Stupid brake question

Please excuse my stupid first post but I did a search and on found about what master cylinder works and which dont, nothing on how they work.

Someone told me that the only difference between a disk.drum master cylinder and a Drum/Drum master cylinder are the check valves since the bore sizes are the same except for one year (1978).

Did some more reading last night and from 1976 on they didn't use the check valve any more in the master cylinder no matter if it was a disk/drum or drum/drum setup. Now why would they do that ???

So does that mean, you think I could use a disk/drum MC on my drum/drum setup as long as I use the drum/drum proportioning valve? Sounds like I would just have a bigger resivour only.

What I was worried about ar the two springs in the MC. It almost looks as if the rear/primary piston moves first and then comes in direct contact with the front/secondary piston. On the secondary pistion there is a return spring but the spring on the primary piston is interesting. If it is a retun spring then it looks as if would have some effect on the force of the secondary spring until the primary piston comes in contact with the secondary spring. A small effect admitally. Now if that return spring on the primary piston sit on a flange inside the bore then there will be no effect. If both pistons are connected together then what is the purpose of the spring on the primary piston? Unless that piston has to travel back further then the secondary piston but then they cannot be connected. In that case are the spring rates different between a drum/drum system vs a Disk/drum setup.

It makes sense what you say about the bore size in the MC but it's those annoying springs that has me worried.

Let say we have a system failure on the rear brakes because you forgot to change the length of your hose after you put you 4" lift on. Then the secondary pistion wil go to the end of trave right away and the primary piston wil be doing all the work. If you have a failure in the primary side because your oversize tires rubbed through your hose then the primary piston will go all the way to the end of travel wich in this case would be the secondary piston which would then actuate the rear brakes right away.

D*mn it see what you did! Now I realize I don't know what is happening and now I'll spend all day looking for a real explination about what is happening in a MC under all conditions. Is there any brake fluid between primary piston and the secondary piston where the primary piston can be exerting an hydralic effect on the scondary piston.??? You should of seen me trying to figure out all those little springs inside the proportioning valve. and now I don't think I got those figured out correctly anymore. [img]images/graemlins/smile.gif[/img]

If I recall correctly there were 3 types of proportioning valves. The first type was for a drum/drum and in that setup the rear brakes had a proportioning valve so that they would not lock up in a emergincy. The second had just a metering valve for the front disk and no proportioning valve in the back. And as you can probably guess, the last type had both the metering valve to the front disks and a proportioning valve to the rear.

[img]images/graemlins/laugh.gif[/img]

Was this to long for a first question post. ???