disc swap 74 into 69 c30

[jdean]

So I'm having a hell of a time finding a backing plate for the 69 drum setup so i'm looking closer at throwing the disc frontend from the 74 I have into the 69. Both are c30's, I think the front of the 74 has the smaller rotor setup on it, the wheels are pretty flat, not dished. Main question is, I have a new 69 master cylinder and brake booster (painted it so I can't return it) and was wondering if I could use it with the disc brake frontend if I used the valve that is in the 74? There is what looks like a proportioning valve on the 69 already right up by the master. The 74 has both sets of lines going right down to the front crossmember to a valve mounted on it. I have also put the rearend from the 74 into the 69. I know i'd be best off to just put the 74 master and booster in, but I'm trying to do this on the cheap. And already have the new master/booster that i'm stuck with. Sorry for the long post. Thanks in advance. I'll try to get pics today

[lolife99]

Sorry but you need a disc brake master cylinder and prop valve.
The 71-72 version would be a bolt on.

[71Dually]

Man I have to type faster, give shorter answers or do less proofreading.

"I missed it by that much" - Maxwell Smart aka Control Agent 86

Originally Posted by jdean

Main question is, I have a new 69 master cylinder and brake booster (painted it so I can't return it) and was wondering if I could use it with the disc brake frontend if I used the valve that is in the 74?

No, you can not use a drum brake master cylinder on a disc brake system. In order to operate properly disc brake calipers require a larger volume of fluid than drum brake wheel cylinders do. A disc brake master cylinder with it's larger reservoir to accommodate the additional fluid volume is required.

Don't know if the 74 has the vacuum or hydraulic brake booster. If it's a vacuum unit check if the new one you have for the 69 is the same unit as the 74. If it is use the new unit. However, if your 74 has a hydroboost system you may want to consider swapping it over to the 69. The hydroboost system provides better braking over the vacuum booster system.

The proportioning valve on the 74 will need to be used, the valve on the 69 is actually a distribution block and won't properly regulate the pressures for the front discs and rear drums.

Originally Posted by jdean

The 74 has both sets of lines going right down to the front crossmember to a valve mounted on it. I have also put the rearend from the 74 into the 69. I know i'd be best off to just put the 74 master and booster in, but I'm trying to do this on the cheap.

Go with the best way, brakes are a critical safety system and my bet is you want them to function correctly. Use everything off the 74 from the booster thru the front brake lines. Install the prop valve into the 69 on the lower crossmember just like it's installed on the 74. Just drop the valve with the 74 lines assembled into position and drill a couple holes to secure the valve to the lower crossmember. Replace the rubber hoses with new ones.

Originally Posted by jdean

...but I'm trying to do this on the cheap. And already have the new master/booster that i'm stuck with.

In my opinion, on the cheap and critical safety systems are not usually a good match. It could cost more in the end.

Are you sure you are stuck with a new master cylinder and brake booster? While they have been painted have they been installed? Have you tried to return them to exchange for the proper parts to do the swap? The worst the parts store could do is say no. But then again they could exchange them for what you need with a store credit. If the parts store won't exchange them you could always sell the new parts albeit at a small loss. But still less of a loss than just sitting on a shelf gathering dust waiting to be used.

[jdean]

Bummer, that's kinda what I figured. Just cause I want to do it cheap doesn't mean I don't wanna do it unsafe . That's why I bought I all new lines, wheel cylinders, master cylinder and booster for the 69. It didn't necessarily need it all, but I was going to change it all. Then I found the rf backing plate was bent out at the wheel cylinder mount. I can't seem to find a backing plate anywhere, so I guess I'll do the front swap. It's all going onto a dumptruck setup that won't get hardly any miles, but I still want it right. I'll have to look around and check out interchange on the booster. both rigs have a vacuum set up. This easy checkup run through on this thing has turned into a nightmare. Oh well, I try to take care of the things I have. Once this is back on the road, i'll be turning my attention to the tractor.

[jdean]

Nice call lolife99. I spent awhile comparing boosters and masters, you're spot on. jb7 brake system, same diaphragm size for the 74 and 72, but with the right mount for the 69, and the 74 and 72 use the same master cylinder. Sucks doing all the work just cause of a backing plate, but obviously it will be easier to find parts for it this way. Thanks guys! Now i'm off to try and move the thing without a rearend in it.