Can you use manual master cylinder in power application??

[yody]

I'm doing a job for someone. They brought me the touring classic front and rear disc brake kit, looks like C4 rotor/C5 caliper front, and LT1 rear. The master cylinder they sent is a manual master cylinder. They informed me that it would work if I only modified the pushrod longer, because the depth of the hole in the back of the master cylinder is deeper. I did a bunch of measuring and made a longer push rod. I am 100% sure it is the correct length. I am also 100% sure of the way i routed my brake lines. Now I get no pressure out of the port for the rear brakes. I bench bled the master cylinder and air did come out and then stop coming out. but I can compeletly take off the line to that port and pump the brakes and fluid isn't coming out, there is also a loud squeeking/rubbing noise once you push the pedal farther than half way(which could have something to do with booster?)



The guy at touring classics assured me that the master would work, but I have this nagging feeling that something about it just wont work. the bore on the master he sent us is 15/16", is it just a bad master or do we need a power master cylinder, if so which one? I have heard a disc/disc power cylinder from a C3 corvette works well? thanks!

[yody]

just thought of something, wondering if this could be a quick take up master???

[chicane67]

Disc-disc from a C3 is going to be around 1 1/8" in diameter. There is a good reason for the recommendation of the 15/16"...... it needs to have the line pressure that a master of that size provides. Its a pressure to volume issue.

You are really trying to achieve 1200 psi no matter what you end up for caliper/master combination. The pedal ratio you have is 6:1. With that to create 1200 psi, you'll need a caliper piston volume of 13 to 14 (or so) square inches..... which would equate the use of roughly a 15/16" diameter master. Anything larger and your pedal effort will increase.

The meaning of 'power' or 'manual' in reference to a master cylinder is really related to one side of the scale or the other in size and the fact that it has a power has a deep and the manual has a shallow push rod hole.

Bear makes an insert for the deep master to convert it to the depth of a shallow. Doesnt make athe requirements of making a new rod..... but its too late for that.... so why did I bring it up !?!! I dunno.....

[yody]

so maybe this master is a quick take up, that would explain no pressure in the rear port, because you are supposed to wait 30 seconds between strokes. How can you identify those? BTW this is on 68 firebird

[Q ship]

Cody, when you look in the bowl of the master, you will see the valve in the outlet. Look at figure 5 & 6 of this link. I have one out of a S10, and it's pretty obvious when you look. If you need a pic, I can go dig it up and post one-it's not on the car yet(heh, nothing is).

[MrQuick]

you didn't modify the combo valve did you?

[chicane67]

Dont you have a Honeymoon to attend or something ??

[yody]

well I got the system to bleed, i don't think it is a quick take up cylinder. Not sure what i did right, but the rears wouldn't bleed at all. So I said screw it and bled the fronts instead, that got the ball rolling and for some reason teh rears decided to bleed after that? I have an iinline adjustable prop valve, and am just using a brass "T" for the front system. Got all the air out of the system. Pedal isn't rock hard, but i believe that is due to the 15/16" bore. If anyone is using powerbrakes I would suggest the 1" bore for a higher firmer pedal, but we also need to wait for the pads to bed in. Booster is still rubbing/squeeking, not sure what that is about. But I will worry about that later.

[MrQuick]

your right Tom, but i can't seem to sleep! LOL check out my post in the outhouse...gotta see Allen SMASHED!!!! ROFL!!!!!