by the way. I also use DOT5.1 as I have for many years.

So if anyone can enlighten me where I have gone wrong I would appreciate it cause at this point I'm lost.

 

[iwannadie]

Did you bench bleed the MC or just fill the reservoir? From the way you said you filled it with fluid to make bleeding easier I think you just put it in the car full. You gotta bench bleed.

 

[tekdeus]

When the master is drained completely, it needs to be bench bled, or it will likely have trapped air no matter how much you bleed the system. You can buy a bench bleeding kit for $5 to $10 at any auto parts store. During bench bleeding, my master took at least 30 pumps to get primed and to get all the air out.

Doing it in a vise lets you get the cylinder level. On the car, the slight upward angle can trap air.

On another note, I did all the system bleeding myself using both a vac pump and traditional 2-man bleeding. I still plan to get my brakes pressure bled at a shop just to see if it makes my brakes firmer. I still feel they are not as firm as they should be.