4g61t.org

I would pay heed to what gregd and myself have already stated, vacuum will put you back on the map and give you a much better application and feel when applying the brakes in all circumstances. Try the reservoir and move on from there.

I am curious how you managed to get more pressure from your brake system by shortening the rod? Was there a constant apply, or was this creating an unwanted consequence?

No constant apply, but the engagement was at the top of the pedal travel, above the clutch pedal.

Shortening the rod gave the pedal and and rod more leverage, physics type stuff that's over my head, so less foot force needed

What is your pedal free play at now?

The amount of travel before it engages components in its path of travel ?

Essentially, the extended system is there to adjust the amount of movement before the pedal travel engages the rod and in turn the booster and through to the master piston apply.

Is this the rod at the booster assembly that contacts the master piston assembly?

Do you have the FSM page for testing brake pedal travel by chance? I have a few booster "nubs" laying around I can swap in and tweak length even more.

Now it feels like every other car on the road - normal. Not pedal on the floor low, but in the sweet spot middle.

With no other changes I can lock the front tires up. On decel it brakes even harder. Harder than ever before. I have that 1 1/16" master on the way. Will report again with a direct comparison between the 1" and 1 1/16".

PDF 1989 Mirage Turbo Chapter 5-9 http://4g61t.org/archive/manuals/89%20C ... arking.pdf

Ok, some more interesting results here.

I had time to install the 3000GT 1 1/16" master cylinder.

With the 4G booster and shortened push rod it is WORSE than the 15/16 master cylinder. With no other changes I'm back to sub-500psi line pressure at the caliper. I think it all revolves around the leverage and shortened engagement going to the thinner 4G booster.

I am going to order a 7/8 master cylinder, original to the 3G Mirage. I'm hoping to get even more line pressure than the 15/16 master which was around 1000psi for moderate pressure and 1500psi as-hard-as-I-could push the pedal.

Side note, apparently the 1.5L hatchback came with a 13/16 master cylinder which is a hair smaller than the 7/8 found on the 1.5L / 1.6L sedans and turbo

Will report back with the results.

The smaller the bore in the cylinder, the higher the pressure created at the other end, but travel increases proportionately at the pedal. So you could go as small as humanly possible but may end up with excessive travel or very low pedal height to operate the system. Traditionally we all try to match calipers to masters, while trying to accommodate an appropriate booster ratio but seeing as how most “sets” don’t physically fit in our platform, we have to mix and match. Pressure goes up and travel at the output goes down/pedal travel increase. Ideally, a larger master would get a higher ratio booster to make up for the pressure apply. If we had 5:1, I’d be a happy camper.

The 1 1/16 was probably the worst feel yet. Rock solid pedal with very little braking power. Very similar to the 15/16 with the unmodified push rod/ball.

The 15/16 with the shortened push rod/ball was OK and pedal travel wasn't too bad. I can always put a longer push rod/ball back in with the 7/8 master if the pedal travel is too long.

I'm at the point where that push rod can't be any shorter, but I can go longer.

Preface: I have a 93-96 Mirage thin brake booster, Wilwood Dynalite front calipers, dsm rear calipers, mirage turbo prop valve, and FP 4R cams (10 mmHg at idle)

7/8 master is installed. I haven't road driven it yet, but I think this one is the ticket.

I am able to get even higher caliper pressure compared to the 15/16.
The only difference is the pedal travel is lower to the floor which was 100% resolved by simply turning the push rod out farther. Now my pedal engagement is right in the middle - basically the same as the 15/16 master and I also have more clamping force.





I talked to a trusted source who said Wilwood calipers aren't really matched correctly to Mitsu/import master cylinders which might be why I'm having issues. For some reason increasing the master size with these Dynalites doesn't work as it should in theory.

The first braking benchmark should be, "Can you lock up your tires if needed in a panic stop?"

If the answer is yes then congratulations. Your brake system is working properly.

If no, then you need to look at the master cyl if you have aftermarket calipers.

The booster should not matter. You should still be able to lock up the brakes if the booster is undersized, or smaller. Heck, you should be able to lock them up without a booster.

Short video: https://youtu.be/0PKSEiu3dUU

Very interesting findings.

I’m onboard with your measurements for pressure apply with the smaller bore 7/8” Master, but I am on the fence about pedal travel that occurs and the “extra slack” being taken up simply with more stub adjustment. In theory you can only adjust for so much before the physics of drag/constant apply will occur due to the natural heating of all components but mostly the fluid, and the pedal will have to sink further by consequence of a smaller bore cylinder used with a larger capacity setup of front piston cups (larger apply area). All in all, I think you are moving forward in a very positive way. Thanks for posting and using pics to back it up. In theory, someone could use the factory turbo 7/8” master, or step up to the 1”, with 1-1/16 being at the lower end of pressure apply. Readers should note the vaccuum level that your car operates at, compared to cars being closer or on par with stock vaccuum levels. With a vaccuum pump, your car would feel like 2009 Mini Countryman setup. Why you say? My dad had one and I’ve never felt brakes that grabbed with so much force for their size. He wore through rotors and pads like clockwork every two years and never had a rotor corrosion issue. I can assure you that system had a very capable piston apply force. It may have used the vaccuum pump that all Mini’s used in that era. I have replaced quite a few of those, and it sounded a lot like your low pressure apply or complete lack of assist with a cam driven pump that wasn’t functioning.

I mention the low vaccuum scenario because street cars operate in low RPM ranges for most of their life, and you need all of the assist available as much as possible to make the brakes feel the same under all circumstances. For most users with smaller cam setups, this can be less of an issue.

Do you have those big radials on your car in that video, that I saw in earlier pictures?

These are regular street tires, 205/45/16

I'll elaborate on this one.

The booster rod is adjusted in conjunction with pedal travel before the booster rod engages the master cylinder piston.

Example 1) The 1 1/16 master engaged at the top of the pedal. I needed to cut down the booster rod to allow the brake pedal to travel further prior to pushing on the piston. More travel in turn should have created more leverage which also would increase caliper pressure as a bonus to moving the brake pedal where the factory wanted it.

Example 2) I installed the 7/8 master and the pedal was close to the floor when using the push rod set for a larger master. I unscrewed the push rod to get the pedal travel back to factory specs. The rod itself never preloaded the master cylinder.