Need to install new master cylinder but the piston depth is diff

[Longhorn 70]

I am finishing up the drum to disc conversion on the truck. I have a question about the depth in the back of the master cylinder piston. The old MC has one that is about 3/4" deep while the new has just a dimple. Will I be OK just screwing the push rod tip in all the way or should I go another route?

I stop fine now but could it be better???

Thanks in advance,

Vince

[jocko]

Did you add a booster during the conversion?

Sounds like you didn't (since the intermediate pushrod isn't adjustable between the booster and mc) - and, you've got a "power" mc that is meant to be used with a booster.

You may not have sufficient adjustability to make it work (without cutting and shortening the pushrod - but that's an option).

I'd recommend adding a booster. You have the correct master cylinder for a booster. Wouldn't improve braking perf, per se, but pedal effort would be redeced and maybe somewhat feel like better brake perf.
Member Captainfab makes a bracket to do the addition, or you could nab a bellcrank setup and and brackets from the junk yard.

Or you could go buy the correct non-power master cylinder. But most disk/drum setups were power from the factory.

[Longhorn 70]

sorry I should have said................ I have Power Brakes now but have the older style booster on the truck. I don't really want to go with the bigger diameter booster if I can avoid it.

I think mine is the 8" booster. I know it isn't the big 11".

[jocko]

Hmm. I'm a little stumped then - if you have a new shallow bore disk/drum mc and a booster, you should be good. Bottom line - since you have a booster, there is no way to "adjust out" your problem via the pedal. You have to have a proper length booster to mc intermediate pushrod. Since you have a deep bore mc piston, you will need a longer one than you have. You can grab an old mc pushrod from a manual brake pedal in a junkyard and cut it down, but measure carefully to ensure it's not engaging the piston or your brakes will drag.

I am still a bit confused. So, when you were drum/drum - was THAT a power setup? If so, the stubby pushrod coming out of the front of your booster should be the correct length to connect to your new shallow bore mc (but then again, your OLD mc should have been shallow bore also...) So, your current setup is shallow bore disk/drum mc (and that sounds like a correct power mc to me) and an older original booster? Is the booster's intermediate (i.e. front) pushrod too long for the shallow bore mc? If so, you can just cut it down the difference between the bore depths. Or replace the booster. If you have a booster with a long intermediate pushrod - i.e. that fits a deep bore piston in the mc, I'm really confused - have never seen that before.

[Longhorn 70]

Here is what my booster and MC look like. I have put it back to stock for photos. The booster and MC have been replaced about 12 years ago so I hate to toss a perfectly good unit. I also like the looks of it better than the 11" units. Probably because I'm used to it. I have a new disc/drum and prop valve to install but want to make sure it isn't going to freak out when I need to jam the brakes.

One part of me says OH Heck just go for it. The other says better safe........

[mgchevyparts]

Some early boosters used the long pin style instead of the short pin commonly found on boosters. See if the booster front pin is the pull out type, if so you can replace it with the short pin.

[Longhorn 70]

Good to know. I do know the booster front pin does pull out. It has come out before. When I put it back in and bolt things up it keeps vacuum so I didn't think it was a big deal....NOW, where can I get the short pin or should I measure and make one? You know which way I am leaning....

[Longhorn 70]

Finally got the disc brake master cylinder and proportioning valve installed properly. I had to modify the brake booster due to the Hydraulic Push Rod being about .600" too long. I cut it carefully and tapped the remaining bore depth to install a new adjustable tip. I had it too long at first and used some of my daughter's modeling clay on the master cylinder piston to gauge depth. I ended up using a clearance of .150" between the end of the push rod the piston.

I then attached the bracket and lines. I bled the lines using a plastic bottle with a clear vynyl hose run down to the bottom.

Now I have great brakes. Maybe too good. I tend to hit them hard due to not being used to them yet.

Now I get to insall the heater core and A/C stuff. Then on to the cab cushions. This is so relaxing.....