HELP!!!! Question regarding master cyl brake booster option

[erikh09]

I am currently converting my 67 GMC 3/4 ton to a 1/2 ton. The truck came with power drum brakes and I want to put discs up front. Can anyone please help lend a suggestion as to which master cylinder and booster I should use (model year) so I can go to the auto parts store rather than order from a fancy custom car shop and overpay for the same part. Also, should it be for a 3/4 ton or 1/2 ton when I spec it out to the auto parts store!! Thanks as usual for all of your help!!

[68gmsee]

You should be able to use the booster you have and just replace the master cylinder with one from 71 on up. That's when power disc brakes showed up. That's what I did on another truck.

[71 super]

You may have to change booster as well. Some master cylinders have a shallow throat and some have deep. If the booster is set up for a shallow make sure that you get like master if deep you will have a harder time finding one. Just went through this on the 70. Converted to 3/4 ton booster and disk up front did not check new master (shallow) no brakes, finally found deep (delco dual bail ) and now it will stand you on your head.

[erikh09]

Thanks for the replys. How can I tell if it is shallow versus deep? What do I look for?

[jocko]

I do not believe you will need to change your booster - the difference lies in power vs. non-power master cylinder. Power brake master has a "shallow" indentation piston, a non-power master cylinder had a deep indentation piston (see 1st picture - this is a NON-power master cylinder - ignore the fact that there is a booster in the same picture - i.e. not a matched set, "these are not the aliens you are looking for...."). The booster has a shallow push rod for mating to the master cyl. So, since you are just going from power manual to power disk, all you should need is the master cylinder for a disk/drum setup (and it should have a "shallow" indent for receiving the stubby power booster push rod. See pic #2 to see how shallow the pushrod is. What I DON'T have a picture of below is a Power master cylinder (i.e. a shallow one) but the below pics should give you an idea of what will NOT work (i.e. learn from my mistake).

To be on the safe side, recommend buy the disk/drum master cyl and check to see if it mates - if not, then buy the booster to match. OR, if you buy from a local parts dealer - buy a matched set (sa TWO separate parts, not a combo) and return the booster if you find you don't need it.

If your booster is at all old, I'd just buy a matched set and call it a day.
Hopefully the pics will help - again, 1st one is a NON-power (deep indent) master and the 2nd one shows the shallow nub on the booster (i.e. picture this mating to a power master cylinder and you can see how shallow the indent in the master cyl's piston really should be.

I BELIEVE the only change you need to make up on the firewall is to go from a same size reservoir master cyl (i.e. drum) to an uneven size reservoir master cyl (i.e. disk).

[erikh09]

Thanks jocko!! The pics were a big help!

[erikh09]

How about the proportioning valve? Are they typically available from an auto parts store or do I need to go catalog hunting again! Thanks all for your help!