brake question

mulescj8

Ok guys someone tell me what to do here.........running 4 wheel disc brakes off a 78 to 80 chevy 3/4 ton truck....3/16 hard and soft lines.............just switched over to 68 corvette master cyl. for 4 wheel manual disc. and running 42" tires.......and i have damn near no brakes,I don't expect it to stop on a dime at 50 or 60 mph........but even at 15 mph it will barely stop.........this is a trail only rig and will never see street use again.........have bled them before the swap and did a bench bleed and have the rear bowl going to the rear brakes and the front bowl going to the front.......I'm open for suggestions or tell me about your setups if you are runing a simalar set up.. :confused:

jerry

on my cj5 i changed to a bigger master cylinder a bigger bore I think I went from 15/16/ to 1 1/8 I can not believe ho well it stop now before there was no paddle you would have to pump them jerry

tufcj

Jerry has part of the answer. How big was the old M/C bore VS. the new. If the Corvette bore is smaller, you may not be moving enough fluid for the HUGE 3/4 ton calipers. Even though the Corvette was 4 wheel disc, the calipers were probably much smaller, so there wasn't the same demand for fluid. The other thing is the actuating rod on the booster. If it's too short, you waste pedal travel before the piston moves. If it's too long, the piston is already partially depressed, and you don't get full travel or move enough fluid. Measure and compare the depth from the mounting surface to the piston on both M/Cs. A friend put 4 wheel discs on a 57 Chevy, replaced every part with new (and the M/C several times, but with an identical replacement) and couldn't get a solid pedal. When I finally got him to upgrade the M/C to a bigger piston unit (universal from a street rod shop), the pedal firmed right up. Bob tufcj

mulescj8

Ok bore is 1 1/8" and i have an adjustable brake rod and it's adjust to where there is no travel before engaging the master cyl. this thing will never see road use again and that's the reason for ditching the booster and going to manual brakes...........just want to be able to stop and have it hold on a steep hill with the motor not running.........or running down a dirt road for a few miles at 30 or 40 and be able to shut it down if i have to.thanks guys........i have this posted on a few other boards so if i find a fix i will let everyone know...........still open to ideas and suggestions

Toy Maker

If your problem is that the brakes require excessive effort to apply or hold, then you will need to go with a smaller diameter bore master cylinder to increase the line pressure or increase the leverage of the pedal over the cylinder. A smaller cylinder will result in a lower pedal position when depressed to move the same amount of fluid volume. If the pedal bottoms out with a smaller master cylinder, then you may have to go back to power. Using an aftermarket racing style dual master cylinder setup with a balance bar may give you the adjustability you need, but at a price.

youdidwhat

if you have the space put the booster back on. those 3/4 ton breaks were designed to have a big old booster pushing them out. unless you can put the breaks from the vette on there with the manual master, youy will not likley get enough force thru the pedal. (massive giant weightlifters excluded). plus with underpowered breaks like that the time you need the extra stopping power on the trail will be the time you also need the other foot for clutch etc. i had this trouble with trying to remove the booster on a 68 cougar. and wound up getting one shipped from Co. just my $.02

mulescj8

thanks guys for all the input...........finnally got everything mounted up but still haven't had a chance tp work on it and finish rebleeding the system........twisted a bleeder off and then broke two ez outs trying to remove it.......so i just went and bought another caliper........will get it swaped out this week and then rebleed everything.........as far as the booster there is room i had a huge one on it and was never happy with the brakes..........since this thing will be a trail only rig i really want to stay away from the power brakes..........i want to beable to have brakes with the motor not running..........it will never see any speeds above 35 or 40 mph ever again

tufcj

The booster is just a helper. When it's running, the vacuum assists you in pushing the pedal. If your engine pulls a good vacuum, and everything else is in good shape, you'll get 1 or 2 pushes of the pedal with assist even with the engine off. No vacuum = manual brakes, just like without a booster.

My CJ is trail only, 37" tires, 4 wheel discs. With proper gearing, I can avoid using the brakes in all but the toughest spots, but I'd still hate to be without power. After some long days, my leg is tired with power brakes, I'd hate to feel it without.

I've had my eye on one of the dual diaphram boosters from www.mpbrakes.com for a while. They also have a pretty good FAQ section in the tech area on the lower left of the home page. Someday.... when I have the time and money.

Bob
tufcj

mulescj8

Bob i got pissed off after two bad master cyl. and broke down and ordered the MP set up last week..................hopefully in a month or two i can report back on how it's doing-JC