E32 booster/master cyl w/02 res...can't bench bleed, no check valve?

[lcoleman]

I have had an e32 735i booster and master cylinder for a long time now, but since it's a junkyard unit I am attempting to replace the with a new one. I also bought the 02 remote reservoir setup as outlined in this thread.

As I understand it, when reinstalling the new MC, a little fluid should dribble out, but there is a check valve to stop all the fluid from draining out. On my setup, this isn't the case. All the fluid begins running out slowly, no sign of stopping until the res sucks air. If you don't get the hard line threaded in fast enough, back to square one. That's where I'm at now, no amount of two-man or pressure bleeding will get the air out. Brakes work ok on the first press, better on the second, and usually like normal on the third. I have completely flushed the system (fluid changed color completely).

I used a cheap Autozone reman'd master, could that cause this issue? The new OEM piece is prohibitively expensive.

Could the new master just be plain faulty?

Is there a secret to bench bleeding I don't know?

Thanks in advance for the help.

[RobertC]

Pump the pedal and apply pressure, now crack the hardline at the master. Fluid will bleed out with any air that might be trapped in the master.

Could be a defective master as well. It's from Vato Zone. They rebuild them by the hundreds and expect a fairly high failure rate, that is why all there shit has a no hassle free exchange. They know its all crap, they expect it to come back.

Basically try to bleed it as I mentioned above and if you have no improvement then take it back and exchange it for another master.

[lcoleman]

That had a pronounced effect, but the issue is still present. (had a few sketchy moments on my Dragon run today) I guess I will swap this one for another and see if I can bench bleed it/the check valves work. Thanks for the input.

[jlevie]

There are no check valves in a master cylinder, nor can there be.

Air in the system causes a soft pedal that never gets any better. If the brakes feel better on the second or third pump the cause is usually a bad master cylinder. Though badly worn brake pads can require an extra pump of the pedal to get full brakes.

[lcoleman]

Thanks Jim. Yeah, pads are still good. Do you have an explanation why all the fluid runs out of this one, but only a small amount comes out of the one it is replacing? I mean, there must be something in there.

I'm just going to get a new TRW/Girling unit. Price sucks, but it would've been totally worth it to have a working one in the first place.

[jlevie]

Thanks Jim. Yeah, pads are still good. Do you have an explanation why all the fluid runs out of this one, but only a small amount comes out of the one it is replacing? I mean, there must be something in there.

I'm just going to get a new TRW/Girling unit. Price sucks, but it would've been totally worth it to have a working one in the first place.

The one you have could (probably does) have bad seals. Pony up for a new (not rebuilt) master cylinder and be done with it.