Disc brake master cylinders

[Ghoste]

I ordered a new master cylinder for my car because the one that is in there is for a drum brake car.  The new one has a longer bore but both resevoirs are the same size.  Shouldn't one be much larger or does the longer bore compensate for the additional fluid needed?  If this one is wrong, will a master cylinder for an E-body with discs work properly?

[Chryco Psycho]

most but not all disc masters have the larger reservoir for the discs , the Key difference is the lack of a residual pressure valve in the disc side of the master cyl , just remeber to check the brake fluid more often

[Ghoste]

Well, here is my dilemma CP.  The one in there is for drum brakes.  At least, I think it is.  It has a short bore (don't know the diameter) and it has the same size resevoirs.  It was fine until I added a linelock to the car.  That extra bit of tubing and the soolenoid were enough that when I want to really hold the brakes, it takes the entire pedal travel.  I don't have fear of stopping the car (at least not until the pads wear some more and it requires that much more fluid to stop), but when I want to hold the car against the converter, I clamp the brakes hard enough.  I can push the car at a pretty low rpm.
How can I check this new one for the residual valve?  Once I put fluid to it, they aren't going to accept it ona return.  It does have a longer bore than the unit on the car now but I don't know the diamter.

[Just 6T9 CHGR]

Mabey this might help???

[Ghoste]

No but thanks just the same Chris.  Actually, that table is from the Mopar Action article and it is what made me start doubting what I have in the first place.  Thanks though.

[Chryco Psycho]

something is wrong , adding a line lock should have no effect on pedal travel , so if the pedal moved 1/2 way down before it should with the line loc in place also , so you either still have air in the system or it is leaking
the residual pressure valve is inside the master cylinder & has to be removed if using a drum brake master on disc brakes , the pressure valve is behind the tapered fitting where the line bolts on so remove the line thread a drywall screw into the brass fitting & using vice grips pull out the brass fitting , behind that si the residual pressure valve , the trick is to not damage the brass fitting badly so it will still seal to the line
right now I`m trying to get hold of more fittings to replace the pulled ones with rear disc brakes being a lot more common conversion now we need to remove the RP valves , if the RP valve is left in place the disc brakes will drag & can warp the rotors 

[Ghoste]

Exactly.  What is wrong is that I have a drum brake master on there and it doesn't have sufficient capacity.  As my pads and rotors have worn, it takes more fluid to fill those old four piston calipers than the drum master on my car was meant to fill.  It was close to the limit before and adding the line lock and a another year's wear is just taking it to the limit.
Are you saying the only physical difference is the RP valve?

[Chryco Psycho]

as you can see on Chris's chart the bore size changes also

[Ghoste]

I picked up a master from NAPA based on a number that was supplied to me by the guy who wrote the article that table came from.  It isn't on the car yet but it is physically larger in almost every dimension so I hope that means it'll push more fluid.
Thanks all.

[General Ryan]

if the RP valve is left in place the disc brakes will drag & can warp the rotors  

Would that also make the pedal very sensitive and touchy to make the brakes grab/almost lock up feel??