Master brake cylinder information -

I compared the 3000GT SOHC 1" master cylinder to my '03 Sport and my big '97 Sport this morning. They look identical with the exception of the fluid reservoir. I think the push rod depth is different between the Sport BMCs and the 3000GT BMCs. Anyway, it looks like all the Gen 1 Sports came with 15/16" BMCs. At least that's what the FSMs say.

On a positive note, the 3000GT BMC fluid reservoir doesn't interfere with anything and isn't in the way on either the Gen 1 or Gen 2 Sports so if the BMC fits, there wouldn't be any need to change reservoirs. I checked this on my '97 3.5L and '03 3.5L. I figured if anything was going to conflict, it would show up on the 3.5L.

I'm going to see if I have a spare Sport BMC so I can compare actual measurements.

3000GT SOHC 3.0L Non-ABS (late '90s)
RAYBESTOS MC39984 (1" bore)

3000GT DOHC/Turbo VR4/Spyder ABS (late '90s)
RAYBESTOS MC39983 (1 1/16" bore)

2003 Limited 3.5L AWD
RAYBESTOS MC390655 (1" bore)

Edward

1997 Montero Sport 3.0L LS 4WD Wide - 15/16"

1999 Montero Sport 3.5L Limited 4WD Wide - 15/16"

2003 Montero Sport 3.5L Limited AWD Wide - 1"

1999 Montero Sport 2WD/4WD - 15/16" - according to the FSMs


Edward

Yea! I have got a stock Gen 1 15/16" BMC in today. All I can say is better never need a BMC TODAY here in Denver. Everyone I talked to has to order it including the dealerships. I also got the 3000GT VR4 BMC in today, too.

I'll pull the calipers out tonight when I get home, but so far eyeballing them ...

The OD of the end that sticks in the booster is cast smaller on the Gen 1 BMC, but has a machined 1/4" shoulder where it centers in the booster. The 3000GT VR4 1 1/16" is machined the entire length and looks like its the same OD as the shoulder on the Gen 1 BMC.

The length of the 'snout' that fits in the booster is the same for both. The depth inside the snout where the piston C clip sits is also the same. I can't accurately see if the depth inside the piston (where the rod sits) is the same without using a gauge.

Bolt pattern is the same, flange is the same thickness, bolt diameter appears to be the same. Outlets are in exactly the same spots as well.

The push-rod between the brake booster and BMC is adjustable. I didn't expect it to not be, but the tech told me that there are some Mitsu boosters that do not have an adjustable push-rod. With my luck I expected that to by something I own.

There is a TSB (TSB-97-35-003) on the '90s - '92-97ish - 3000GT, Montero Sport, Montero and a couple other things I can't remember, brake booster master cylinder push-rod adjustment. The adjustment procedure changed from using a ruler and block to using a Mitsu 'special tool' - P/N MB991714. By some sheer stroke of luck someone was getting rid of one on eBay Friday so I snagged it. I've used the ruler technique before. I looked through the 'new' instructions using the tool which are in the '98+ Sport FSMs. This has GOT to be easier than doing it with a ruler.

I guess none of this is probably earth shattering news to anyone but me. I'm happy its looking more and more like a direct swap. The 1 1/16" should take care of the only thing anyone ever complains about on my '97 SAS and I'm keeping my fingers crossed that this will be the ONLY thing - besides swapping proportioning valves for the Wilwood 4-way - that I'll have to do on the '99 that's brake related.

Anyway, I'll get all the dimensions posted up tomorrow for the 15/16", 1" and 1 1/16".

Oh, BTW, there is (sometimes) a difference between the non-ABS and ABS brake master cylinders. Its the bore size. ABS usually - but not always - comes with a BMC with a larger bore. According to the 3000GT guys this is the ONLY difference and basically the only way to tell the 1" and 1 1/16" BMCs apart. I can't argue with them there - I have both for the same year 3000GT and they're otherwise identical. Good thing they cast the size on them.

And the Gen 1 Sport ABS is 3-channel, not 4-channel like I thought.

Edward

Raybestos MC390156, Mitsubishi MB699830
1997-2003 Montero Sport, 1992-2000 Montero - non-ABS/ABS
15/16" Bore

Bore OD - 1.37"
Piston depth - .9614"
Bolt width - 3.1465"
Bolt hole dia. - .36"
Piston 'snout' length - .6155"
Fittings - M10x1


Raybestos MC39983, Mitsubishi MB699681
1991-1999 3000GT VR4, Spyder, SL - w/ABS
1 1/16" Bore

Bore OD - 1.375"
Piston depth - .9244"
Bolt width - 3.151"
Bolt hole dia. - .353"
Piston 'snout' length - .5930"
Fittings - M10x1

The piston depth is not entirely accurate. I measured it with a flat ended depth gauge and the inside of the cup is slightly rounded so I expect this is off by a couple 1/1000s. I did measure everything with the same tool, the same way, though.

By my measurements, there's only a 0.0145" difference between the Sport and GT cup depths measured from the mounting flange. Little enough that the push-rod can be adjusted if necessary.

Nothing else was off by more than manufacturing tolerances.

Oh, I didn't bother posting the 1" specs because I already know that the 1" and 1 1/16" are interchangeable. Figured that would be a little redundant.

Edward

Good tech dude!

def good tech here!

Thanks guys.

DANG! I just realized ... I measured Piston Depth from the end of the snout (that goes into the booster). I laid a 1/64" straight edge across the casting and measured the depth from that minus 1/64".

I think that normally, the depth of the piston is measured from the flange. So, in that case it'd be the distance from the flange to the bottom of the piston cup.

Anyway, a little math with the above numbers will get that figure if anyone needs it. I just wanted to clarify.

If someone wants a different measurement, let me know. The 15/16" will be going back shortly and I'm planning on putting the 1 1/16" in maybe this weekend. Won't have anything to measure from. Again.

Edward

I mocked up all three brake master cylinders - 15/16", 1" and 1 1/16" - on a '98 booster. They all fit fine. If I could find a 1" BMC for a Sport - without taking the one off my '03 - I'd try that, too. Brake master cylinders are hard to get here without ordering it and at this point its not worth the time to mess with. IMHO, if someone needs a 1" BMC for a '90s something they could just as easily use the 3000GT non-ABS BMC.

Pictured below is the Mitsubishi tool used to measure and set the brake booster push rod for the master cylinder. It is WAY easier to use than a block and ruler.

I checked the factory setting on the 1" BMC that came on the booster. It was actually off by just a hair, but it was noticeable. I checked the 1 1/16" MC and it was almost dead on - maybe 1/4 turn. Then I checked the Gen 1 Sport BMC and it looked to be off by about what I came up with by doing the math. I'm guessing maybe two turns.

The only other thing I found that was noteworthy was the instructions in the FSMs. They say to use a tiny 1/4" ratchet and deep socket (or extension) to turn the adjuster while holding the base inside the booster with an angle nosed long needle-nose pliers. Someone might have some other tool that might work but I don't. Glad I bought a pair a while back.

Didn't get a chance to mount the new BMC on the big Sport this weekend. I decided that before pulling everything off and swapping BMCs, I wanted to take measurements of fr/rr brake line pressure at the caliper so I know where I'm starting from especially if I have to install an adjustable proportioning valve. Since I'll be replacing the OEM valve entirely, I'd like to know what OEM looks like so I can set the new valve to something I know works. Anyway, didn't have the measuring tools 'till today and forgot the MC bleeder kit at the office Fri. Sigh.

Mitsubishi Brake Booster Push-rod Adjustment Tool MB991714-01







1998 Mitsubishi 3000GT N/A non-ABS Brake Booster






Edward

Uh, does any one know what this is? 3000GT 1 1/16" BMC PN# 54311 in the pic below. It looks like a cup and it slides down inside and loosely sits in the brake fluid reservoir. It sits above the fluid level sensor.

I was looking at the new BMCs yesterday, trying to decide whether to swap fluid reservoirs with the Montero Sport reservoir and noticed the 1 1/16" BMC had this 'cup' thing inside. It immediately made me think of fuel cell baffling for anti-slosh or starvation. Neither of which make sense to me on a car. I know the GT will pull 1G+, but .... This would make a lot more sense on an off road vehicle if its what it looks like.



Edward

I think it's a float for the sensor.