4th gen master cylinder bore?

[JPrevost]

Anybody know the bore? I'm looking at replacing the 190k mile MC with something a little less messy and maybe even designed to give a firmer pedal... I like the 4th gen cap instead of snap Tupperware cover and if it's got a larger bore I'd a very happy camper.

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[Dave_Jones]

I just disassembled a used '94 MC that I'd bought to answer the same question. It's a 1.00" diameter bore straight through (not a stepped bore like the thirdgen MC).

The replacement parts catalogs indicate that the '98-02 MC is different from the '93-'97. How it's different, I don't know, since I haven't examined one of those yet.

[JPrevost]

Dave, what do you mean stepped bore. Don't tell me the primary and secondary are different bore's.... is that just the disc/drum mc or all? Do you have those dimensions?
I confirmed tonight the bore is 1" for 93-97 from a buddy's old mc. Now I need a newer LS1 mc. I'm glad they've got 1" bore. Now to find the cheapest new/rebuilt.

[Dave_Jones]

All thirdgen MCs that I'm aware of were stepped bore.

In the late 70's, for fuel economy reasons, GM began using "low drag" calipers where the caliper piston seal was redesigned to help pull the caliper piston back away from the rotor when not under pressure. Obviously, this increased the piston travel necessary to apply the brakes. So, to compensate for this increased piston movement (i.e. fluid volume), they introduced something called a "quick take up" MC -- I've also heard it referred to as a "fast fill" type. The intent of which is to supply a large volume of fluid in a very short amount of pedal travel, then function like a normal MC beyond that.

Basically, you have a very large diameter piston at the back of the MC bore, that does the initial take up, then a bypass valve opens and vents the fluid pressure to the main MC bore, so that the MC functions as normal. Actually, you can see the quick take up valve at the bottom of the fluid reservoir -- it's that funny little plastic thing down there in the rear hole.

As far as thirdgen master cylinders go, I believe the bore diameters were as follows (first number is the main bore, second number being the quick take up bore):

PBR ALUMINUM CALIPER REAR DISC and ALL REAR DRUM
24 & 31.75MM CYL BORE (i.e. .944" main bore)
MC P/N: GM 18030555, or Delco 18M313

IRON CALIPER REAR DISKS
25.4 & 36MM CYL BORE (i.e. 1" main bore)
MC P/N: GM 18030556, or Delco 18M314

[Dave_Jones]

This might help explain things.

[MurcoRS]

Originally posted by JPrevost
Now to find the cheapest new/rebuilt.

Go with new, never rebuilt, and stick with Wagner, Bendix, or Raybestos parts. My company has experienced a failure rate of over 50% with rebuilts and that backs up the perception I had working on parts counters for 8 years.

[JPrevost]

Dave, so it's not really a differential bore it's just a quick fill. I thought you ment the primary was a different size than the secondary (front/rear). I guess the quick fill mc on our cars would explain why the rear drums didn't have the residual pressure valve!
Thanks.
Yeah, new is likely unless the price difference is 3x. I don't mind working on my car, what I don't like doing is overspending so I walk that fine line. I've never, and I do mean never, had a rebuilt part fail on me. I can't see how people screw up rebuilding a mc. Then again most of the rebuilt parts I buy are from circle track guys or I do it myself... The other reason I was considering the rebuilt was for the material. A few posts have covered this but the new mc's for OUR cars have been iron and the rebuilds OEM aluminum... what about the 4th gens. Are their "new" mc's iron as well?

[Dave_Jones]

FWIW, I recently purchased a used '98-02 MC, for comparison.

It's also a 1" bore. The MC body is practically identical to the '93-'97 style, and even carries the same casting number.

Internally there's some minor differences, none of which are terribly significant IMHO. (Specifically, the secondary piston can move a bit further in the '98+ MC before it hits its stop, the primary piston is somewhat redesigned, and there's a bit of difference in the piston springs but I doubt anyone would be able to tell once it's all assembled and installed.

The biggest difference is the reservoir. The '98-02 reservoir is quite a bit larger, has a couple of extra baffles, and also has provisions for a low fluid warning switch. Also, the '98-02 MC is tapped for an M10 fitting on the primary side, and an M12 on the secondary side, while the '93-97 was tapped for M10 on both.

HTH,

[ebmiller88]

Good info fellas, thank you much..

Ed

[Mark A 91Formula]

Are the external dimensions of the LS1 master any differant than the stock master cylinder? Any room that can be saved around the master cylinder for me the better. My blower intake sits right in line with the master.

[Dave_Jones]

LT1/LS1 MC is a little more compact than the thirdgen style.

I'll try to snap a pic or two for comparison.

[Mark A 91Formula]

Sweet, gonna look in to that then, hell even if it's only half an inch it will be well worth swapping in. Pictures appreciated

[JPrevost]

The 4th gen mc should bolt in so long as the pedal assemblies relativity to the ground is the same as that of our 3rd gens. If it isn't, then the angle might be or or the pushrod length could be different because of the power assist. I think it would provide a nice alternative.

[Dave_Jones]

Originally posted by Mark A 91Formula
...hell even if it's only half an inch it will be well worth swapping in.

It's more of a difference than that, so it sounds like it might work for you.

[Dave_Jones]

Here's the LS1 MC (top) next to the LT1 MC (bottom), along with the internals.

[n2omike]

so can the 4th gen MC be swaped to a 3rd gen ?

[Dave_Jones]

Yes. IIRC, there are a few people on this board running that combo.

[Mark A 91Formula]

Wow, that will make alot more room, looks like it'll be time to redo the blower intake hoses.

[iansane]

So what are the effects of the two styles? LS1vsLT1?

[JPrevost]

Originally posted by Dave_Jones
, the '98-02 MC is tapped for an M10 fitting on the primary side, and an M12 on the secondary side, while the '93-97 was tapped for M10 on both.

HTH,

Dave, what are the stock 3rd gen fittings, I'm assuming M10 on both but if you knew off the top of your head it would save me a trip.

[Larry Burd]

Originally posted by Dave_Jones


PBR Rear Disc or Rear Drum cars: 25.4 & 36.0mm (i.e. 1" main bore)
(MC P/N: GM 18030556, or Delco 18M313)

Delco Morraine (Iron) Rear Disc: 24.0 & 31.75mm (i.e. .944" main bore)
(MC P/N: GM 18030555, or Delco 18M314)

the bigger bore units was used in 88,
the smaller bore masters were used from 89-92

is bigger better?
would it make sense to put an 88 big bore master in my 89.
My 1LE brakes suck in my 89, and it must be the master.. so I'm tempted to use the bigger bore master from 88....
what are your thoughts??

[Jetmeck]

Originally posted by Dave_Jones
FWIW, I recently purchased a used '98-02 MC, for comparison.

It's also a 1" bore. The MC body is practically identical to the '93-'97 style, and even carries the same casting number.

Internally there's some minor differences, none of which are terribly significant IMHO. (Specifically, the secondary piston can move a bit further in the '98+ MC before it hits its stop, the primary piston is somewhat redesigned, and there's a bit of difference in the piston springs but I doubt anyone would be able to tell once it's all assembled and installed.

The biggest difference is the reservoir. The '98-02 reservoir is quite a bit larger, has a couple of extra baffles, and also has provisions for a low fluid warning switch. Also, the '98-02 MC is tapped for an M10 fitting on the primary side, and an M12 on the secondary side, while the '93-97 was tapped for M10 on both.

HTH,

There was a thread couple weeks ago about this. I will be following along to see if the 4th gen MC works for you. I am interested in using the provision for the warning switch to run the idiot brake light in the dash as I plan to remove the troublesome combo/proportioning valve and put an adjustable valve in line rear along with a line lock in front when I do ED's c4 upgrade.

[MurcoRS]

Originally posted by Jetmeck
I plan to remove the troublesome combo/proportioning valve and put an adjustable valve in line rear along with a line lock in front when I do ED's c4 upgrade.

You'd be better off getting a new 1LE combo valve unless you plan on a racing-only set-up. The combo valve meters pressure to the rear upon initial brake application promoting even wear front to rear, the internal prop valve in the 1LE combo is also the highest pre-ABS rear-biased GM valve ever produced, and an adjustable prop-valve running straight off the rear chamber will limit the total amount of pressure available to your system overall. I've seen a few do it here and report success but check back with them after 10-15K miles of use.

[iansane]

Originally posted by MurcoRS
an adjustable prop-valve running straight off the rear chamber will limit the total amount of pressure available to your system overall. I've seen a few do it here and report success but check back with them after 10-15K miles of use.

How does it limit pressure to the whole system? What would someone be looking for 10-15k miles later?

[6speedIROC]

The delco part numbers listed above by Dave_Jones are WRONG. They are reversed. Here are the correct ones:

With RPO REAR DRUM BRAKES POWER BRAKES 24 & 31.75MM CYL BORE
(MC P/N: GM 18030555, or Delco 18M313)

With RPO REAR DISC BRAKES POWER BRAKES 25.4 & 36MM CYL BORE
(MC P/N: GM 18030556, or Delco 18M314)

The GM numbers were reversed..thisis straight from the Delco on-line parts site (and the box beside me)

Dave_Jones, can you update that post?

[Dave_Jones]

Yikes, you're right, I did switch the numbers. And it was more than just transposing the GM part numbers, I also reversed the applications.

Hope that didn't mess anyone up too much!

How's this look?

PBR ALUMINUM CALIPER REAR DISC and ALL REAR DRUM
24 & 31.75MM CYL BORE
MC P/N: GM 18030555, or Delco 18M313

IRON CALIPER REAR DISKS
25.4 & 36MM CYL BORE
MC P/N: GM 18030556, or Delco 18M314

[iggy1991]

So which MC would give the best performance? The 98+ has the same fitting sizes as the stock 3rd gen MC lines correct? Is its better to have an adjustable Proportioning valve or just the stock valve from a 4wheel disc setup?

[bluegrassz]

Is the 98-up fittings the same size as the thirdgens?


Would the LS1 mc help in the case of a disk/drum setup?

[89 Iroc Z]

Originally Posted by bluegrassz

Is the 98-up fittings the same size as the thirdgens?


Would the LS1 mc help in the case of a disk/drum setup?

The 98-2002 LS1 MS has the same fittings as the 84-92 F-body. I suggest the 84-88 rear disc brake master cylinder over the LS1 MS due to a larger bore size

84-88 F-body MS
Step Bore
Front brake bore: ~1.42" 36mm
Rear brake bore: 1" 25.4mm
18030556 GM
18M314 (Ac-delco)

Check out my site for a pull list of MS: http://www.lukeskaff.com/car/brakes/...Clyinder_Specs

Also you can use this setup with rear drums.

[bluegrassz]

Originally Posted by 89 Iroc Z

The 98-2002 LS1 MS has the same fittings as the 84-92 F-body. I suggest the 84-88 rear disc brake master cylinder over the LS1 MS due to a larger bore size

84-88 F-body MS
Step Bore
Front brake bore: ~1.42" 36mm
Rear brake bore: 1" 25.4mm
18030556 GM
18M314 (Ac-delco)

Check out my site for a pull list of MS: http://www.lukeskaff.com/car/brakes/...Clyinder_Specs

Also you can use this setup with rear drums.

So the 84-88 will be a direct swap even though I a running rear drums? I dont have to change the proportioning block correct.

That along with Earls brake hoses and good pads/shoes should help the complete system.