Drum 2 Disc swap: Which master cylinder do I need?

[thecatseye66]

Thanks for checking to see if you can answer my question.
I just finished changing the entire front crossmember from a 1977 2500 Trailering Special into my '68 C-20 396 that already had an original power booster. Now, I can't get the brakes working right.
When I took the parts off the '77, I took the booster and master cylinder, all the brake lines and the proportioning valve from the donor truck too, but I couldn't use the booster or the m/c from it because 1) booster mounts differently to the firewall and it has a solid, long rod that connects to the pedal rod directly through the firewall (instead of the booster already on the truck which has a lever-type connection), and 2) booster/master cylinder has a short pushrod setup that won't let the m/c work with my original booster.
I used the lines and the proportioning valve from the '77 but I used the original master cylinder and booster from the '68 and, well, it isn't working right.
I bench-bled the m/c and I've got a solid stream of fluid at all four bleeders (a lot of volume from the front calipers), but the pedal goes all the way to the floor before it will hold the truck at idle.
I'm pretty sure I need a master cylinder especially for the disc/drum configuration (is it because the the discs need more pressure or volume?) but I don't know what one to buy that will work with the stock '68 booster?
Will the master cylinder from a 71/72 disc/drum bolt up to my '68 booster and solve the problem?

[thecatseye66]

anyone?

[krue]

You need a 71 or 72 master cylinder to make it work right. Or some have done the mods and used the later model master cylinders on their trucks.

[thecatseye66]

Originally Posted by krue

You need a 71 or 72 master cylinder to make it work right. Or some have done the mods and used the later model master cylinders on their trucks.

Thanks Krue.

What modification do I need to do to use the newer m/c? Does someone make a shorter inner booster rod that will work?
The newer m/c has a short dimple on the piston where the rod from the booster meets it, but the older m/c has a recessed hole that allows the rod from the booster to go in an inch or more before it touches the piston.
When I try to bolt the new m/c to the older booster it just presses the piston in before it can even be attached. I tried to use the rod from the newer booster in the older booster but it is too short.

[72longbed]

I just did this swap on my 72 K20 but I went to a disk/disk setup. I have some parts left over from the conversion that I am willing to sell. A Classic Performance Parts (CPP) 1972 Disk/Drum Master Cylinder with all new lines to the Prop. valve and a new bracket that holds a new CPP Disk/Drum Proportioniing Valve. I also have the proper length MC rod (shorty) that goes with the 11" power booster and this MC. This is for a C20 or K20 with disk brakes in front and drums in the back. All of these parts are a little over one year old and work great for disk/drum trucks.

Give me an e-mail and let me know if you have an interest, I'll let it all go real reasonable.

Thanks,
72longbed
suprtuff@fast.net

[thecatseye66]

Krue,
I tried to use a standard '72 m/c with my '68 booster, but that won't work either...there wasn't really any difference between the '72 m/c and the one I have from the '77.

72Longbed,
Just a bit too late!
I just ordered a '72 m/c and booster combo from autozone for $109.00 yesterday. At first, I thought I'd try to fabricate a rod to use the '77 booster and m/c--or fabricate a shorter booster pushrod to the m/c piston so I could use the '77 m/c with my original drum/drum booster, but I didn't really want to mess with it when a newly rebuilt combo wasn't going to break the bank...

dennisbrooks,
Thanks for the pm

[Blue85]

$109 for the booster and m/c damn thats a good deal.

[thecatseye66]

Originally Posted by Blue85

$109 for the booster and m/c damn thats a good deal.

True that!
I thought the guy was looking at the wrong part. I almost dropped my wallet getting my debit card out.
I forgot to mention the 35 dollar core charge, but still it was 60 bucks less than NAPA. They both have lifetime warranties. I wonder if they will take my spare '68 drum/drum booster and m/c as a core...