Master Cylinder Frustration

[AMXguy]

I'm about to scream. I've been trying to get the right M/C for my power disc brake '70 Challenger from the local parts houses for over a month, all 6 incorrect.  so I looked at Summit, they don't have one, so as a last resort I bought a brand new one off ebay, it is correct.

So now I cannot get it bled, I've tried everything.  the front compartment  locks up and you have to stick a pick in the brass cone where the brake line goes to let air in so it will let the plunger go back to rest position.  there is definetly something rubber behind the front brass cone, is this right? it's like a check valve. I assume there is no way to remove the cone without destroying it.

I pulled the piston out and everything looks perfect, it's just in that front cone.  I assume I have a screwed up master after all this, but I want to make sure before I see if I can return it and start all over.

[AMXguy]

After more research I find this is a residual check valve and it's supposed to be there.  some guys say to remove it and avoid trouble.

I guess I could remove it, but if I can get it bled I could live with it.  anyone know any tricks to get this done?  I'm trying to bench bleed it with the tubes stuck back in the reservior that doesn't work. I tried vaccum bleeding it too and that doesn't work.

[nqkjw]

I assume you have front discs?
That check valve is in there for drum brakes.
Are you sure your front and rear lines aren't on back to front?
If they are correct then REMOVE the rubber check valve.This valve always needs to be removed  from the rear compartment when doing a rear disc conversion.
To remove the brass cone just screw in a self tapper and pull it out with pliers or visegrips.It's easy.

[AMXguy]

Yes I have front disc.  the valve is in the forward reservoir compartment, which is for the rear drums I think?

I guess I can remove it but how can it work anyway?  I pushed the piston in and it created a vaccum in the front which held the pistin all the way in overnight, then I push on the valve with a pick and the piston snaps back. something has to be wrong.

What seals the brass cone? after pulling it is there something to replace behind it?

[AMXguy]

Another thought, maybe I'm looking at the residual valve when that's not the problem.

When you push the piston in, where does it take back fluid to release the vacuum and return?  is it from the brake line ? or is it from inside the reservoir? if so maybe that hole is plugged, if it's from the brake line the check valve is probably at fault .

What gets me is I can't even pull fluid through it with a vacuum bleeder, but that could still be the valve.

[johannes]

Yes I have front disc.  the valve is in the forward reservoir compartment, which is for the rear drums I think?

I guess I can remove it but how can it work anyway?  I pushed the piston in and it created a vaccum in the front which held the pistin all the way in overnight, then I push on the valve with a pick and the piston snaps back. something has to be wrong.

What seals the brass cone? after pulling it is there something to replace behind it?

First, the FRONT chamber is for the REAR brakes.... If you have drumbrakes at the rear, the check-valves should be installed.
Fluid shall go OUT of the reservoir...only.
The orientation of the cup (piston seal ring) shall enable the piston to return back...forced by the return spring.....the cup acting as a check valve.
I posted an illustration in another tread related to brakes and master cylinder....my guess is that you have the cup installed the wrong way ??
NB. Remember to lubricate the cylinderwall and seals using brakefluid when installing the parts together....

[Chryco Psycho]

The brass insert can be removed , just insert a drywall screw & gently pull it out with vice grips , remove the rubber & insert the brass insert , there should never be a residual pressure valve in the line to the disc brakes only drum

[AMXguy]

Okay I got another new master and excat same problem, if it's a problem.

the checkvalve is in the front, which is for the rear drums.  BUT, that valve creates a suction which makes the piston slow to return, and it never returns the last 3/16"  of an inch unless I poke the checkvalve with a pick and then it snaps back. with this shortened stroke the rear brakes will get fluid, but just a little squirt.

I screwed up the seat in the other master and couldn't find another anywhere. anyone know where to buy them if I have to try it again?
 
So,  is a shortened stroke and a squirt enough maybe it's fine? and if so do I adjust my push rod from the booster to where the piston actually returns to ? or where it will if I release the checkvalve with a pick?