Does this new master cylinder look correct?

[Toro]

Take a look at the photos comparing the original MC on my 72 with power brakes, to the new AC Delco 18M1036 I just ordered. Should I be concerned that the rear reservoir is so much smaller? The bore size/depth, fittings, etc are all the same. I didn't expect to find an exact match, but just wanted to double check. Also, any ideas what that 5/16 bolt is for in the bottom of the front on the original? Never noticed it until today when I pulled it off.

[Sheepdip]

I don't believe the one you pulled off is the original, your new replacement would be more like the original. Those double big potted reservoir M/C's back then were referred to as a Corvette M/C because of the front and rear disc brakes.

The larger pot is for the front disc and the smaller for the rear drums. Disc brakes as they wear move outward and keep fluid in the caliper piston bores, rear drums have automatic adjusters and return springs so piston movement is minimal and less fluid required.

My hunch on the bolt is it holds the plunger in....just a SWAG.

[71CHEVYSHORTBED402]

Originally Posted by Sheepdip

I don't believe the one you pulled off is the original, your new replacement would be more like the original. Those double big potted reservoir M/C's back then were referred to as a Corvette M/C because of the front and rear disc brakes.

The larger pot is for the front disc and the smaller for the rear drums. Disc brakes as they wear move outward and keep fluid in the caliper piston bores, rear drums have automatic adjusters and return springs so piston movement is minimal and less fluid required.

My hunch on the bolt is it holds the plunger in....just a SWAG.

Really good info.

To add, if it's your first MC job, bench bleed it and be sure the push rod is positioned right. You probably know to bleed the brakes no doubt.

[Toro]

Originally Posted by Sheepdip

I don't believe the one you pulled off is the original, your new replacement would be more like the original. Those double big potted reservoir M/C's back then were referred to as a Corvette M/C because of the front and rear disc brakes.

The larger pot is for the front disc and the smaller for the rear drums. Disc brakes as they wear move outward and keep fluid in the caliper piston bores, rear drums have automatic adjusters and return springs so piston movement is minimal and less fluid required.

My hunch on the bolt is it holds the plunger in....just a SWAG.

Okay, thanks. So, if this one isn't original, do you know what the correct bore depth for power brakes should be? If it was replaced before with a different type, I just want to be sure the new one is correct. The new and old both measure about 1 3/4" from deepest point back to rear edge of bore. Thanks again.

[michael bustamante]

prettyy sure the bore is 1 and 1/8th
your old mc is for a drum drum system
youre new one is correst
i just replaced mine too. took forever to bench bleed! lol

[jocko]

Your new mc reservoir sizes are good to go for a disk/drum C10 setup, but your mc is for manual (non-power) brakes - this is the bore depth I think you are asking about in post #4(?). Pic in post #4 shows the deep bore in the mc piston. Power brakes require shallow depth hole to receive the standard intermediate pushrod (the little stub pushrod that protrudes from your booster). Recommend return your mc to swap for a power brake mc. There are other ways around this (custom intermediate pushrod) but getting the correct mc is easier in the long run, no guesswork involved. Sorry, probably not what you wanted to hear.

Out of curiosity - was this a power brake truck from the factory?
Also, can you confirm whether this is for a C10 or C20?

[Toro]

Originally Posted by jocko

Your new mc reservoir sizes are good to go for a disk/drum C10 setup, but your mc is for manual (non-power) brakes - this is the bore depth I think you are asking about in post #4(?). Pic in post #4 shows the deep bore in the mc piston. Power brakes require shallow depth hole to receive the standard intermediate pushrod (the little stub pushrod that protrudes from your booster). Recommend return your mc to swap for a power brake mc. There are other ways around this (custom intermediate pushrod) but getting the correct mc is easier in the long run, no guesswork involved. Sorry, probably not what you wanted to hear.

Out of curiosity - was this a power brake truck from the factory?
Also, can you confirm whether this is for a C10 or C20?

Thanks. I'm sorry for my lack of knowledge on this, but if you see the picture in my first post on this thread it shows the MC I removed with a deep bore, that's what I thought was the original MC, but sounds like it was replaced at some point with something else.

This is factory power brakes on a 72 K5 Blazer. So the original MC with power brakes should have been the shallow depth? If that's the case, the PO must have changed the brake push rod to a longer one to use the MC I removed. Look at the pictures below of the rod in the booster. Even if I had a shallow depth MC, I don't think it would go on just looking at how far this rod sticks out. Can you tell if that has been changed as well? What should factory setup look like? Again, I'm sorry for all the questions, but I want to put it back correctly. Thanks!!

[jocko]

Thanks, that's a helpful set of pics! Yes, I do believe your original would have had a non-adjustable intermediate pushrod and a shallow bore mc - and that someone has likely installed an adjustable pushrod when they changed out the mc before (so someone has already gone thru exactly what you're going thru now). But then again, who knows... (maybe someone else has an original setup just like this?? - all the intermediate pushrods I've seen from the factory were non-adjustable, but could just be the trucks I've owned)

So, you may be ok - just do some careful measuring to see if the adjustable pushrod will extend "almost" all the way into the bore. If it does, you're probably just fine and I'd press on with the mc you have now. If you have everything adjusted as far out as it can go and still have a good amount left before it bottoms out in the bore (like 1/4"-1/2"-ish??) then you may want to re-wicker this setup to use a correct mc and intermediate rod. My guess is that if your brakes worked before and you're just doing a brake job, you'll be just fine with what you have (i.e. if previous mc and new mc were both deep bore and the previous one worked, then that means your intermediate rod is probably already well-adjusted. But I'd still measure just to make sure - and it might be a good idea to measure the pushrod when the engine is running and vac source attached to booster - without brake pedal applied. I don't know if idle vac signal results in minor reposition of the pushrod compared with no vac signal off the top of my head).

[Toro]

I really appreciate the help. Yes, it was working before but the MC was worn out and needed to be replaced.., that's when all of this was discovered. The depth on the new MC is the same as the old one that was removed, but I will still measure for fitment. A few more rookie questions, for adjustment, it appears the threads on the rod are for that purpose? Also, it seems the pushrod is just sitting on a rod inside the booster and not actually secured, meaning that I can pull the rod out towards front and it comes off only being held in by the flange and the grommet on the booster. To put it back you sort of have to feel around until it slides back in place. Does that sound normal? And finally, is there supposed to be an o-ring or gasket that goes between the flange on the MC and the booster? Old one didn't have one, and there wasn't one in the new box.

[KQQL IT]

Your new one looks like the one i replaced the other day.

[jocko]

Originally Posted by Toro

I really appreciate the help. Yes, it was working before but the MC was worn out and needed to be replaced.., that's when all of this was discovered. The depth on the new MC is the same as the old one that was removed, but I will still measure for fitment. A few more rookie questions, for adjustment, it appears the threads on the rod are for that purpose? Also, it seems the pushrod is just sitting on a rod inside the booster and not actually secured, meaning that I can pull the rod out towards front and it comes off only being held in by the flange and the grommet on the booster. To put it back you sort of have to feel around until it slides back in place. Does that sound normal? And finally, is there supposed to be an o-ring or gasket that goes between the flange on the MC and the booster? Old one didn't have one, and there wasn't one in the new box.

You're asking good questions, not rookie at all
- yes, the threaded end of the pushrod is for making minor rod length adjustments, if needed.
- good observation on the pushrod seeming "unattached" to the booster - but that is normal, it's how they are made, just center it and stuff it in there.
- as for the o-ring or gasket between the mc and booster - very good question (because the flat "groove" would sure make one think there would be a seal there...) but, no there is not an external o-ring or gasket that that goes in between the mc and booster (at least I've never seen one on any mc's I've removed). But there should be no need for one. Brake fluid is retained in the mc by the cup seal on the mc piston (see part #6). Vacuum seal is provided by an internal o-ring (see part #7). Would be worth knowing if anyone else has ever seen an "external" seal or o-ring between mc and booster(?) Hope that helps