My '26 T coupe. 4 wheel disc. GM metric calipers. Rears are the ones with the built in emergency brake. Underfloor Corvette MC w/1" bore. 6:1 pedal ratio. 2# RPV's front and rear, right at the MC. No leaks in the system. Have had 3 different MC's in it. All with 1" bores. All with the same results. No matter how hard I stand on the pedal it will not slide the tires. There is no "pump up" to the pedal at all. I am 99.999% sure all the air is out of the system. I have run gallons of brake fluid through it. Vacuum bleeding and the old pump up and hold method. I get no air. Pretty sure there is none. The pedal has a slightly long throw. Seems like it should catch quicker. Don't get me wrong, the brakes work very well and will stop the car quickly, but it is like it has ABS and will not slide the tires. Can't figure it out. I think the system is configured correctly with parts that should work together. This thing ought to be able to lock up. So where is the problem?
Locking up the tires is not a good thing to do. Brakes don't slow cars down very well when the tires are sliding. Is everything else OK other than that? Are you able to make emergency stops? How does your braking performance compare to the table at the end of this article? https://nacto.org/docs/usdg/vehicle_stopping_distance_and_time_upenn.pdf
I get confused on this. 7/8" bore would require less pedal pressure and more pedal movement to achieve the same braking as the 1"? So, the same pedal pressure on the 7/8" cylinder should give more brake effect than the 1" MC?
Yes, a smaller bore will increase pressure with the same input force. I have a thread I did for tech week that may help. https://www.jalopyjournal.com/forum/threads/tech-week-57-fargos-brakes-101.1229211/
I have a 7/8” bore MC, disc/drum, not quite 6-1 pedal, no power 3600 pound car, and can slide the tires in a test. Your MC is to big. Contact manufacturers like Wilwood and ask them.
That is a great article. I think I will lay out 100 feet on my country road and do a 45 mph stop with a marked braking point to eliminate reaction time from the equation. According to that chart I should be able to stop in 97 feet. It does make good stops. I realize that sliding the tires can be bad in most situations from thousands and thousands of laps going in circles . But I've had to pitch it sideways with the brakes locked more than once. I'd just like to know that the car is capable of that. I guess if I had to pick on something in the brake system, the pedal feel is not the firmest I have ever felt. It feels a slight bit mushy.
Yeah, mine is not power either. But my T is 2500# soaking wet. I'm going to look into a 7/8" MC. I hate to call somebody like Wilwood when I am not running any of their stuff. Unless maybe they have an MC to fit my needs. I'll check. Thanks guys. Can someone confirm or deny this just to get my head straight?
Oh Yeah! Sorry I missed that! Thanks! Also great post on brakes. Just skimmed it, but will go back and read it thoroughly.
In terms of braking effectiveness, locked brakes are not a good idea. But an old school brake system that won't lock up all four corners at the far end or extreme of braking is most definitely not setup or adjusted correctly.
Wilwood offer at least 2 dual MC’s I’m aware of. I believe they have at least 3 bore sizes. I did not know of the smaller bowl size until I had purchased the larger one.
I have basically the same setup and this was my first thought after getting them done. After putting a few miles on them, the pads seem to have bedded in and now work much better. The brakes feel much better overall now, not soft feeling like they were at first and they'll lock up if I want them to. It may be your pads just need to bed in.
What are we fixing here? Why do you want to go into a skid? If locking up the brakes were an important thing to do, then why did the automotive industry spend billions-of-dollars designing and perfecting systems to prevent it from happening? If it hard-stops in an acceptable fashion, and you can retain control of the vehicle while doing so, the job here is done.
I don't care how many billion$ they spent to make a brake system that I absolutely hate. ABS ****s. It is for people who can't drive. I know how to feather a brake. We are fixing a brake system that is not working properly. This system should be able to lock up all 4 tires, but it doesn't. So there is a flaw somewhere in the system. Knowing this does not generate a lot of confidence while driving.
Okay then. Since the other components match, you choice here is a smaller bore master cylinder, but be aware, the pedal effort will feel softer, and you will need longer pedal travel to accomplish the same task. On a side note: Do you have a parking brake hooked up, or are the rear calipers just going along for the ride? If you do not have a means of actuating them, try to manually cycle the levers as many times as you can before you can no longer move them. Actually, do that even if you have a lever attached.
I saw this post about a stepped master cylinder. Not sure if it applies here. https://www.jalopyjournal.com/forum/threads/7-8-bore-master-cylinder-oem-applications.216030/
I do have the emergency brake hooked up and keep it pumped up. In fact, the instructions with the calipers mentioned to actuate the E brake occasionally for this reason. I had never seen calipers with E brakes built in until I bought these. I have wondered if these could be part of the problem. They seem to be pumped up tight. Yes I realize the consequences of changing to the 7/8" MC. That is kind of the problem. The system now takes more pedal movement to actuate than I like. Since the 7/8" MC takes less effort with more stroke, maybe shortening my 6:1 pedal ratio to 5:1 or maybe a little less would account for that. With the 4 wheel disc and no power, I don't want to get the required pedal pressure too high. Changing the pedal ratio will be a chore. I built the nicely bead rolled, 20 gauge floor around the pedal and like a dummy didn't leave room to slide the pedal off of it's pivot stud. Last resort. I think I will try the 7/8" MC first and see how that goes.
Anyone know about the stepped master cylinder mentioned in post #20 above? Does that apply to @jaw22w ? @6sally6 ?
Pad material composition, (and rotor material) makes a huge difference. A lot of people don't like any noise or brake dust. Some people even purchase "Racing" brake pads thinking they will stop really well for their grocery getter. They do, though not when cold, which is probably 90% of their driving. Also "bedding in" new pads, if everything gets glazed over they won't give good braking.
I'm of the wanna lock them up school, just to know I'm getting all of the brakes I can. I'm also of the school that says if you lock up your rear brakes first, you will spin out. Never wanna have that, either. If you have a bias adjuster, locking up the rears first is the only way to know you have gone too far.
The MC I eventually used on my off-topic "M-word" car that used S-10 calipers was recommended by one of the VERY knowledgeable guys here on the HAMB! Like most well-trained home mechanics I did NOT write down part number or anything! that would help find the MC or which one to order. Sorry It was a Chevy MC for front discs and rear drums. The HAMB 'brake-guy' pointed out that GM has been using this type of MC for a long time. (that makes me think an S-10/Camero/starting in the pre 90's should work with these particular calipers). Maybe he will see this and jump in. I do remember he was from Oz if that narrows the search any?! 6sally6
What calipers are you using ? have you adjusted the rear calumets after being installed ? Not just using the e brake to ratchet it up ? power brakes or manual ? disc /disc master or disc drum ? what type of pads snd rotors have you used ? Material and composition make a difference . peddle ratio and free play ? Go with a smaller master cylinder and modify your brake pedal accordingly to get the ratio correct, can you add a booster if you don’t have one ? harder to lock up 4 wheel disc then a drum brake set up. With everything else being equal.
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