4g61t.org

I found this interesting.

Common contributors to "hard pedal, won't stop" issues are an oversized master cylinder bore and/or inadequate pedal lever ratio. Another contributing factor is the "aggressiveness" of the pad compound being used. Disc brakes require approximately 900-1200 psi at the caliper for effective functioning. We recommend that you use a Wilwood Quick Check Pressure Gauge to measure your pressure at the caliper. If you are not generating the required pressure, we recommend increasing your pedal ratio, and/or going to a smaller bore master cylinder

260-0966 is the Wilwood part number for the gauge that threads into the caliper. Nice way to find out if each of your calipers are getting the right PSI. This will provide good data on your brake setup that will allow you to tweak your system.

I figured the Euro crowd had a way of testing their caliper application pressures for their MOT/road worthiness inspections.

I use the 1-1/16" 1991 Mitsubishi 3000GT master cylinder as found at Rock Auto - it is a Beck Arnley part I likely would have ordered, but either way it is identical to this link:

BECK/ARNLEY 0728956 {#MB699681, MC39983}

https://www.rockauto.com/en/catalog/mit ... inder,1836



You could use a 1" master cylinder too, and yes, it would apply more pressure due to the size of the diameter of the piston - more force on a small area end to end essentially, versus the same force spread over a larger area, like with the 1-1/16" setup. This is why the GVR4 crowd source a smaller bore slave cylinder when working with the clutch setup, there is a particular color internal alum piece/bore size that is most compatible with high pressure apply setups, versus not getting enough release at the fork.

I think most find the pedal travel issue moot, as it is maybe noticeable in the overall application of the brakes, but if adjusting your pressure apply rod connected between your pedal box and the booster, just be sure to leave room for things to heat up - it can cause constant apply otherwise!

I would prefer some setup that allows a pressure sensing block to the installed into the travel zone of a caliper, and thus with a caliper flipped up, the piece could be inserted and held in place for testing. Just need a small enough main block and some extra sections that bolt in place, to make it wider and deeper. Long story short, I wouldn't want to have to spin the gauge into the caliper just to test it, but if it is as simple as removing the bleeder, then it isn't so bad!

Yes, you remove the bleeder then screw this in. The bleeders on the wilwoods are up front and easily accessed. I'll report back when I know more.

I currently am running a 1" master cyl from a 2G DSM.

If not enough vacuum is the cause of all this I might have to install an electric vacuum pump for most booster assist.

What were your vacuum specs at hot idle (I assume you have no cold high idle device on your current setup because you don't have an OEM TB?) and hot decel at higher RPM's?

If it is close enough to what others have then assuming your hydraulic setup is fine, we would be moving towards vacuum booster assist.

Throttle body is stock 1G with ISC. The coolant lines are not hooked up. Will check vacuum readings in the AM.

So idle is -9 inHg.

Hot idle in my VR4 with 272's is 1-10 to -11hg, so he is right there, especially if he has bigger cams.

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Thom
1991 GVR4 1025/2000 (PTE 1200's, 16g, DSMLink v3, gm-maf, eagle/ross, .020 over, 272's, EVO VIII Wheels)
1992 GVR4 866/1000 (getting everything from above)

Jesus, that is low vacuum! My HKS 264/272 combo now yields 15-18 inHg and this combination works perfectly with that. I think you are missing half of the required "pull" to operate the diaphragm properly or anywhere near optimal.

I vote to install a vacuum pump. Now I say that knowing that my cams are installed straight up (even with AM cams gears - not confirmed lift or timing mind you!) and they could be dialed in so that it yields their desired positioning which could in turn equal more vacuum or not. I am not sure how far your cam tuning has gone Jason, because it is quite a tedious process, especially if you are constantly changing it and seeing how it runs on a dyno - but that is the only way to confirm other than how most are doing it, and that is to install them straight up and take for granted the potential for the grind to be one side of perfect alignment or the other from what the spec card says.

For shits and giggles, temporarily rig up a vacuum pump from something like a VW diesel of some other car that is easy to get a hold of parts from, and drive the car with all that zip tied under the hood and have it triggered with a switch to come on and off. This may or may not wreak havoc on the tune because of the changes, but if the engine runs stable enough then the change in vacuum will have a direct effect on pedal feel and brake output. I don't know what the actual pressure at your caliper is now, with X amount applied to the pedal and XX for vacuum, but if you install that gauge to the caliper, then you sure could see a change without driving anywhere, just on stands. Now a dynamic test on decel will be another story, because motors can reach a real decent vacuum range when engine RPM is sufficient enough and that TB blade is shut, or so it should happen in my books.

I vote, more vacuum. You are in territory where drag racers have had to install an extra boost to their brake system for just this reason. Picture those giant motors with huge, lopey cams .... ain't no vacuum happening there on par with what is needed.

Look what I found: http://www.superchevy.com/how-to/brakes ... uum-pumps/

They talk of adding a vacuum canister for the pump to run less, and a sensor inside the device to only have it running when it is really needed ... not sure on adjustment but if it has a physical spring of some sort, then it translates to real world parts swapping and so forth.

I think we are here!

Well I seemed to have found a problem with the 1" master cylinder. Not getting nearly enough pressure at the caliper.

Based on the results below I need to go to a SMALLER master cylinder, maybe a 7/8"

First value is normal foot pressure, every day braking. (should be 950-1000) The second higher value is as hard as I could push the pedal down (should be 1400-1500psi). Without the car running the values are a couple hundred lower.

I know Volvo's have had those pumps for a long time as a safety feature.

Try a vacuum reservoir or pump first.

The reservoir tees inline between check valve and booster.
You can get em from summit or jegs.

Revving the engine to 4 or 5 k, the sharply taking foot off gas pedal will really up the vacuum. The reservoir will trap it. (For lack of a better term) then see where you sit on brake pressure.

It's something domestic guys have dealt with for many years and big cams. Hence the reservoir.

It will build vacuum during normal driving, and be used when needed.

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If speed kills, then i shoulda been dead awhile ago. There is no such thing as "Too Much Power". There is no excuse for a lost race. Do you view the rev limiter as a fun limiter?or as a shift point? And we all know, more boost=more fun.

Even braking under full deceleration is not right. It's next to impossible to lock up the wheels. I'll do some more tests tomorrow and compare the pressures with the car off vs. on.

I think maybe because of the low vacuum and smaller booster going to a smaller master is the ticket. At this point, increasing the master would just make things worse.

Ok, I gained 500psi by shortening the booster rod. Now that I'm at the right pressure I can try a 1 1/16 master.