Before you swap out the vette MC, do you want to do a little further debugging? I keeps sounding like you might be bottoming out a piston in the MC which would mean something is taking too much fluid. It would be useful to see if that's the case and which one. You could do this relatively easily by creating a 'plug' for each port of the MC. Then with great caution, try out the brakes with one port plugged at a time. See what it does to pedal travel and stopping for each one. If you need some ideas for the plugs, let me know.
I plan on doing more troubleshooting, but on this one - the pedal stroke of the vette MC - it's 100% as a result of the MC. During a regular stop, it almost goes to the floor which is too long of a stroke for me. I swapped out the MC yesterday so I am back to the cop one. I may get the OEM one back out and try that, like you suggested.
The panic-stop/brick wall issue is what I am trying to solve. On ALL of the MC's, this has been the issue. Regular driving - pedal travel and braking power is ok. It was the best with the vette MC, but too much travel. Panic stops - the pedal stops almost immediately and gets very hard to push. Braking power is also massively diminished when this happens. It's why the PV was (and continues to be) my primary suspect.
You can try with the current MC too. Have you been able to do the MC swaps without a total rebleed? I know you replace the power booster, but that hard pedal sounds like a symptom of a bad one OR, how much cam you have in that 383? Maybe not enough vacuum for the rapid deployment. Maybe a restriction in the rubber hose feeding it?
All good suggestions, and all have been tested. The cam shouldn't be the issue - it still pulls a good vac at idle, and I've replaced the booster and the vac lines. The problem is unchanged between boosters and calipers, so that rules that out. There is good fluid flow at all 4 corners. The booster was actually my first suspect, too. Yes, the MC swaps have been dead simple and not required re-bleeding.
Faulty logic. There is a difference between how the two different boosters amplify user force or GM would not stock two different parts and the aftermarket would not stock the two types. You have made no reference to the different brake pedal designs or which booster you are using. See posts #30,31, and #33. Leverage and boost force are important to braking.
The panic-stop/brick wall issue is what I am trying to solve.
Measuring and reporting pedal stroke might help. Others might then measure/report their pedal stroke if you make the effort to post yours first.
The most definitive test would be a test pressure gauge. Then you would know what is happening to system pressure when the pedal gets hard and stops in travel. There are "bleeder repair kits" . You drill the stuck bleed valve then tap it for a new smaller one. So you could make your own test set by getting a spare bleed valve, tap it for a hose and gauge. A grease gun hose would take the pressure and has threaded ends.
Your system should work, anyone would be concerned and frustrated. What should get things working is a logical look at what is different or out of specification.
No, it kinda does rule it out because I left out all the other **** I've tried. For clarity:
Bought car. Kore3 C5 Z06 calipers up front. Navy Lifer DBC in back with vette calipers and 335 rotors. Hard pedal all around. Very hard pedal in panic stop. Stock booster, stock pedal.
Changed both front calipers with new GM calipers. No change. (Full bleed)
Changed pads all around to ceramic
Changed both rear calipers out. No change (Full bleed)
Changed MC to cop version. No change, but pedal uptake was a bit better so I kept it.
Tested vac at booster and manifold. All good. Changed out all vac lines anyway. No change
Changed booster with AC Delco booster. No change.
Changed MC again to 'vette MC. No change in panic stop. Regular driving brake "grab" is much improved, but I can't deal with the long pedal travel.
Cop MC is re-installed. (Full bleed) No change
Driving around town in normal braking, its fine. Good, even. The pedal effort is high, but nothing I can't deal with and its transparent to me. Its only when I jump on them that I'm like "**** - I may not stop". Foot pressure required on the pedal increases ten-fold and pedal travel decreases to about 1-2 inches and will go no further. Thankfully, I drive like a grandma 99% of the time so this rarely happens. To me, that screams "proportioning valve problem" because the PV only comes into play in a panic stop situation. However, lord knows I am not one to shy away from admitting when I've done something stupid and wasted my time and money, so I am fully prepared to be wrong.
Given what I've done, the booster and the calipers are not the issue. The MC is not the issue either. It's either a clogged brake line that only "clogs" in a panic stop, or its something in the the combi valve. I dunno. I'm purchasing a new (new to me) combi valve so we shall see.
That Motive bleeder is worth its weight in gold.....
I've seen brake hoses trap pressure in the caliper and then they won't release. Could be happening the other way upon a panic stop, but it would have to be both front hoses at the same time, repeatedly, which seems near impossible as I type this out.
Maybe for fun take the guts out of the proportioning valve so the same pressure goes everywhere in the brake system and (carefully) try a panic stop to see if you get the solid pedal/short stroke.
I would like to be wrong but I can not think of a way the proportioning valve could restrict the front brakes from working properly unless it was attempting to block all fluid due to a sensed flow problem.(bad bleed or line or caliper leak) This would light the "brake warning light"
OP has not mentioned any uneven braking front to rear, only overall hard petal. OP has mentioned ABS does not operate so one assumes front or back is not locking up and the OP has not mentioned lock up as a problem.
When I first bought my 95 SS, I couldn't get it to lock up or even get into the ABS. Very hard pedal. I bought the car anyway and straight away took it to a mechanic friend. He hooked up to it, could find no issues, cycled the ABS pump, and finally we took it out for a drive. I told him to drive and see if he could get it activate the ABS. He got up to speed and stomped the pedal. Hard! Worked as advertised. I've done both stealth bolt mods and put ceramic pads all around and it simply has a harder pedal than anything else I own. Requires a serious foot to stop, but stop it will. My take is that it's just the nature of the beast.
Update: Changed out all the flexible lines - which were no-name steel-braided - to Russell braided lines. The old lines were on there when I got the car 5 years ago so I don't really know how old they were. Brakes are much improved. The initial push takes a lot less pedal pressure and it has significantly more bite. The second push is hard, but that's expected due to the low vacuum from the cam.
I still have not been able to get to lock-up/abs activation, so there's more troubleshooting to be done.
Good to hear at least notably better. I caught a Scotty Kilmer vid and it jerked my memory to this thread. He's talking on mystery disappearing fluid from leaking past mc seals and filling up the booster tank. I know it's out there, and don't recall whether you had any leaking, - but there it is as a possible culprit if fluid is robbing available space for vacuum.
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