I have read everything I could find on this site and others regarding the Corvette 4-wheel disc manual brake master cylinder.
http://www.mpbrakes.com/mc390360pfull.JPG
I installed one in my CJ8. I am running lines with a tee to the front, and straight line with Wildwood adj prop to the rear. I have no residual or combination valve. I used the original manual brake actuator rod from the CJ8 and added 3/8" shim inside the Corvette MCs hole. There is absolutely no play between the pedal and the master. The front calipers are '79 Wagoneer and the rear are '85 ElDorado w/e-brake. The e-brake cables are installed, and the piston and adjuster arms have been adjusted. I have gravity bled the system. I tried pressure bleeding, but this made the pedal mushy. I am not sure if the actuator rod is supposed to physically attach to the master or if the rod can just sit in the hole. I would appreciate some constructive suggestions. I have been working on this brake system for close to 2 weeks with various different masters/boosters/prop etc...

Thanks,
Tom

R your calipers upside down? Hell i went thru the same thing then realized I had those damn son of a bitch motha fawkers upside down. turned them around and had great pedal!
It's always the little things!
Bob

Calipers are bleeder screw up. I even took them off the brackets and shook/banged/etc.. while bleeding. I can't believe there is any air in the system. Besides I thought when you had air that you could pump the brakes for good pedal. I can't. The Jeep stops okay, but it barely passed inspection today. The rule is stop in 20 ft. from 20 mph. I think it should be working much better than it is. My CJ7 will almost put you through the windshield with disc/drums and a GM booster. I am assuming that the Corvette setup should provide quick stopping power.

Later,

is your booster holding vacuum?

I don't have a booster, just MC, Calipers and a adj rear prop valve.

now i understand.....massive effort to stop....right?

i would install a booster at this point.

No, even with massive effort it doesn't stop fast. I tried this master with a booster already, and I don't have enough pedal ratio to make it work. I know there are several guys running the same setup I now have. I was just looking for opinions or suggestions as to why my brakes don't stop on a dime even when I get my foot into it.

Thanks,

have you tried a residual on the rear?

i think that 20 foot thing sucks,the new ram 1500 takes 196ft from 60:eek: .

go to www.mc-brakes.com and check out their trouble shooter guide it may help. did you bench bleed the master before install?
later
bob

I have a 2lb residual I can install on the rear line. I was just concerned that it might make the pads drag. Also, I can lock the rears up no problem if I adjust the proportioning valve all the way in (80-100% pressure available). i just don't think I am moving enough fluid, or the 1-1/8" bore is too big so I am not building pressure fast enough. The larger the bore the lower the overall system pressure.

Thanks,

2lb is what you should be running with rear discs. that should solve the probs when you had the booster on there IMO.probably had a lotta pedal travel without much braking,with the booster and no residual.

Thanks Larry,

That is what I was thinking, but the pads are really close to the rotors and I didn't think it would make that much difference. What do you think about 2lb residuals front and rear?

i would start with the 2lb on the rear and adjust your bias with the adj. you have.and put the booster back in if you have it.

I saw one thing in your posts that caught my eye. Zero freeplay at the master actuating rod. That could be blocking up the passages by not letting the piston return fully.
Working on motorcycles and racebikes in particular I learned a really unlikely trick that works great. I could bleed and bleed, and bleed some more and the brake lever would never really get as hard as the rider wanted. Pumping them for 30 seconds then duct-taping the lever with lots of pressure for a few hours always worked.
Maybe you could try the same thing with the Jeep. Pump up the brakes and rig up a bar to hold the pedal, then come back a few hours later.
Worth a shot :confused:

Just to share my experiance so far:
I have that exact same master w/ 1/2ton Chevy front & Wilwood 4 piston rear calipers. I tried bleeding w/ the stock master & combo valve. It would stop, but the pedal went to the floor. Since I only had a few days before a club trip this last weekend, I took it to a shop i know & trust to have them power bleed it. Still no pedal, so I had him get the Vett master & install it. Now my pedal is high & firm, but it takes a lot of effort (no booster). It worked OK for this trip. My next step will be to eliminate the combo valve as you did. Maybe I'll put in the 2 lb. residuals while I'm at it. I'll keep an eye on this post & let you know if I get mine working better before you. Probably wo'nt work on it till next weekend though.

Yeah I am really tired of having DOT3 all over everything. I am going to try finishing my other little projects, and then hit the brakes again this weekend. I will install the 2lb rear then 2lb front, and then the booster again. If I still don't have brakes after that I will probably have to kick my own ass. I thought about taking it to a shop, but I doubt the would understand what I am doing. They would probably say "68 Corvette what?, Wagoneer what?, adjustable what?, that won't work..."

I am almost done with the Scrambler. :D
Here are some pics:

http://home.off-road.com/~tom85cj7/scrambler.htm

Later,

I had the same problem with the Corvette M/C when I put discs on the rear Scout 44 in my CJ. I bled and bled and had very little pressure on the front brakes. What the problem was is that the rod from the brake pedal to the M/C was on a slight upward angle, (pin on pedal higher than M/C). This caused the M/C piston to not return all the way after pressing on the pedal. I could bench bleed it great and get good pressure on the bench, but as soon as I put it back into the Jeep no pressure. The solution was to make a slight bend in the rod right where it entered the M/C piston, this took care of the interference problem between the rod and the piston, now I have great brakes.

Hope you get thing worked out, I know I was extremely pissed when I was working on mine.

Tom

TJS80:
So if the pedal rod is holding the piston slightly open then you won't have good brakes? I noticed that when I tried pressure bleeding the system the fluid would travel back toward the master even with the pedal released. I did a gravity bleed and everything seemed alright. Maybe my pedal rod is slightly holding the MC open. I have a 3/8" shim I can reduce/remove. I was also wondering if the pedal rod needs to be attached to the MC somehow. That is how it is on lots of other manual MCs.

Thanks,

Tom,

Here is what I'm guessing is wrong, I didn't bother to read the rest of the responses so forgive me if someone said this. Unhook the e-brake cables from the lever on the rear calipers. Then, push the lever forward, then pull it back, go forward again, do this until you can't go any further and re-hook the cables back up. What is happening is the piston is too far from the rotor and when you get on the pedal it is moving the piston but it isn't pushing against the rotor until the end of it's stroke...which is when your pedal is on the floor. This procedure will tighten everything up and make things much easier for you.

CJ

Originally posted by tom85cj7
TJS80:
So if the pedal rod is holding the piston slightly open then you won't have good brakes? I noticed that when I tried pressure bleeding the system the fluid would travel back toward the master even with the pedal released. I did a gravity bleed and everything seemed alright. Maybe my pedal rod is slightly holding the MC open. I have a 3/8" shim I can reduce/remove. I was also wondering if the pedal rod needs to be attached to the MC somehow. That is how it is on lots of other manual MCs.

Thanks,

That will do it because the brake system will never close, ever. You'll always have fluid share (Im tired so I cant explain well) . I've also see spongy brakes when the lines were hooked to the wrong res of the MC (ie front to front, rear to rear instead of the opposite)

Don't know if it helps but I cut off some of the push rod to get a full return stroke at the master cylinder. Works great now.

:usa: :usa:

Tom-
From your picture it looks like the MC was designed for a booster.
And you are probably right ... manuals are generally hooked up mechanically.
That little indent in yours is supposed to accept a straight shot plunge from a booster and my Vanco HydroBoost Kit R & D experiments confirm that even the slightest pressure on your MC's button will engage the brakes.
I think you'all have touched on the keys here...

I don't recall you saying you have a rock hard petal ever.
Make sure your pushrod is well aligned with the MC piston's center line had has a good, non binding swing.
Remember, your petal end of the rod swings in an arc so you need to make sure there is not too much side force.
Then give the rod just a little "play" to ensure there is no engagement.

If that doesn't help, it still sounds like it could be a bubble in your new MC.
Van told me a good trick for fixing this ...

Master Cylinder Bleeding
1. Get a helper to jump in the Jeep and work the petal.
2. Back off the fittings for your brake lines on the MC one turn (both of them).
3. Have your helper push the brake pedal to the floor.
4. Tighten both brake lines and have the helper release the brake.
Repeat 2-4 until the fluid streams out of both ports without "sputtering".

And Good Luck!


:p

Just a question. Did you run the 1/2-20 port for the front?

I think I responded via email but,

I run a 68 MC mines an 1 1/8 bore. There are two tyes of masters out there, ones the one I have which is made for 4 wheel discs manual or power off a vette. The other is a 7/8s bore made for manual discs only.

The masters are reverse as far as ports go. The one closes to the fire wall is the front and the one furthest away is the rear.

I run no residual valve, no poportioning valve, just straight lines to each end and mine will put you through the windshield if I step on them hard. I dont have to apply alot of pressure to get them to work properly.

you have one of two problems,

Your push rod is to long.

Or

Your lines are switch, which I am betting is the problem.

Steve

two things, swap the lines. If that doesnt work try gravity bleeding the system. I've run em with the lines hooked up both ways, and while there was a marked improvement with them hooked up correctly it still stopped fine with them hooked up backwards.

I don't run the vett master but I do run vett calipers in the rear and the standard ford 2 piston calipers in the the front. I run the willwood pedals and willwood masters and both are 3/4 bore. I don't run any proportioning valve or residule valve just the proportioning bar on the pedal assembly. I also run 3/16 brake lines and nothing larger. My brakes work very good and it is all just by leg power no powere assist.
So did you bench bleed the master before you hooked up the lines or just gravity bleed it. If you havent blead the master then you really need to do that in order to get it to work right. I also think that the pedal isn't letting the master fully return and that will cause the problem too because it cant let in a new shot of fluid so it won't ever pump up.

I checked with power brake exchange here in San Jose and they said the 1/2-20 port on the vette MC is for the front brakes. The 1/2 and 9/16 ports are the reverse location of the CJ MC, so some tweaking of the lines is required.